Preparation and budding growth of whiskers in a homogeneous system

Ca₁₀(PO₄)₆(OH)₂ (HAP) and NH₄Al(OH)₂CO₃·H₂O (AACHH) whiskers were prepared by a homogeneous precipitation method based on urea hydrolysis reaction. To clarify the growth process of whiskers in the homogeneous system, XRD and SEM results of the products obtained at different reaction time were investigated in detail. A novel observation about budding growth in preparing both whiskers was described. It was indicated that the growth of whiskers went through three stages, which were oversaturation, nucleation, and budding growth. The growth units of whiskers budded from the surfaces of substrates, which were crystallized flakes for HAP preparation and amorphous spherical nuclei for AACHH preparation. Subsequently, the whiskers grew up accompanying with the disappearing substrates. One-dimensional whiskers with fine morphology and large slenderness ratio were finally obtained. Besides, according to the crystal growth and the interface diffusion theories, the effects of the templates and the budding growth mechanism were discussed.     

Polymer-assisted growth of molybdenum oxide whiskers via a sonochemical process

Whiskers of molybdenum oxides with high aspect ratios were synthesized from peroxomolybdate precursor solutions in the presence of small amounts of poly(ethylene glycol) (PEG) via a sonochemical process at temperatures of 25-70 degrees C. Irradiation with ultrasound reduces the time needed for the growth of micrometer-sized whiskers from weeks to a few hours. The simplicity of the sonochemical approach also compares favorably to a hydrothermal/solvothermal process. The morphology, crystal structure, and other characteristics of the whiskers were characterized by scanning electron microscopy, transmission electron microscopy, selective area electron diffraction, energy-dispersive X-ray spectroscopy, wide-angle X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and the Brunauer-Emmett-Teller method. The surface area of the calcified molybdenum oxide whiskers (55.4 m2/g) was found to be much higher than those of molybdenum oxide nanofibers (35 m2/g) or nanorods (13.4 m2/g) The growth rate of various crystal faces could be postulated to be controlled by the binding of peroxomolybdate ions to pseudo-crown ether cavities formed by PEG. The reduction of molybdenum oxide to produce mixed-valent oxides and their growth could also be controlled by the reducing ability of PEG. The aspect ratio of the molybdenum oxide whiskers increased with decreasing concentration in the initial peroxomolybdate precursor solution. Whether the precursor solution species was H2Mo2O3(O2)4(H2O)2, H2MoO2(O2)2, or MoO2(OH)(OOH), the peroxide group in all the species disproportionates to give the final product MoO3 by a catalytic process. On the basis of experimental evidence of the dual role of glycols, a mechanism for the growth of the molybdenum oxide whiskers is proposed.     

STUDIES ON BaF_2 PROMOTED PRASEODYMIUM OXIDE CATALYSIS FOR THE OXIDATIVE COUPLING OF METHANE

IntroductionTheoxidativecouplingofmethane(OCM)toetheneandethaneisanattractivereactiontoconvertnaturalgasintovaluablechemicals.SincethepioneeringworkbyKellerandBhasin[1],OCMhasbecomeasubjectofworldwideintenseresearchinrecentyears.Almostallofelementsinthepe…     

Growth of single-crystal Ca10(Pt4As8)(Fe(1.8)Pt(0.2)As2)5 nanowhiskers with superconductivity up to 33 K

Single-crystal Ca(10)(Pt(4)As(8))(Fe(1.8)Pt(0.2)As(2))(5) superconducting (SC) nanowhiskers with widths down to hundreds of nanometers were successfully grown in a Ta capsule in an evacuated quartz tube by a flux method. Magnetic and electrical properties measurements demonstrate that the whiskers have excellent crystallinity with critical temperature of up to 33 K, upper critical field of 52.8 T, and critical current density of J(c) of 6.0 × 10(5) A/cm(2) (at 26 K). Since cuprate high-T(c) SC whiskers are fragile ceramics, the present intermetallic SC whiskers with high T(c) have better opportunities for device applications. Moreover, although the growth mechanism is not understood well, the technique can be potentially useful for growth of other whiskers containing toxic elements.     

METHANE OXIDATIVE COUPLING: SYNERGISTIC PROMOTION OF BINARY METAL FLUORIDES TO THORIA BASED CATALYST ThO_2/LaF_3/BaF_2

Introductionlnthcstud}ofmcthaneoxidativccoupIing(0CM),aseriesofcatalystshavebeendevelopedandox}'genspeciesoncatal}'stst"ereinvestigatedextensively.Alotofpapershavebeenpublishedonthisarea.Anongkindsof0CMcatalystsmetalhalidespromotedmctaloxides.cspcciall}'Li /Mg0catal}'stl1~3l.havcbecnmostoftenstudied.Themostpromisingindustrialoncshavebeenthoughttoberarcearthoxidesbasedcatalysts.H.L.Wanelajl#Ihavesuccessfull\'madeuseofmetalfluoridesin0CM,andfoundthebcttcrpron1otingcffectofmctalfluoridcstomc…     

丙三醇-变频微波-水热法制备氢氧化镁晶须

以MgCl₂·6H₂O和NaOH为原料, 采用丙三醇-变频微波-水热法制备了优质氢氧化镁晶须. 采用XRD, SEM和TEM进行了物相、粒度、晶体形貌和结构分析. 考察了丙三醇-变频微波-水热法工艺对氢氧化镁晶须晶形、结构及分散性的影响. 实验发现, 在水热反应体系中加入体积分数为15%的丙三醇, 用变频微波加热, 在180 ℃反应6 h, 可获得粒度分布均匀、晶形和分散性好、表面光滑及缺陷少的优质氢氧化镁晶须. 在透射电镜下估算晶须直径约为0.1～0.3 μm, 长度约为80～110 um. 初步分析了变频微波和丙三醇对氢氧化镁晶须生长的作用.     

Self-catalysis: a contamination-free, substrate-free growth mechanism for single-crystal nanowire and nanotube growth by chemical vapor deposition

A unified mechanism for the growth of a wide variety of long, uniform, single-crystal nanowires and whiskers, including III-V and II-VI binary, ternary, and quaternary nanowires and whiskers, without the use of any substrate and catalyst has been presented. While elucidating the mechanism, attempts have been made to provide a kinetic and thermodynamic rationale for the growth. Various features of the growth mechanism, including the formation of liquid droplets and seeds, nucleation, and creation of products, have been discussed. Extensive studies of illustrative examples provide the validity of the proposed mechanism. The influence of various parameters such as growth temperature and chamber pressure on the growth mechanism has been studied. The advantages and disadvantages of the proposed mechanism, and its superiority to the well-known vapor-liquid-solid mechanism, have been elucidated. Means to improve the mechanism to obtain self-aligned nanowires and whiskers have been suggested. Based on these, it has been demonstrated that the present mechanism is indeed a powerful self-catalytic growth mechanism uniquely suited to the growth of a wide variety of single-crystal nanowires and whiskers. It can be very useful also for the growth of single-crystal nanotubes.     

Ice nucleation on BaF2(111)

The mechanism of heterogeneous ice nucleation on inorganic substrates is not well understood despite work on AgI and other materials over the past 50 years. We have selected BaF(2) as a model substrate for study since its (111) surface makes a near perfect match with the lattice of the basal face of I(h) ice and would appear to be an ideal nucleating agent. Two series of experiments were undertaken. In one, nucleation of thin film water formed from deposition of vapor on BaF(2)(111) faces was explored with the finding that supercooling to -30 degrees C was required before freezing occurred. In the other series, nucleation of liquid water on submerged BaF(2) crystals was studied. Here supercooling to -15 degrees C was needed before ice formed. The reason why BaF(2) is such a poor nucleating agent contains clues to realistic mechanisms of heterogeneous nucleation. Our explanation of these results follows the model of Fletcher [J. Chem. Phys. 29, 572 (1958)] who showed that heterogeneous ice nucleating ability depends on how well ice wets a substrate. In this view, a smooth BaF(2)(111) face is poor at nucleation because ice only partially wets its surface. In an extension of Fletcher's model, our calculations, consistent with the experimental results demonstrate that the pitting of a submerged BaF(2) crystal dramatically improves its ice nucleating ability.     

New boron nitride whiskers: showing strong ultraviolet and visible light luminescence

Boron nitride whiskers with a special structure have been synthesized by a thermal reaction process. The as-prepared BN whiskers have a length of tens of micrometers and a mean diameter of 500 nm. High-resolution TEM analysis shows that the as-prepared BN whiskers can be described as a nanofiber-interweaved network. Infrared and electron energy loss spectra reveal that the BN whiskers are composed of both sigma-sp2 and sigma-sp3 chemical bonds. The UV-vis absorption spectrum displays the energy band gap of the BN whiskers and multiple fine absorption peaks of the phonon-electron coupling. Both photoluminescence (PL) and cathodoluminescence (CL) measurements show the specially structured BN emits strong UV and visible luminescences, which is a promising material for deep-blue and UV applications.     

In situ transmission electron microscopy observation of the growth of bismuth oxide whiskers.

Growth of bismuth oxide (most probably Bi2O3) was observed in situ in a transmission electron microscope. Bi liquid particles were dispersed on the substrates of diamond or SiO2. Introduction of oxygen up to 5 x 10-4 Pa resulted in formation of bismuth oxide (most probably Bi2O3) whiskers. The growth mechanism of the whisker was discussed in terms of a vapor-liquid-solid (VLS) mechanism. It is suggested that the liquid droplet of Bi acts as a physical catalyst for growth of bismuth oxide (most probably Bi2O3) whiskers.     

SrF2和BaF2熔化温度的分子动力学模拟

利用壳层模型分子动力学方法,考虑萤石结构分子中的预熔化现象,对SrF2和BaF2的分子动力学模拟熔化温度进行修正,获得了高压下SrF2和BaF2的熔化温度．同时给出了300K、0．1MPa-7GPa和10．1MPa-3GPa时SrF2和BaF2的状态方程,与已有研究结果的最大误差分别为0．3％和2．2％．计算所得SrF2和BaF2常压下的熔点与已有的实验结果符合较好．对于SrF2和BaF2分子体积变化和已有的熔化模拟的差别也做了比较和讨论．     

Physicochemical and functional peculiarities of metal oxide whiskers

Practical aspects of preparation and prospects for practical use of a series of the metal oxide whiskers were studied. The procedures for the synthesis were proposed, and the phase composition, micromorphology, and electrochemical and sensor characteristics of the macroscopic (up to 5–10 mm long) whiskers in the Ba-V-O, Ba-Mn-O, and Sn-O systems were analyzed. The electroconducting BaV₈O_21-δ whiskers were prepared by the hydrothermal treatment. These whiskers possess stable electrochemical characteristics appropriate for the development of novel secondary current sources. The protonated form of the Ba₆Mn₂₄O₄₈ whiskers produced by the isothermal vaporization of chloride fluxes is a mixed conductor with the proton and electron conductivity at a level of mS units at 25 °C. A new procedure by the thermal disproportionation of tin(ii) oxide under nitrogen was proposed for the growth of SnO₂ whiskers of various morphology. The produced whiskers have substantial sensor sensitivity toward a series of toxic components of the gaseous medium, such as nitrogen dioxide. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1023–1034, May, 2008.     

Preparation of novel saw-toothed and riblike alpha-Si3N4 whiskers

alpha-Si(3)N(4) whiskers with novel saw-toothed and riblike structures have been synthesized in a high yield by a carbothermal reduction and nitridation route. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and selected-area electron diffraction (SAED). The results show that the saw-toothed whiskers with one smooth surface and another toothlike surface have lengths about several tens of microns and widths in the range of 600-1200 nm. The riblike whiskers are composed of Si(3)N(4) rod-arrays, which grow closely packed perpendicular to the central axial whiskers with uniform diameter and length. The growth mechanism of the products can be considered as a combination of VS mechanism and secondary epitaxial nucleation process. The photoluminescence (PL) spectrum of the whiskers shows a strong blue light emission peak at 406 nm and a weak peak at 485 nm, suggesting their potential applications in light and electron emission devices.     

Sol–gel process of ZnO and ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript> coated aluminum borate whiskers

Zinc nitrate and citric acid were used to prepare ZnO sol. ZnO and ZnAl₂O₄ coated aluminum borate whiskers were separately prepared by a sol–gel process. The results show that ZnO forms when ZnO xerogel is calcined at 500 °C and it does not undergo any phase transformation in the range of 500 and 1000 °C during calcinations. In ZnO xerogel coated aluminum borate whiskers system, a large amount of heat, gas and pores are produced during the heating process. When ZnO xerogel coated aluminum borate whiskers are calcined at 500 °C, ZnO can be uniformly coated on the surface of the whikers and the coated whiskers can be easily dispersed in distilled water through an ultrasonic vibration apparatus. During the calcination of ZnO coated whiskers at 1000 °C, ZnO reacts with the whiskers and ZnAl₂O₄ forms on the surface of aluminum borate whiskers.     

Thermal dissociation of iron carbonyls during growth of iron whiskers

Methods for the preparation of iron whiskers in chemical transport reactions of thermal dissociation of iron penta- and dodecacarbonyls and carbidocarbonyl clusters Fe₅C(CO)₁₅ were described. The morphology, structure, and chemical composition of the whiskers were studied. The main factors determining the growth rate and mechanical properties of the whiskers were revealed. A model for the mechanism of thermal dissociation of iron carbonyls was proposed. This process was shown to be a chain radical ion reaction initiated via the scheme of activating complex formation. Analogies between the thermal dissociation of iron carbonyls in the adsorption layer and the known radical ion processes in the liquid and gas phases were found.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1826–1836, September, 2004.     

IV-VI semiconductor growth on silicon substrates and new mid-infrared laser fabrication methods

This paper reviews results from research conducted at the University of Oklahoma on the development of new IV-VI semiconductor (lead salt) epitaxial growth and laser fabrication procedures that can ultimately lead to dramatic increases in mid-IR laser operating temperatures. Work has focused on growth of IV-VI semiconductor laser structures on silicon substrates using buffer layers that contain BaF2. Recent experiments show that it is possible to obtain high crystalline quality IV-VI semiconductor layer structures on (111)-oriented silicon substrates using molecular beam epitaxy (MBE) or on (100)-oriented silicon using a combination of MBE and liquid phase epitaxy (LPE). Experimental data for IV-VI semiconductor layer structures grown on silicon substrates including crystalline quality information as determined by high resolution X-ray diffraction (HRXRD) measurements and absorption edge information as determined by Fourier transform infrared (FTIR) transmission measurements are presented. Results show that these materials can be used to fabricate lasers that cover the 3 microns (3333 cm-1) to 16 microns (625 cm-1) spectral range. Removal of IV-VI semiconductor laser structures from the silicon growth substrate by dissolving BaF2 buffer layers with water is also demonstrated. This allows epitaxially-grown laser structures to be sandwiched between two heat sinks with a minimum of thermally resistive IV-VI semiconductor material. Theoretical modeling predicts that IV-VI lasers fabricated this way will have maximum continuous wave (cw) operating temperatures at least 60 degrees higher than those of IV-VI lasers fabricated on PbSe or PbTe substrates.     

Controlled electrochemical growth of silver microwires

The cathodic deposition of silver whiskers as microwires in the electrochemical cell Ag/AgBr/Ag has been investigated. From growth experiments observed with an optical microscope, we conclude that the growth process occurs exclusively at the interface of the silver wires with the solid electrolyte. The basal contacts of the microwires show different morphologies without clear correlation to the substrate. The silver crystals often show a hexagonal morphology, but XRD gives no definite information on the structure of the wires. The measured exchange current density is in the order of 100 mA/cm². The growth rate and the diameter of the microwires can be controlled by variation of the current.     

Mineralization of simulated flue gas to prepare CaCO3 whisker using suspended CaSO4

CaCO₃ whiskers were synthesized controllably by introducing simulated flue gas containing CO₂ and N₂ into a CaSO₄ suspension. The effects of solution pH, reaction temperature, and simulated flue gas on the formation of CaCO₃ whiskers were studied. The growth mechanism and growth model of CaCO₃ whiskers had also been provisionally recommended. The reaction product was characterized by scanning electron microscopy analysis, thermogravimetric analysis, and X-ray diffraction analysis. CaCO₃ whiskers with 97% purity were synthesized at pH 7.5, 80°C, 0.1 L/min CO₂ flow rate, and 16.7% CO₂ purity, with a length between 15 and 20 μm and an aspect ratio of about 12.     

Optimization of the isolation of nanocrystals from microcrystalline cellulose by acid hydrolysis

The objective of this work was to find a rapid, high-yield process to obtain an aqueous stable colloid suspension of cellulose nanocrystals/whiskers. Large quantities are required since these whiskers are designed to be extruded into polymers in the production of nano-biocomposites. Microcrystalline cellulose (MCC), derived from Norway spruce (Picea abies), was used as the starting material. The processing parameters have been optimized by using response surface methodology. The factors that varied during the process were the concentration of MCC and sulfuric acid, the hydrolysis time and temperature, and the ultrasonic treatment time. Responses measured were the median size of the cellulose particles/whiskers and yield. The surface charge as calculated from conductometric titration, microscopic examinations (optical and transmission electron microscopy), and observation of birefringence were also investigated in order to determine the outcome (efficiency) of the process. With a sulfuric acid concentration of 63.5% (w/w), it was possible to obtain cellulose nanocrystals/whiskers with a length between 200 and 400 nm and a width less than 10 nm in approximately 2 h with a yield of 30% (of initial weight).     

A kind of carbon whiskers in new structure and morphology

Carbon whiskers with new structure and morphology were observed when heating the milled graphite. Transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) show that carbon layers are almost perpendicular to the growth axes of carbon whiskers. Field emission scanning electron microscopy (FESEM) indicates that there are spirals appearing on the surface of the whiskers. The structure analysis shows that the growth mechanism of carbon whiskers is related to the trace amount of ZrC in the heated samples.