Synthesis of magnesium borate (Mg2B2O5) nanowires,growth mechanism and their lubricating properties

Large quantities of single crystalline magnesium borate nanowires of the form Mg₂B₂O₅ with typical diameter about 120–180 nm and length about 0.2 mm have been successfully synthesized by a new and simple method of heating the mixed tablet of Mg(BO₂)₂ and graphite directly in vacuum at 1200 °C for 1 h. The products have been characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectrometry and selected area electron diffraction. The process of the nucleation and the growth of nanowires have been analyzed by VS mechanism. The results of the lubricating properties show that the friction coefficient of the oil is significantly decreased by the addition of Mg₂B₂O₅ nanowires. Our results indicate that the new method we use is effective in synthesis of Mg₂B₂O₅ nanowires and the nanowires can be used as additive to antiwear nanodevices.     

Catalyst-free synthesis and shape control of CdTe nanowires

The procedure reported here allows for the size and shape control of CdTe nanowires by means of colloidal chemistry. Thus, ultrathin, straight, saw-tooth-like and one-sided branched nanowires with zinc blende structures could be synthesized. Their formation does not require any catalyst and is most likely due to the oriented attachment of nanoparticles formed in the beginning of the reaction. The use of oleylamine as a solvent turned out to be crucial in order to achieve CdTe nanowires. The reaction between oleic acid and oleylamine in the presence of CdO proved to be essential, not only to activate the Cd precursor but also to provide reaction conditions facilitating nanowire formation by oriented attachment.      

Perovskite oxide nanowires: synthesis, property and structural characterization

Perovskite oxide materials display a wide spectrum of functional properties, including switchable polarization, piezoelectricity, pyroelectricity, and non-linear dielectric behavior. These properties are indispensable for application in electronic devices such as non-volatile memories, sensors, microactuators, infrared detectors, microwave phase filters, and so on. Recent advances in science and technology of perovskite oxide materials have resulted in the feature sizes of perovskite oxides-based electronic devices entering into nanoscale dimensions. At nanoscale perovskite oxide materials exhibit a pronounced size effect manifesting itself in a significant deviation of the properties of low-dimensional structures from the bulk and film counterparts. In the last decade low-dimensional perovskite nanosized oxides have been received much attention because of their superior physical and chemical properties. Among them, perovskite oxide nanowires are especially attractive for nanoscience studies and nanotechnology applications. Compared to other low-dimensional perovskite oxide systems, perovskite oxide nanowires are not only used as the building blocks of future nanodevices, but also they offer fundamental scientific opportunities for investigating the intrinsic size effects of physical properties. In the recent years, much progress has been made both in synthesis and physical property testing of perovskite oxide nanowires, which have a profound impact on the nanoelectronics. In this work, an overview of the state of art in perovskite oxide nanowires is presented, which covers their synthesis, property, and structural characterization. In the first part, the recent literatures for fabricating perovskite oxide nanowires with promising features, are critically reviewed. The second part deals with the recent advances on the physical property testing of perovskite oxide nanowires. The third part summarizes the recent progress on microstructural characterizations of perovskite oxide nanowires, to improve their crystalline quality, morphology and uniformity. Finally, this review concludes with some perspectives and outlook on the future developments of perovskite oxide nanowires.     

Catalyst-free synthesis of silicon nanowires by oxidation and reduction process

A new process has been developed to grow silicon (Si) nanowires (NWs), and their growth mechanisms were explored and discussed. In this process, SiNWs were synthesized by simply oxidizing and then reducing Si wafers in a high temperature furnace. The process involves H₂, in an inert atmosphere, reacts with thermally grown SiO₂ on Si at 1100 °C enhancing the growth of SiNWs directly on Si wafers. High-resolution transmission electron microscopy studies show that the NWs consists of a crystalline core of ~25 nm in diameter and an amorphous oxide shell of ~2 nm in thickness, which was also supported by selected area electron diffraction patterns. The NWs synthesized exhibit a high aspect ratio of ~167 and room temperature phonon confinement effect. This simple and economical process to synthesize crystalline SiNWs opens up a new way for large scale applications.     

Synthesis and characterization of nanocrystalline Mg2CoH5 obtained by mechanical alloying

The stoichiometric mixture of 2MgH₂ + Co was ball milled under a hydrogen atmosphere to synthesize nanocrystalline metal hydride Mg₂CoH₅. Upon milling, the mixture was analyzed by X-ray powder diffraction (XRD) and thermal methods employing the techniques of differential scanning calorimetry (DSC), thermogravimetry (TG) and differential thermal analysis (DTA). Hydrogen absorption and desorption measured by pressure-composition-temperature (P-C-T) curves indicated that the capacity loss was small after 20 consecutive cycling tests. The enthalpies associated with hydride formation and decomposition were measured to be –69.5 and –83.2 kJ mol^–1 H₂, respectively. At the temperatures of this study (553 to 653 K), hysteresis decreases with increasing temperature.     

Effects of TiO2 coating on microstructure and mechanical properties of magnesium matrix composite reinforced with Mg2B2O5w

The interface between the reinforcement and the matrix is very important for metal matrix composites. The effects of TiO₂ coating on microstructure and mechanical properties of Mg₂B₂O₅w-reinforced AZ91D magnesium matrix composite fabricated by squeeze casting technique were studied. The results indicate the flexural strength and flexural modulus of Mg₂B₂O₅w/TiO₂/AZ91D composite is 40% and 35% up on that of Mg₂B₂O₅w/AZ91D composite. The reason is that the MgO interfacial product resulting from the reaction between the TiO₂ coating and the liquid Mg can enhance the interfacial bonding strength and increase the load transfer from the matrix to the reinforcement, which leads to higher mechanical properties of Mg₂B₂O₅w/TiO₂/AZ91D composite.     

Facile synthesis and characterization of potassium-doped MnO2 nanowires

A new type of potassium doped manganese oxide nanowires were synthesized using a simple hydrothermal route. The reduction of MnO4- in the presence of acetate species led to the formation of the Multi-filamentous nanowire structure. Detailed TEM and chemical characterizations indicated that potassium ions were homogeneously distributed in the nanowires. XPS results show a clear binding energy shift (1 eV) for K(2p) peak in nanowires compared with its starting material of KMnO4. Detailed synthetic condition investigation indicated that the presence of acetate ions played an important role in the formation of such a type of nanowires other than layered structures.     

Catalyst-free MOCVD growth of ZnO nanorods and their structural characterization

We have synthesized ZnO nanorods on ZnO-coated Si(100) substrates without a metal catalyst by a reaction of a diethylzinc (DEZn) and oxygen (O₂) mixture. By adjusting the argon (Ar)/O₂ gas flow ratio, we have obtained ZnO nanorods of various densities at a temperature of 450°C. The ZnO nanorods had an average diameter of 30–70 nm, and transmission electron microscopy (TEM) showed a single crystalline structure.     

Fabrication, structural and electrical characterization of VO2 nanowires

The structural and electrical properties of VO₂ nanowires synthesized on Si₃N₄/Si substrates or molybdenum grids by a catalyst-free vapour transport method were investigated. The grown VO₂ nanowires are single crystalline and rectangular-shaped with a preferential axial growth direction of [1 0 0], as examined with various structural analyses such as transmission electron microscopy, electron diffraction, X-ray diffraction, and X-ray photoelectron spectroscopy. In particular, it was found that growing VO₂ nanowires directly on Si₃N₄ deposited molybdenum transmission electron microscopy grids is advantageous for direct transmission electron microscopy and electron diffraction characterizations, because it does not involve a nanowire-detachment step from the substrates that may cause chemical residue contamination. In addition to structural analyses, VO₂ nanowires were also fabricated into field effect transistor devices to characterize their electrical properties. The transistor characteristics and metal-insulator transition effects of VO₂ nanowires were investigated.     

助熔剂工艺制备硼酸镁（Mg2B2O5）晶须的扩试研究

对以六水氯化镁、硼酸、氢氧化钠为原料,氯化钠为助熔剂制备硼酸镁(Mg2B2O5)晶须的工艺进行了百吨级扩试。研究了温度、硼镁比例、助熔剂含量等对晶须组成和形貌的影响。应用容量分析、X射线衍射仪(XRD)、扫描电镜(SEM)以及颗粒统计系统对制备的晶须进行了检测和表征。通过工艺优化得到了硼酸镁晶须的最佳合成条件:温度为880℃,硼镁的最佳摩尔比为1.4、助熔剂与镁的最佳摩尔比为3.0。XRD分析表明制备的产品为单一相三斜结构硼酸镁(Mg2B2O5);SEM电镜分析表明制备的晶须形貌规整,无团聚;颗粒统计系统分析表明直径在1～5μm之间的晶须占统计总数的96.36%,长度在40～60μm之间的晶须占统计总数的69.25%。该扩试的完成为推动硼酸镁晶须的应用起到了积极的作用。     

Catalyst-free synthesis and characterization of boron nitride whiskers and nanotubes

Catalyst-free transformation of h-BN in an optical furnace in the flow of nitrogen results in formation of nanotubes, whiskers and negligible quantity of melted drops on the surface of heated samples. The thread-like structures and equiaxial particles were precipitated on a silicon substrate. The phase composition of the produced material is a mixture of h-BN, two boron-enriched tetragonal phases of BN (B_51,2N and B₂₅N), tetragonal and rhombohedric phases of pure boron and amorphous phase.An approximation of the spectral dependence of optical absorption versus photon energy of an incident light was explained in terms of absorption of the corresponding phases. Tetragonal phase of B_51,2N, tetragonal and rhombohedric phases of pure boron may correspond to band gap 3.5 eV, tetragonal phase of B₂₅N - 3.8 eV and hexagonal phase of BN - 4.8 eV. This fact confirms a theoretical assumption for the effect of boron excess in BN on a band gap.     

草酸盐法制备硼酸镁晶须及其生长机理

以MgCl2·6H2O、H3BO3和H2C2O4·2H2O为原料,利用草酸盐法制备出长度为4.0～8.0μm,直径为0.6～1.5μm,长径比约为10,形貌良好的硼酸镁晶须。采用XRD、SEM及TG-DTA等分析手段,研究了烧结温度、烧结时间、n(B)/n(Mg)、n(H2C2O4)/n(Mg)对硼酸镁晶须的生长过程和质量的影响,探讨了其最佳制备条件。结果显示,硼酸镁晶须的最佳制备条件为:烧结温度为800℃,烧结时间为4.0h,n(B)/n(Mg)=1.2,n(H2C2O4)/n(Mg)=1.2。根据XRD和TG-DTA分析,MgC2O4前驱体对硼酸镁晶须的形成和生长有较大的促进作用,分解产物不引入其它杂质,使得制备的晶须纯度较高。晶须的生长机理为S-L-S机理。     

Optical properties of zigzag twinned geometry of Zn2SnO4 nanowires

High-density single-crystalline Zn2SnO4 nanowires have been successfully synthesized by using a simple thermal evaporation method by heating a mixture of ZnO and SnO2 nano powders. The products in general contain various geometries of wires, with an average diameter of 80-100 nm. These nanowires are ultra-long, up to 100 microns. The transmission electron microscopy study showed that these nanowires exhibited zigzag twinned geometry, and grow along the (111) direction. Low-temperature photoluminescence properties of the nanowires were measured, showing a strong green emission band at about 515 nm and a weak peak corresponding to UV emission at about 378 nm, which have not been reported before.     

Mg_2B_2O_(5W),SiC和Gr颗粒增强6061Al基复合材料的摩擦磨损行为

采用粉末热挤压法制备2%Mg2B2O5w/6061Al,2%Gr/6061Al,2%SiCp/6061Al,2%Mg2B2O5w+2%Gr/6061A,2%Mg2B2O5w+2%SiCp/6061Al,2%Mg2B2O5w+2%Gr+2%SiCp/6061Al单一及混杂增强的铝基复合材料,并对其耐磨性和摩擦行为进行研究。结果表明:随着载荷的增大,各种复合材料的磨损率均增大,石墨的添加增大了铝基复合材料的磨损率;复合材料的摩擦因数随载荷的增大而降低并趋于稳定,摩擦因数均介于0.22~0.32之间。未加入石墨的复合材料的磨损机制以磨料磨损和轻微的黏着磨损为主,加入石墨后复合材料的磨损机制转变为剧烈的黏着磨损。     

Novel low-temperature synthesis of glasses and glass-ceramics in the B2O3-SiO2-P2O5 system

In recent years considerable progress has been made in electronic packaging substrate technology. The future need of miniaturization of devices to increase the signal processing speed calls for an increase in the device density requiring the substrates to be designed for better thermal, mechanical and electrical efficiency. Fast signal propagation with minimum delay requires the substrate to possess very low dielectric constant. Several glasses and glassceramic materials have been identified over the years which show good promise as candidate substrate materials. Among these, borophosphate and borophosphosilicate glass-ceramics have been recently identified to have the lowest dielectric constant (3.8). Sol-gel processing has been used to synthesize borosilicate, borophosphosilicate and borophosphate glasses and glass-ceramics using inexpensive boron oxide and phosphorus pentoxide precursors. Preliminary results of the processing of these gels and the effect of volatility of boron alkoxide and its modification on the gel structure are described. X-ray diffraction, differential thermal analysis and Fourier transform-infrared spectroscopy have been used to characterize the as-as-prepared and heat-treated gels.     

Hydrothermal synthesis and characterization of diaspore,β-Al2O3 · H2O

An acicular single crystal of diaspore was synthesized under the conditions of 1 N NaOH, 450 ° C and 1200 kg cm^–2 and characterized by means of X-ray diffraction, scanning electron microscopy, electron diffraction and infrared spectroscopy. The reactivity of three kinds of natural ores containing diaspore which were used as a seed crystal for synthesis of diaspore was also compared.     

退火处理对硼酸镁晶须增强铝基复合材料性能的影响

采用挤压铸造方法制备了硼酸镁晶须增强纯铝基复合材料,在450℃下对其进行了不同时间的退火处理,研究了不同退火时间对复合材料的室温力学性能、热膨胀行为及基体织构的影响.试验结果表明,退火处理能有效提高复合材料的抗拉强度,随着退火时间的增加材料的拉伸强度也逐渐增加,在5 h时达到最大值295 MPa,7 h时略有下降,为285 MPa.复合材料的热膨胀曲线表明,退火处理会使材料的热膨胀系数变大,随着退火时间的增加,材料的热膨胀系数先增加后减小,但始终高于未退火处理的材料.     

Optical, structural and surface characterization of CdO:Mg films

In this work, we have tried to improve some physical properties of CdO films by Mg doping. Ultrasonic spray pyrolysis technique has been used to obtain the films. Thicknesses and refractive indices of the films have been determined by Spectroscopic ellipsometry technique using Cauchy-Urbach model for fitting. Transmission and reflectance spectra have been taken by UV Spectrophotometer, and band gap values have been determined by optical method. X-ray diffraction patterns have been used to study the structural properties. Texture coefficient, grain size and lattice constants have also been determined. AFM images have been taken to see the effect of Mg doping on surface topography and roughness of CdO films. Finally, it has been concluded that Mg doped CdO films (especially at 4%) have improved properties and are good candidates for photovoltaic applications.