Synthesis and tribological properties of laminated Ti3SiC2 crystals

Laminated Ti₃SiC₂ crystals are prepared of Ti, Si, C and Al powders by the method of hot isostatic pressing with NaCl additive in argon at 1350 °C. The laminated morphology of Ti₃SiC₂ is presented through the SEM and TEM observations. The results of high resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) patterns combined, it can be seen that the layers are of Ti₃SiC₂ crystals. The growth mechanism of Ti₃SiC₂ crystals, controlled by two‐dimensional nucleation, is also explained. The tribological properties of Ti₃SiC₂ crystals as additives in HVI500 base oil are investigated by a UMT‐2 ball‐on‐plate friction and wear tester. The study shows that under determinate conditions, the friction coefficient of the base oil containing Ti₃SiC₂ crystals is lower than that of pure base oil, and it decreases with the increase of mass percent of Ti₃SiC₂ nanolayers when its proportion is lower than 5wt. %. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

SiC crystal growth from transition metal silicide fluxes

SiC crystal growth in transition metal silicide melts was investigated by using spontaneous infiltration and solution methods. In the infiltration experiments, SiC powder preforms were infiltrated with Fe_xSi_y (Fe₃Si, Fe₅Si₃ and FeSi) and CoSi melts. The dissolution and precipitation of SiC led to SiC crystals growth in the infiltrated Fe₅Si₃ and CoSi melts, SiC particles coalescing in FeSi and free carbon precipitation in Fe₃Si. In the solution experiments, carbon from the graphite crucible dissolved in and reacted with FeSi₂ and Ti_2.3Si_7.7 to form SiC crystals. Scanning electron microscopy (SEM), X‐ray diffraction (XRD) and Raman scattering spectrometer were employed to investigate SiC crystals growth. Based on the investigation, the effect of solution content on the SiC crystal growth, the growth mechanisms in both methods and prototypes of the SiC crystals are also discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

SiC单晶生长热力学和动力学的研究

升华法生长大直径碳化硅(SiC)单晶一直是近年来国内外研究的重点,本文对Si-C系中的Si,Si2,Si3,C,C2,C3,C4,C5,SiC,Si2C,SiC2等气相物种的热力学平衡过程进行了研究,发现SiC生长体系中的主要物种为Si,Si2C,SiC2.生长初期Si的分压较高,从而SiC生长为富硅生长模式.对外加气体进行研究发现,氩气为最好的外加气体,它既可以有效地抑制Si物质流传输,又可以减缓扩散系数随温度升高而递减的趋势.建立了简单一维传输模型,对三个主要物种的动力学输运过程进行了研究,计算得到了两个温度梯度下的主要物种的物质流密度.     

Growth of nanowires from annealing SiBONC nanopowders

Silicon carbide (SiC) nanowires were prepared by the gas pressure annealing of SiBONC powders, which were synthesized by pyrolysis of a polymeric precursor. The yield, morphology and composition of the nanowires were influenced by the Si/B ratio in the original ceramic powders, annealing temperature and atmosphere. Annealing temperatures between 1500 and 1600 °C and Si/B molar ratios between 70:30 to 60:40 were suitable for growth of the nanowires. When annealing in an argon (Ar) atmosphere, the SiC nanowires contained little oxygen (O); and the diameters ranged from 20 to 200 nm. Then annealing in a nitrogen (N₂) atmosphere, the nanowires were thicker and rougher, and consisted of a relatively high level of nitrogen. Varied shapes and morphologies of the nanowires were observed for different synthesis conditions. The present novel method makes possible the large-scale fabrication of β-SiC nanowires.     

Crystallography and cathodoluminescence of ultra‐long GaN nanowires

Ultra‐long GaN nanowires have been synthesized via a simple thermal evaporation process by heating mixed GaN and Ga₂O₃ powders in a conventional resistance furnace under ammonia gas at 1150 °C. The average length of GaN nanowires is estimated to be more than 100 μm after 30‐min growth, corresponding to a fast growth rate of more than 200 μm/h. Scanning electron microscope (SEM) observation indicated that the diameter of GaN nanowires was rather uniform along the growth direction and in the range of 100–200 nm. X‐ray diffraction (XRD) and transmission electron microscope (TEM) measurements confirmed that the GaN nanowires are crystalline wurtzite‐type hexagonal structure. Room‐temperature cathodoluminescence (CL) measurement indicated that an obvious red‐shift of the near band‐edge emission peak centered at 414 nm of the ultra‐long GaN nanowires and a wide shoulder in the range of 600–700 nm were observed. Possible reasons responsible for the red‐shift of the near band‐edge emission of the ultra‐long GaN nanowires was discussed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rapid synthesis and catalytic performance of α‐Mn2O3 single‐crystal nanowires

Single‐crystal α‐Mn₂O₃ nanowires were prepared via a “self‐sacrificing template” route, simply by calcining the prepared α‐MnO₂ nanowire precursors at 550 °C for 1.5 h. XRD, TEM, SEM and HRTEM characterizations show that the as‐prepared α‐Mn₂O₃ samples are all phase pure and the nanowires have uniform diameters of approximately 15‐30 nm and lengths up to several micrometers. The catalytic performances of the prepared α‐Mn₂O₃ nanowires were studied in the degradation of coking wastewater with H₂O₂ as the oxidant, and the technological conditions were optimized by single‐factor and orthogonal experiments. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

TEM analysis of the container effect of Au‐based catalyst droplets during vapour‐liquid‐solid growth of axial ZnTe/CdTe nanowires

Axial heterostructure nanowires (NWs) of ZnTe/CdTe were grown by vapour‐liquid‐solid growth realized in a molecular beam epitaxial chamber. By alternative supply of Zn or Cd and constant Te the heterostructure was generated. The liquid phase is provided by a Au‐based eutectic droplet which stays at the tip of the NW during the entire growth. For structural and chemical characterization by TEM the NWs were harvested from the substrate and transferred to a holey carbon film. The NWs exhibit an expansion of the diameter correlated with the interface region between ZnTe and CdTe. Idiomorphic growth of the CdTe is evident from electron diffraction experiments. The growth rate of CdTe appears to be smaller compared to that of ZnTe at the same temperature. Both, quantitative high‐resolution TEM and energy dispersive X‐ray spectroscopy line scans reveal a smeared ZnTe/CdTe interface along about 200 nm. The smearing is due to both, the liquid catalyst which buffers the supply of Cd instead of Zn at the liquid/solid interface and to the strain which is induced by the lattice mismatch. It forces the system to consume the remnant Zn for the NW growth in favour of Cd. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microstructure and cathodoluminescence study of GaN nanowires without/with P‐doping

In this work, P‐doped GaN nanowires were synthesized in a co‐deposition CVD process and the effects of P‐doping on the microstructure and cathodoluminescence (CL) of GaN nanowires were studied in details. SEM observation and CL measurments demonstrated that P‐doping has led to a rough morphology evolution and a depression of the band‐gap emission of GaN nanowires, whereas the visible emission of GaN nanowires was obviously enhanced. Finally, the corresponding morphology transition and optical properties of GaN nanowires with P‐doping were discussed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

One‐pot fabrication of large‐scale ordered NiTe nanosheets and its application in lithium‐ion batteries

Large‐scale sheet‐like nickel telluride (NiTe) nanocrystals in situ grown on nickel foam with three‐dimensional network structure were successfully synthesized by reacting elemental nickel with tellurium powder. The reaction was conducted via a facile solvothermal method in a mixed solvent composed of 1 mL ethylenediamine and 15 mL ethylene glycol at 170°C for 24 h. The growth process and the possible formation mechanism were discussed based on the intermediate products obtained at different reaction stages including different reaction times and temperatures. When the as‐synthesized NiTe nanosheets were employed as anode materials for lithium‐ion batteries, this work enriches the electrode materials for energy materials, and can serve as a good reference for the fabrication of desired materials.     

Large‐scale synthesis of submicron gallium oxide hydrate rods and their optical and electrochemical properties

Large‐scale submicron gallium oxide hydrate (GaOOH) rods have been synthesized by a simple hydrothermal process and their optical, electrochemical properties have been analyzed. The diameter of the GaOOH nanorods with good single crystalline structure is 80‐800 nm and length of less than 4 μm. The tips of the GaOOH structures are composed of nanorods with the diameter of less than 10 nm and length of more than 100 nm. Hydrothermal temperature, time and the Ga‐contained starting materials have important roles on the formation and growth of submicron GaOOH rods. The submicron GaOOH rods exhibit good UV‐vis absorption ability. The electrochemical analysis shows that the submicron GaOOH rods have good detecting ability for ascorbic acid and cysteine in PBS and NaCl solution exhibiting promising potential for electrochemical sensing application. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Factors affecting the formation of misoriented domains in 6H‐SiC single crystals grown by PVT method

Misoriented domains (MDs) are common defects in 6H‐SiC single crystals. We performed an experimental study on the formation of MDs in 2‐inch 6H‐SiC single crystals. Micro‐Raman spectroscopy revealed that the polytype of MDs was mainly 4H‐SiC. By changing growth conditions, it was found that the MDs' formation was closely related to growth rate and the position of highest temperature relative to growth interface. When the growth rate of ingots was relatively high the MDs were more likely to form. Furthermore, the nearer growth interface the position of highest temperature was, the larger the size of the MDs. Based on our experimental findings we suggested that the MDs' formation and the polytype switching from 6H‐ to 4H‐SiC were due to too large axial and/or radial temperature gradients.The results would be helpful to improve the quality of SiC single crystals grown by PVT technique. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A simple glucose reduction route for the synthesis of sheet‐like copper dendrites

In the current paper we designed a simple glucose reduction route for synthesis of sheet‐like Cu dendrites on a high yield, using CuSO₄ as the starting material. The reaction was carried out at 180 °C for 18 h in the absence of any structure‐directing agent. The product was characterized by X‐ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and electron diffraction (ED). Some factors influencing the shapes of Cu microcrystals, including the reaction temperature, time, and the concentration of the starting CuSO₄, were investigated. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tunable synthesis of hierarchical TiO2 spheres consisting of rutile nanowires for dye‐sensitized solar cells and photocatalysis

Hierarchical TiO₂ spheres consisting of rutile nanowires (HTS) are successfully prepared in the bulk solution via a non‐polar solvent/polar solution interfacial syntheses strategy. The effect of reaction temperature on the morphology, size, BET surface area and monodispersion of HTS is studied systematically. Increasing reaction temperature decreases the diameter of spheres in micro‐scale whereas increases the diameter of nanowires in nano‐scale for these HTS. Monodisperse HTS are formed at 230 °C for 20 h, which can suitably be served as light scattering structure for dye‐sensitized solar cell with the additional advantage of rapid electron transport rate. A high efficiency of 8.54% was achieved for the composite DSSC consisting of nanoparticles and monodispersed HTS (synthesized at 230 °C), showing 31.38% improvement compared with that (6.5%) of the DSSCs made from pure P25 nanoparticles at the similar thickness. Furthermore, the photocatalytic performances of HTS for the degradation of methyl orange were also investigated. Contrary to the effect on the performance of DSSC, the HTS synthesized at 150 °C show higher photocatalytic degradation efficiency due to larger BET surface area.     

A template‐free route for controlled synthesis of dumbbell‐like Sb2S3 microcrystals

Large amounts of dumbbell‐like Sb₂S₃ microcrystals were synthesized via a simple solvothermal treatment method. Various techniques such as x‐ray diffraction (XRD), field‐emission scanning electron microscope (FESEM), high‐resolution transmission electron microscope (HRTEM), selected area electron diffraction (SAED), and photoluminescence spectrometry (PL) have been used to characterize the obtained products. The results showed that the products belong to the orthorhombic Sb₂S₃ phase, and the dumbbell‐like Sb₂S₃ microcrystals were composed by uniform microrods. Besides, the morphologies of Sb₂S₃ microcrystals could be changed from microshperes to dumbbell‐like microcrystals by only adjusting the reaction solvent. The solvent effects are discussed in detail. Furthermore, the PL properties of the obtained Sb₂S₃ microcrystals clearly show shape effects. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

合成温度对碳热还原法合成碳化硅晶须形貌的影响

以SiO2微粉为硅源,炭黑为碳源,氧化硼为催化剂,采用碳热还原法分别在1500 ℃、1550 ℃、1600 ℃制备了SiC晶须.通过扫描电镜,电子探针和透射电镜等分析手段,研究了合成温度对SiC晶须形貌的影响,探讨了晶须的生长机理.结果表明:当合成温度为1500 ℃时,所合成的SiC晶须形貌呈竹节状,选区电子衍射分析发现孪晶等面缺陷在晶须的生长方向上周期性出现;当合成温度在1550 ℃以上时,哑铃状晶须的数量会急剧增多,分析表明晶须表面包裹的串珠小球为β-SiC.在晶须的顶端发现催化剂熔球,由此推测生长机理为VLS机理,但当合成温度超过1550 ℃时,SiC会以VS生长机理沿径向沉积生成哑铃状晶须.     

Large‐scale production of well‐dispersed submicro ZrB2 and ZrC powders

ZrB₂ and ZrC powders were synthesized via metallothermic reduction route using ZrO₂, B/C, and Mg as raw materials. Low packing density of the green mixture and high heating temperature of the furnace are crucial for the formation of well‐dispersed submicro powders. Optimum reaction time helped achieve good crystallization and high purity. The as‐prepared samples were characterized by XRD, FESEM, TEM, and particle size analyzer. Results showed that well‐dispersed ZrB₂ powders with mean particle size of 0.534 μm and ZrC powders with mean particle size of 0.376 μm can be obtained. Oxygen content can be controlled lower than 1.0 wt%. The bench‐scale output is about 10 kg/d.     

Catalyst‐free thermally‐evaporated growth and optical properties of ZnO nanowires on Si,GaN and sapphire substrates

Vertically well‐aligned zinc oxide nanowires (NWs) with high density were successfully synthesized on Si, sapphire and GaN/sapphire substrates by thermal evaporation of zinc powders without catalysts or additives. The growth behavior of ZnO NWs was strongly dependent on the substrate materials. The effects of the substrate position on the structures and properties of ZnO NWs were primarily discussed. The morphology and crystallinity of the resultant NWs were studied by scanning electron microscope, transmission electronic microscope and X‐ray diffraction. The photoluminescence (PL) characteristics of the ZnO NWs on the different substrates were studied. The results showed that the as‐grown ZnO NWs exhibit a sharp and strong ultraviolet emission at 3.27 eV and a very weak green emission at around 2.48 eV, indicating that the a‐synthesized NWs have excellent PL properties with good crystalline quality and can be an ideal candidate for making luminescent devices. By comparison of PL spectra, we revealed that the green‐to‐UV emission intensity ratios were considerably dependent on the substrate materials, which was explained by the difference in the structural morphology of the produced nanowires.     

Hydrothermal synthesis of nano‐crystalline BaMoO4 under mild conditions using simple additive

Large‐scale high‐quality BaMoO₄ nanocrystals have been synthesized in aqueous solutions under mild conditions with citrate as a simple additive. The crystals have bone‐like, spindle‐like and wheatear‐like morphologies assembled from nanoparticles, nanofibers and have been characterized by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) techniques. The results showed that experimental parameters had great influences on the shape evolution of products. The adjustment of these parameters such as room temperature stirring time, reaction temperature and reaction time of hydrothermal reaction, can lead to obvious morphology changes of products, and the growth mechanism has been proposed. Room‐temperature photoluminescence indicated that the as‐prepared BaMoO₄ nanocrystals had a strong blue emission peak at 481.5 nm. This facile route could be employed to synthesize more promising nanomaterials with interesting self‐assembly structures. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and self‐assembled mechanism of CuO peony‐flowers by a composite hydroxide‐mediated approach at low temperature

A composite‐hydroxide mediated (CHM) method was utilized for the synthesis of CuO peony‐flower nanostructures under temperatures ranging between 25 and 160 °C. The CHM mechanism was confirmed through X‐ray Powder Diffraction (XRD) and a Thermo‐Gravimetric Differential Scanning Calorimeter (TG‐DSC). Cu(NO₃)₂ was shown to transform into Cu(OH)₂ in the mixed alkalis (NaOH/KOH); the reaction was facilitated by the solvent properties of the mixed alkalis. Cu(OH)₂ subsequently consumed H₂O in the adsorption of the mixed alkalis at 25∼65 °C. At higher reaction temperatures (>65 °C), the Cu(OH)₂ was seen to decompose at an accelerated rate. Therefore, crystalline CuO could be obtained not only above 65 °C but also at 25 °C. The crystal morphology and structure of CuO were examined through Filed Emission Scanning Electron Microscopy (FE‐SEM) and Transmission Electron Microscopy (TEM). It was determined that the CuO peony‐flower had a polycrystalline structure composed of single crystalline CuO petals. Using the Selected Area Electron Diffraction (SAED) results, the rings were indexed as (002), (111), (112), (202) and (−113), which was in agreement with the XRD results. With increasing temperature, the CuO flower petals self‐assembled through random aggregation and gathered CuO nanorod parts, which led to incomplete CuO flower petals through orientated aggregation. Prolonged reaction time led to the growth of CuO flower petals in the direction of [001]. An ideal CuO flower structure was observed through TEM observation.     

Low‐temperature urea‐assisted hydrothermal synthesis of Bi2S3 nanostructures with different morphologies

Different morphologies of single‐crystalline orthorhombic phase bismuth sulfide (Bi₂S₃) nanostructures, including sub‐microtubes, nanoflowers and nanorods were synthesized by a urea‐assisted hydrothermal method at a low temperature below 120 °C for 12 h. The as‐synthesized powders were characterized by X‐ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high‐resolution transmission electron microscopy (HRTEM) and UV‐vis spectrophotometry. The experimental results showed that the sulfur sources had a great effect on the morphology and size of the resulting powders. The formation mechanism of the Bi₂S₃ nanostructures with different morphologies was discussed. All Bi₂S₃ nanostructures showed an appearance of blue shift relative to the bulk orthorhombic Bi₂S₃, which might be ascribed to the quantum size effect of the final products. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)