Transmission electron microscopy and theoretical analysis of AuCu nanoparticles: atomic distribution and dynamic behavior

Though the application of bimetallic nanoparticles is becoming increasingly important, the local atomistic structure of such alloyed particles, which is critical for tailoring their properties, is not yet very clearly understood. In this work, we present detailed study on the atomistic structure of Au-Cu nanoparticles so as to determine their most stable configurations and the conditions for obtaining clusters of different structural variants. The dynamic behavior of these nanoparticles upon local heating is investigated. AuCu nanoparticles are characterized by high resolution transmission electron microscopy (HRTEM) and energy filtering elemental composition mapping (EFECM), which allowed us to study the internal structure and the elemental distribution in the particles. Quantum mechanical approaches and classic molecular dynamics methods are applied to model the structure and to determine the lowest energy configurations, the corresponding electronic structures, and understand structural transition of clusters upon heating, supported by experimental evidences. Our theoretical results demonstrate only the core/shell bimetallic structure have negative heat of formation, both for decahedra and octahedral, and energetically favoring core/shell structure is with Au covering the core of Cu, whose reverse core/shell structure is not stable and may transform back at a certain temperature. Experimental evidences corroborate these structures and their structural changes upon heating, demonstrating the possibility to manipulate the structure of such bimetallic nanoparticles using extra stimulating energy, which is in accordance with the calculated coherence energy proportions between the different configurations.     

Molecular dynamics simulation on the tribology properties of two hard nanoparticles (diamond and silicon dioxide) confined by two iron blocks

The tribology behaviors of diamond and silicon dioxide (SiO₂) nanoparticles were examined via molecular dynamics simulations; four cases were simulated. At low velocity and low load, the nanoparticles separated the two blocks from each other and acted as ball-bearings. The plastic deformation, temperature distribution, and friction force were all improved due to the action of the nanoparticles. However, the crushing of the SiO₂ nanoparticles was accompanied by deformation-induced loss of the rolling effect, when the load was increased. Without nanoparticles, a transfer layer formed at high velocity and low load. The two nanoparticles provided support for a certain duration. However, at high velocity and high load, the support effect of these nanoparticles was lost in a short sliding time.     

Controlled Synthesis and Optical Properties of Pure Gold Nanoparticles

Abstract

Gold nanoparticles were synthesized by laser ablation of a gold metallic disc at wavelengths of 532 nm and 355 nm with 7 ns pulse duration in the pure water. The colloidal gold nanoparticles were characterized by ultraviolet-visible absorption spectroscopy, transmission electron microscopy, and fluorescence spectrometry. The presence of a surface plasmon resonance peak around ～ 524 nm indicates the formation of gold nanoparticles. The formation efficiencies of gold nanoparticles in colloids were found to increase when ablating the gold metallic disc with a laser having a longer wavelength. The size distributions of the gold nanoparticles thus produced were measured by transmission electron microscopy. A reduction in mean diameter of the particles was observed with a decrease in the laser wavelength under the irradiation at a high fluence of 25 mJ/pulse. The fluorescence spectroscopy demonstrated that these gold nanoparticles are fluorescent, showing a strong blue emission intensity at 458 nm.     

Laser surface hardening of 42CrMo cast steel for obtaining a wide and uniform hardened layer by shaped beams

For laser surface hardening (LSH) of large-sized workpieces, a wide and uniform hardened layer of a single track is pursued. In this study, two kinds of shaped laser beams were used in LSH of 42CrMo cast steel to obtain the required hardened layer. One is a stripy spot with uniform-intensity array spots and the other a stripy spot with intensity blowup in the edge of the whole array spots. As a comparison, a Gaussian laser beam was also adopted. A three-dimensional finite element model was used to simulate the thermal history of specific points by the latter shaped beam and the Gaussian laser beam. The surface morphology, microstructure, microhardness, and uniformity of hardened layers were studied. The results showed that a wider and more uniform hardened layer could be obtained using the latter shaped beam at relative higher scanning velocities and laser power. The thermal history of a material has an important effect on the microstructure and microhardness finally formed. Due to the high peak temperature and heating rate caused by the latter shaped beam, a higher value of microhardness in the transformation hardened zone was found.     

Silver nanoparticles synthesis using Wedelia urticifolia (Blume) DC. flower extract: Characterization and antibacterial activity evaluation

Silver nanoparticles (AgNPs), synthesized by green methods with the property to kill microbes, are highly valuable in medical sciences. So, the current study was aimed at using the flower extract of Wedelia urticifolia for synthesizing AgNPs with antibacterial properties. The AgNPs were produced by adding the extract to three different AgNO₃ concentrations (1, 10, and 100 mM) in nine possible flower extract to metal salt ratios (9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, and 1:9). The formation of brown color and the presence of a peak at 431 nm in the UV–Vis spectrum of the colloidal solution indicates the synthesis of AgNPs, which were also characterized by dynamic light scattering (DLS), Fourier transform infrared spectroscopy, X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy‐dispersive X‐ray spectroscopy. The DLS results exposed that the smallest sized AgNPs were obtained in 10 mM AgNO₃ solution and 4E6M was the optimized extract to metal salt solution ratio. The characterization techniques revealed that the synthesized AgNPs were spherical shaped and crystalline with a diameter of less than 30 nm. Furthermore, the synthesized nanoparticles were tested against two Gram‐positive (Klebsiella pneumonia and Staphylococcus aureus) and two Gram‐negative (Pseudomonas aeruginosa and Escherichia coli) bacterial strains for their antibacterial efficiency. Although the studied strains showed limited growth, overall, the effect of nanoparticles was found to be insignificant. It is concluded that the current study is advantageous over other previous studies because the AgNPs were synthesized at room temperature from 10 mM AgNO₃ concentration in only 2 hours. Additionally, the present work is simple, ecofriendly, and in‐expensive.     

Experimental investigation on convective heat transfer and friction factor in a helically coiled tube with Al2O3/water nanofluid

In this study, the heat transfer and friction factor of a shell and helically coiled tube heat exchanger using Al₂O₃ / water nanofluid at 0.1%, 0.4% and 0.8% particle volume concentration were tested. The test was conducted under laminar flow condition at 5100 < Re_i < 8700. It is found that the overall heat transfer coefficient, inner heat transfer coefficient and experimental inner Nusselt number are 24%, 25% and 28%, respectively, higher than water at 0.8% particle volume concentration of nanofluid. It is observed that the presence of nanoparticles further intensify the formation of secondary flow and proper mixing of fluid when nanofluid passes through the helically coiled tube. Apart from further flow intensification, higher thermal conductivity of nanofluid and random movement of nanoparticles contribute to the enhanced heat transfer coefficient. Also found that the friction factor increases over particle volume concentration and this is due to increased nanofluid viscosity while increasing particle volume concentration.     

Friction and wear behavior of laser composite surfaced aluminium with silicon carbide

The present study concerns the wear behavior of laser composite surfaced Al with SiC and Al + SiC particulates. A thin layer of SiC and Al + SiC (at a ratio of 1:1 and dispersed in alcohol) were pre-deposited (thickness of 100 μm) on an Al substrate and laser irradiated using a high power continuous wave (CW) CO₂ laser. Irradiation leads to melting of the Al substrate with a part of the pre-deposited SiC layer, intermixing and followed by rapid solidification to form the composite layer on the surface. Following laser irradiation, a detailed characterization of the composite layer was undertaken in terms of microstructure, composition and phases. Mechanical properties like microhardness and wear resistance were evaluated in detail. The microstructure of the composite layer consists of a dispersion of partially melted SiC particles in grain refined Al matrix. Part of the SiC particles are dissociated into silicon and carbon leading to formation of the Al₄C₃ phase and free Si redistributed in the Al matrix. The volume fraction of SiC is maximum at the surface and decreases with depth. The microhardness of the surface improves by two to three times as compared to that of the as-received Al. A significant improvement in wear resistance in the composite surfaced Al is observed as compared to the as-received Al. The mechanism of wear for as-received vis-à-vis laser composite surfaced Al has been proposed.     

Investigation of Annealing Temperature on Structural and Morphological Properties of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanoparticles for Humidity Sensor Application

Cr₂O₃ nanoparticles have been prepared for precipitation technique at reaction temperature 50 °C. The prepared samples were annealed different temperatures at 500,700 and 1000 °C. Synthesized powders were characterized as X-ray diffraction, optical, transmission electron microscope, SEM with EDAX, humidity sensor, FTIR. The annealing temperature has been found to be playing a crucial role in the controlling particle size. XRD study shows the rhombohedral crystal structure of highly preferential orientation along (1 0 4) direction. FTIR reveals that the presence Cr–O bonds in the structure. The TEM images show that the size of NPs of Cr₂O₃ varied from 26 to 60 nm with average crystalline size 43 nm. UV–visible spectrum shows the absorption band of Cr₂O₃ nanoparticles at 400 nm. The humidity sensor of the Cr₂O₃ nanoparticles was studied by two temperature method. 1000 °C annealed Cr₂O₃ nanoparticles show better sensing properties and exhibits good linearity in response than 500 °C. SEM images show the clusters and agglomeration of nanoparticles. EDAX spectrum confirms the presence of Cr₂O₃ nanoparticles. Each samples have been characterized as sensing materials to determine relative humidity in the range of 20–90%. The humidity sensing property increased with increasing of annealing temperature and the resistance was decreased.     

Design and evaluation of high-pressure nozzle assembly for laser cutting of thick carbon steel

Laser cutting of carbon steel is extensively used across a range of industries, due to its advantage of high speed, low kerf and high quality. Currently, a 1-kW carbon dioxide (CO₂) laser with its subsonic nozzle assembly can be used only to cut steel plates up to around 10 mm. This paper aims to design and evaluate a high-pressure supersonic laser cutting nozzle assembly, which can enable a 1-kW CO₂ laser to cut steel of up to 50 mm thickness. Basic gas dynamic and compressible flow equations were used to design the supersonic nozzle assembly. The flow of the high-pressure gas jet inside the nozzle assembly was investigated using computational fluid dynamics (CFD), and the structural integrity of the high-pressure nozzle assembly was ensured using finite element analysis (FEA). The gas flow pattern at the exit of the nozzle assembly was computed and compared with the experimental observation made through a shadowgraph technique. Laser cutting experiments were performed with the developed supersonic nozzle assembly to demonstrate cutting of 50-mm-thick low carbon steel with 1-kW CO₂ laser.     

Scanning electron microscopy of bio‐fabricated Fe2O3 nanoparticles and their application to control brown rot of citrus

Citrus is the leading fruit crop of Pakistan and exported to different parts of the world. Due to suitable weather condition, this crop is affected by different biotic factors which seriously deteriorate its quality and quantity. During the months of November 2018 to January 2019, citrus brown rot symptoms were recurrently observed on sweet oranges in National Agricultural Research Centre (NARC), Islamabad. Causal agent of citrus brown rot was isolated, characterized, and identified as Fusarium oxysporum. For environment‐friendly control of this disease, leaf extract of Azadirachta indica was used for the green synthesis of iron oxide (Fe₂O₃) nanoparticles. These nanoparticles were characterized before their application for disease control. Fourier transform infrared spectroscopy (FTIR) of these synthesized nanoparticles described the presence of stabilizing and reducing compounds like alcohol, phenol, carboxylic acid, and alkaline and aromatic compounds. X‐Ray diffraction (XRD) analysis revealed the crystalline nature and size (24 nm) of these nanoparticles. Energy dispersive X‐Ray (EDX) analysis elaborated the presence of major elements in the samples. Scanning electron microscopy (SEM) confirmed the spinal shaped morphology of prepared nanoparticles. Successfully synthesized nanoparticles were evaluated for their antifungal potential. Different concentrations of Fe₂O₃ nanoparticles were used and maximum mycelial inhibition was observed at 1.0 mg/ml concentration. On the basis of these findings, it could be concluded that Fe₂O₃ nanoparticles, synthesized in the leaf extract of A. indica, can be successfully used for the control of brown rot of sweet oranges.     

类富勒烯碳薄膜的结构演变及摩擦学性能研究

通过等离子体增强化学气相沉积技术,以不同沉积时间在硅表面上制备类富勒烯碳薄膜,探究类富勒烯碳薄膜结构演变和摩擦学性能随沉积时间变化规律。利用拉曼光谱和透射电子显微镜,考察类富勒烯碳薄膜微结构和表面形貌随沉积时间的变化。结果表明:碳薄膜内类富勒烯结构含量随沉积时间先增加后保持不变;采用沉积时间为3 h的类富勒烯碳薄膜组成摩擦配伍对,当载荷从8 N增加到14 N时,摩擦因数从0.013降至0.006,即随载荷的增加实现了由低摩擦向超滑的转变。这是因为摩擦诱使类富勒烯碳薄膜发生结构转变,并形成有利于减少摩擦的类球状或外部石墨壳层闭合的纳米颗粒。     

Characterising the surface and interior chemistry of core-shell nanoparticles using scanning transmission electron microscopy

A method for extracting core and shell spectra from core-shell particles with varying core to shell volume fractions is described. The method extracts the information from a single EELS spectrum image of the particle. The distribution of O and N was correctly reproduced for a nanoparticle with a TiN core and Ti-oxide shell. In addition, the O distribution from a nanoparticle with a Cu core and a Cu-oxide shell was obtained, and the extracted Cu L_2,3-core and shell spectra showed the required change in EELS near edge fine structure. The extracted spectra can be used for multiple linear least squares fitting to the raw data in the spectrum image. The effect of certain approximations on numerical accuracy, such as treating the nanoparticle as a perfect sphere, as well as the intrinsic detection limits of the technique have also been explored. The technique is most suitable for qualitative, rather than quantitative, work.     

Tribological behaviour of MoS2 and inorganic fullerene-like WS2 nanoparticles under boundary and mixed lubrication regimes

Abstract

For several years different types of nanoparticles have been considered and studied as potential friction modifying lubricant additives. Some nanoparticles can reduce the friction coefficient by 30–70%, depending on the base oil and the experimental conditions. In the present study, an experimental analysis on tribological properties of inorganic fullerene-like metal dichalcogenides was performed in comparison with MoS₂ 2H layered structures. Tribological tests were carried out on a pin on disc tribometer in ambient air. Several contact conditions are analysed in order to realise boundary and mixed lubrication regimes. The experimental study was performed on a mineral base oil, and particle concentration effects were analysed. Antifriction properties were evaluated by measuring the friction coefficient and are presented as generalised Stribeck diagrams. Inorganic fullerene-like WS₂ and MoS₂ nanoparticles present interesting friction reduction properties when tested in boundary and mixed lubrication.     

Laser Based Flexible Fabrication of Functionally Graded Mould Inserts

A CO₂ laser-based freeform fabrication process with emphasis on difficult-to-shape and functionally effective materials was investigated with regard to fabrication of dies and moulds. Square and circular moulds were built by use of a material additive process of layers of TiC and Ni-alloy composite. The effects of laser processing on the quality, microstructures, and hardness of the moulds were determined. Additionally, the mould performance was evaluated in die-casting, injection moulding, and in a thermal fatigue environment. The TiC core-Ni-alloy shell mould outperformed moulds made of hardened H13 die steel, 304 stainless steel, and TiC-coated stainless steel in withstanding stresses and in retaining dimensional stability at elevated temperatures.     

High Lubricity of Re-Doped Fullerene-Like MoS2 Nanoparticles

The electrostatic effects in tribological systems have been studied in the past, especially with regards to data storage media. Nanoparticles (NP) of WS₂ and MoS₂ with fullerene-like structure (IF) have been studied in the past and showed very good tribological behavior. Being semiconductors, their electrical properties can be controlled by, e.g., substituting the lattice Mo (W) atoms with Re (n-type conductivity) and Nb (p-type conductivity) atoms. In this study doping of IF-MoS₂, and to a lesser degree IF-WS₂, NP with small amounts (< 1 at.%) of rhenium atoms has been studied. For this purpose two new synthetic approaches have been pursued. The doped nanoparticles were characterized by various techniques. In particular, the doping density was determined by ICP-MS technique. The resistivity of the nanoparticles was shown to decrease significantly with increasing doping level. In contrast to the undoped nanoparticles, the doped NP were shown to exhibit reduced agglomeration and produce stable suspensions in PAO-4 and PAO-6 oils. Extensive tribological measurements with these PAO oils formulated with 1 wt % of the doped NP showed friction coefficients as low as 0.01 in mixed lubrication conditions and negligible wear. Microscopy analysis of the tribological surfaces reveal very smooth but discontinuous and dense film of the doped NP on the tribological surfaces. It is proposed that the doped NP are negatively charged at their surface eliciting mutual repulsion, which has a remarkable influence on their rheological properties and their tribological behavior.     

飞秒激光参数对硅表面微结构影响的研究

为了制备出表面具有准规则排列的微米量级锥形尖峰结构的黑硅材料,在SF6气体氛围中,用一定能量密度的飞秒脉冲激光照射单晶硅片表面。针对激光通量和激光峰值功率这两个参量分别进行实验,具体分析了15fs和130fs脉冲宽度的飞秒激光脉冲作用下硅表面微结构的形成,不同实验条件下制备出的硅微纳结构也有明显的差异。研究表明,在同一背景气体下,激光的峰值功率对硅表面微结构的形成起着决定性的作用。     

Soft X-ray contact microscopy with nanosecond exposure times

High resolution (better than 20 nm) contact micrographs have been produced with exposure times of about a nanosecond. The illuminating source was a short-lived carbon plasma produced by focusing a single short (～1 ns) 100 J pulse from the Vulcan laser at the Rutherford Appleton Laboratory (RAL) to a 300 μm spot on a graphite target. This plasma emits strongly in the soft X-ray region, particularly at the CVI (3.37 nm) and CV (4.03 nm) lines. The specimens were behind a 100 nm thick Si₃N₄ window, at atmospheric pressure in an environmental cell. The images of diatoms recorded on X-ray resist showed features down to the limit of resolution of the SEM used to view the developed resist, which was about 20 nm.     

Experimental investigation and numerical analysis for machinability of alumina ceramic by laser-assisted grinding

Alumina (Al₂O₃) ceramic has been widely used in various fields, but it has certain difficulties in machining as a hard and brittle material. While laser-assisted grinding (LAG), an alternative and novel method for fabrication of alumina ceramic, can utilize laser beam to locally heat the workpiece before the ceramic is removed, thereby reducing fracture toughness and keeping the surface integrity. In this paper, a thermal model is established to study and understand the processing mechanism of the LAG process. Meanwhile, an orthogonal experiment is designed and implemented to optimize the grinding process. Then, by analyzing the surface topography, the advantages of LAG are strongly proved. It is found that the temperature modelling results matches experimental results well. The processing parameter that has greatest impact on surface roughness is laser power, followed by grinding depth and wheel speed, and feeding speed at last. The optimal surface roughness value can be obtained by certain processing parameters. Also, compared to conventional grinding (CG), the removal method of alumina ceramics alters from brittle fracture to plastic fracture. Overall, this study clearly elucidates that LAG of alumina ceramic is a very promising machining method, and can be potentially utilized for various industrial, aerospace and automobile applications.     

基于碳纳米管的偶氮纳米复合材料的制备及光电导性能研究

通过改进的液相直接沉淀法制备了2,5-双(1'-偶氮基-2'-羟基-3'-萘甲酰基-邻-氯苯胺)噁唑(AZO)的纳米微粒,并将AZO纳米级颗粒固定在多壁碳纳米管上(MWCNT-AZO).利用透射电镜、X射线衍射、X射线光电子能谱和紫外-可见光吸收光谱表征了AZO纳米级颗粒和MWCNT-AZO纳米复合体系的结构.以AZO纳米微粒或MWCNT-AZO纳米复合材料作为载流子发生材料,制备单层光电导体,采用光致电压衰减法研究光电导体性能.结果表明,由于纳米尺寸效应,AZO纳米微粒的光电导性能要比本体AZO颗粒好的多.同时发现,相对于本体AZO和MWCNT/本体AZO复合材料,MWCNT-AZO纳米复合材料表现出更佳的光电导性能和更宽的光谱响应范围,这是MWCNT-AZO纳米复合材料的纳米尺寸效应以及AZO纳米微粒向MWCNT的电荷转移引起的.     

片式微波组件激光软钎焊气密封装技术研究

文中对片式微波组件激光软钎焊气密封装工艺开展研究,优化了影响焊缝成形的焊接结构、热台温度、助焊剂、离焦量、激光功率、焊接速度等工艺参数,分析了激光软钎焊密封接头微观组织,测试了激光软钎焊密封组件的气密性。结果表明:激光软钎焊封盖过程是一个温度场不断变化的动态过程,因此激光功率和焊接速度两参数也应在一定范围内通过程序设计动态调节。钎料可完全填充壳体与盖板之间形成的间隙,钎料中无空洞和裂纹,钎料未溢流到壳体内部。钎料与镀Ni 层接触并润湿良好,在钎料与壳体和盖板界面未形成粗大脆性的金锡金属间化合物。接头上部钎料基本保持(Sn)+(Pb)共晶组织状态,接头下部镀Ni层与钎料之间形成薄且均匀连续的金属间化合物层AuSn和AuSn₂。采用优化后的工艺参数激光软钎焊密封的组件气密性满足机载有源天线阵面应用要求。