Chemical Vapor Synthesis Research Articles

Preparation of copper–silicon dioxide nanoparticles with chemical vapor synthesis

Atmospheric pressure chemical vapor synthesis was used to produce copper nanoparticle composites in an amorphous silicon dioxide, i.e., either copper nanoparticles coated with amorphous silicon dioxide or copper nanoparticles embedded in amorphous silicon dioxide matrix. Synthesized metal–organic copper(I) complex was used as a precursor that provided well-defined ratio (1:2) of copper and silicon. The thermal decomposition of the Cu(I) complex molecule leads to homogenous nucleation and formation of copper nanoparticles which are subsequently coated with Si/SiO2 in the gas phase. The decomposition was greatly enhanced when reductive atmosphere, i.e., H2/N2 10 v% were used instead of pure nitrogen. A narrow size distribution with the geometric mean diameter of the particle agglomerates around 30 nm was observed while the primary size of the copper core particles was around 5 nm.

Atmospheric pressure chemical vapour synthesis of silicon–carbon nanoceramics from hexamethyldisilane in high temperature aerosol reactor

Silicon–carbon nanoceramics have been synthesised from hexamethyldisilane (HMDS) by the atmospheric pressure chemical vapour synthesis (APCVS). Direct aerosol phase synthesis enables continuous production of high purity materials in one-stage process. The particle formation is based on the decomposition of the precursor in a high temperature reactor. Reaction of the gas phase species leads to homogeneous nucleation and formation of the nanoparticles with a narrow size distribution (geometric mean diameter range of particle number size distribution 160–200 nm with 1.5–1.6 geometric standard deviation at reaction temperatures 800–1200 °C). A systematic investigation of the influence of the process temperature on the powder characteristics, including the particle size, crystallinity, chemical structure, surface and bulk composition and surface morphology, was carried out. At the reactor temperature of 800 °C, the synthesised nanoparticles were amorphous preceramics containing mostly SiC4, Si–CH2–Si and Si–H units. The composition of the powder turned towards nanocrystalline 3C–SiC (crystal size under 2 nm) when the reaction temperature was increased to 1200 °C. The reaction temperature appeared to be a key parameter controlling the structure and properties of the synthesised powders.

High temperature stability of nanocrystalline anatase powders prepared by chemical vapour synthesis under varying process parameters

Systematic variation in the high temperature stability of nanocrystalline anatase powders prepared by chemical vapour synthesis (CVS) using titanium (IV) isopropoxide under varying flow rates of oxygen and helium was obtained by progressively shifting the decomposition product from C 3H 6 to CO 2. The as-synthesised powders were characterised by high temperature X-ray diffraction (HTXRD), simultaneous thermo-gravimetric analyses (STA), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). It was observed that the anatase to rutile transformation temperature progressively increased for samples synthesised at higher O 2/He flow rate ratios. The improved anatase stability was attributed to the presence of incorporated carbon within the titania structure and confirmed by a high temperature carbon desorption peak.

Formation mechanism of TiO 2 nanoparticles in H 2O-assisted atmospheric pressure CVS process

Atmospheric Pressure Chemical Vapor Synthesis (APCVS) route was used for the synthesis of titania (TiO 2) nanoparticles. The mechanism of nanoparticles formation were investigated by transmission electron microscopy (TEM), X-ray diffraction analysis, nitrogen adsorption technique (BET) and TG–DTA results for as-synthesized powders. The effect of precursor temperature, H 2O effect and effective reaction (ER) zone temperature on the phase structure, nanoparticle size, agglomeration, coagulation, coalescence and nanoparticle morphology were also studied. Also, the effect of thermal velocity on the rate of powder formation, coagulation and coalescence of nanoparticles were discussed theoretically and experimentally. With introducing H 2O, the appropriate rate of powder formation increased and size, coalescence and coagulation of nanoparticles decreased, significantly. Also, by using H 2O vapor, the crystallinity of nanoparticles sharply increased. The minimum temperature for the synthesis of full anatase phase in atmospheric pressure was obtained to be 700 °C. With increasing precursor temperature, thermal velocity and the rate of powder formation increased. Also, no phase transformation was observed but size, coagulation, coalescence and agglomeration of titania nanoparticles increased whereas the morphology of nanoparticles was similar.

Synthesis of an aluminum nitride–yttria (AlN–Y2O3) composite from nano-sized porous AlN and YCl3

In order to enhance the thermal conductivity of aluminum nitride (AlN) with sintering additives including yttria (Y2O3), it is necessary to form yttrium aluminate garnet (YAG) and secondary phases both within and around the boundaries of AIN drains. Nano-sized porous AlN particles were produced to form YAG and secondary phases within AlN grains, after which a AlN–Y2O3 nano–nano composite was formed from AlN and amorphous Y2O3. Porous AlN powders were first successfully synthesized by the chemical vapor synthesis (CVS) method. Highly crystalline and nano-sized porous AlN powders were synthesized at 1,200 °C. Brunauer–Emmett–Teller (BET) analysis showed that these powders had very large surface areas, suggesting that the particles approached nano-scale sizes with very small pores. To form composites of Y2O3 and AlN, we prepared a yttrium source solution that infiltrated the nano-sized pores of the AlN particles. Such an infiltration of AlN with amorphous Y2O3 was expected to effectively reduce the residual oxygen content by facilitating the formation of YAG and secondary phases during the sintering process. We characterized the composite powders of AlN–Y2O3 and the sintered bodies using BET, XRD, SEM, TEM, and thermal conductivity analyses.

Chemical Vapor Synthesis and Structural Characterization of Nanocrystalline Zn1−xCoxO (x = 0−0.50) Particles by X-ray Diffraction and X-ray Absorption Spectroscopy

Nanocrystalline Zn1−xCoxO (x = 0−0.50) particles are produced by chemical vapor synthesis using laser flash evaporation as a novel precursor delivery method. The crystal and local structure of the samples is studied using X-ray diffraction and X-ray absorption spectroscopy. A single wurtzite phase is observed in samples with cobalt contents as high as x = 0.25 (actual content is 0.33 determined by atomic absorption spectroscopy). X-ray absorption spectra show that the Co2+ ions are incorporated into the ZnO wurtzite lattice substituting Zn2+ ions for cobalt contents between x = 0.001 and x = 0.20. Only small lattice deformations are observed in these solid solutions.

Synthesis of porous nano-sized AlN by chemical vapor synthesis

We report the synthesis of AlN powders by the chemical vapor synthesis of an AlCl3–NH3–N2–H2 system. The starting material was gasified AlCl3, generated using a preheating system. AlCl3 gas was transported to a tube furnace in NH3–N2–H2 atmosphere. We synthesized highly crystalline AlN at over 900 °C. The average size of the spherical AlN particles decreased from 250 to 40 nm with increasing reaction temperature in the tube furnace. Porous nano-sized particles synthesized at high reaction temperatures have low oxygen contents and high thermal stability.

Influence of Nucleation Rate on the Yield of ZnO Nanocrystals Prepared by Chemical Vapor Synthesis

ZnO nanocrystals were synthesized by chemical vapor synthesis from two different precursors—diethylzinc and bis(2,2,6,6-tetramethyl-3,5-heptanedionate)zinc—at different temperatures in a tubular hot wall reactor. It is observed that the yield of ZnO nanocrystals is temperature dependent, and it decreases with increasing reactor temperature for both precursors. The nucleation rate was calculated by classical nucleation theory and was correlated with the experimental yield of ZnO nanocrystals.

Properties of Nanosize Aluminum Nitride Powders Synthesized by Chemical Vapor Synthesis

AlN powders by the chemical vapor synthesis (CVS) process in the AlCl3-NH3-N2-H2 system were successfully synthesized. Gasified AlCl3 as a starting material was generated by pre-heating system and transported to the tube furnace in NH3-N2-H2 atmosphere. High crystalline AlN was synthesized at over 900°C. The average particle size of spherical AlN powders decreased from 250 to 40nm with increasing the reaction temperature of the tube furnace. Porous nano-size particles synthesized at high reaction temperature have low oxygen contents.

Vertically Aligned Boron Nitride Nanosheets: Chemical Vapor Synthesis, Ultraviolet Light Emission, and Superhydrophobicity

Boron nitride (BN) is a promising semiconductor with a wide band gap ( approximately 6 eV). Here, we report the synthesis of vertically aligned BN nanosheets (BNNSs) on silicon substrates by microwave plasma chemical vapor deposition from a gas mixture of BF(3)-N(2)-H(2). The size, shape, thickness, density, and alignment of the BNNSs were well-controlled by appropriately changing the growth conditions. With changing the gas flow rates of BF(3) and H(2) as well as their ratio, the BNNSs evolve from three-dimensional with branches to two-dimensional with smooth surface and their thickness changes from 20 to below 5 nm. The growth of the BNNSs rather than uniform granular films is attributed to the particular chemical properties of the gas system, mainly the strong etching effect of fluorine. The alignment of the BNNSs is possibly induced by the electrical field generated in plasma sheath. Strong UV light emission with a broad band ranging from 200 to 400 nm and superhydrophobicity with contact angles over 150 degrees were obtained for the vertically aligned BNNSs. The present BNNSs possess the properties complementary to carbon nanosheets such as intrinsically semiconducting, high temperature stability, and high chemical inertness and may find applications in ultraviolet nanoelectronics, catalyst supports, electron field emission, and self-cleaning coatings, etc., especially those working at high temperature and in harsh environments.

Chemical vapor synthesis of fluorine-doped SnO2 (FTO) nanoparticles

The synthesis and properties of nanocrystalline SnO2 particles and the effects of doping with fluorine are reported in this work. Simultaneous thermal decomposition of tetramethyltin and difluoromethane in the chemical vapor synthesis process was employed. The nanoparticles were analyzed with respect to their structure using X-ray diffraction followed by Rietveld refinement, transmission electron microscopy, nitrogen adsorption, X-ray photoelectron spectroscopy, and Fourier-transformed infrared spectroscopy. Based on the experimental results, a point defect model is proposed, which is supported by density functional theory calculations. At low fluorine concentrations, fluorine substitutes oxygen on a lattice site, while fluorine is located interstitially at high concentrations. The formation of an associated fluorine substitutional–interstitial pair is observed instead of isolated interstitial fluorine.

Computational fluid dynamics simulation of chemical vapor synthesis of WC nanopowder from tungsten hexachloride

The chemical vapor synthesis (CVS) reactor for the preparation of WC nanopowder from tungsten hexachloride was simulated by a two-dimensional multiphase computational fluid dynamics (CFD) model. The model solves the gas-phase governing equations of overall continuity, momentum, energy, and species mass transport inside a tubular reactor system. The population balance model is coupled with the gas-phase equations to describe the formation and growth of WC nanoparticles. The model has been validated with experimental data in terms of average particle size and concentration of unreacted precursor at the outlet. The contours of temperature, velocity, species concentration and particle size distribution (PSD) inside the tubular reactor were computed.

Computational fluid dynamic modeling of a chemical vapor synthesis process for aluminum nanopowder as a hydrogen storage precursor☆

A chemical vapor synthesis (CVS) process for synthesizing nano-sized aluminum powder as a precursor for various hydrogen storage materials was simulated by the use of computational fluid dynamic modeling. The fluid flow, heat transfer and chemical reaction phenomena taking place inside the reactor were analyzed together with particle formation and growth in the CVS process. The temperature, velocity and particle size distribution fields inside the reactor were computed. Chemical reaction rate and population balance model were used to calculate the particle formation and growth. The particle size computed by the program was compared with the experimental data, and the calculated average size of the final product particles was consistent with those obtained in the experimental work.

Chemical Vapor Synthesis of Zinc Oxide Nanoparticles: Experimental and Preliminary Modeling Studies

The chemical vapor synthesis of ZnO tetrapods from zinc metal has been studied using a combination of experiments and fluid dynamics modeling. On one hand, an experimental study allowed production of ultrapure ZnO particles whose mean lengths (250-450 nm) and diameters (14-27 nm) depended on the reactor configuration (i.e., parallel flow/crossflow), but not on the position of air injection. On the other hand, the yield of the reaction depended both on the reactor configuration and on the position of air injection. We then developed an original kinetic model implemented in the computation fluid dynamics code FLUENT. Within the limits of certain assumptions, the model successfully predicts the experimental yield of the reaction for all the conditions tested. This good agreement shows that the kinetics of nucleation/growth of ZnO nanoparticles are probably very rapid compared to the reaction of oxidation of Zn vapor. The combination of the experimental and simulated results led to a better understanding of the heat- and mass-transfer phenomena involved. Finally, several processing parameters, such as argon and air flow rates, position of air injection, and reactor diameter, were varied in the simulations to find optimized reaction conditions for maximum yield and production rate. For the crossflow configuration, a yield of 71% and a production rate 7 times higher than the nominal value have been obtained.

Electrical properties of aluminum-doped zinc oxide (AZO) nanoparticles synthesized bychemical vapor synthesis

Aluminum-doped zinc oxide nanoparticles have been prepared by chemical vapor synthesis,which facilitates the incorporation of a higher percentage of dopant atoms, farabove the thermodynamic solubility limit of aluminum. The electrical propertiesof aluminum-doped and undoped zinc oxide nanoparticles were investigated byimpedance spectroscopy. The impedance is measured under hydrogen and syntheticair between 323 and 673 K. The measurements under hydrogen as well as undersynthetic air show transport properties depending on temperature and doping level.Under hydrogen atmosphere, a decreasing conductivity with increasing dopantcontent is observed, which can be explained by enhanced scattering processesdue to an increasing disorder in the nanocrystalline material. The temperaturecoefficient for the doped samples switches from positive temperature coefficientbehavior to negative temperature coefficient behavior with increasing dopantconcentration. In the presence of synthetic air, the conductivity firstly increases withincreasing dopant content by six orders of magnitude. The origin of the increasingconductivity is the generation of free charge carriers upon dopant incorporation. Itreaches its maximum at a concentration of 7.7% of aluminum, and drops forhigher doping levels. In all cases, the conductivity under hydrogen is higher thanunder synthetic air and can be changed reversibly by changing the atmosphere.

Chemical vapor synthesis and characterization of aluminum nanopowder

Aluminum is a component in many promising hydrogen storage materials such as aluminum hydride and complex aluminum hydrides. In this research, Al and TiAl 3-containing Al nanopowders were prepared by a chemical vapor synthesis (CVS) process using Mg as the reducing agent. XRD and EDS results indicated that the produced powder was composed of Al or Al with TiAl 3. The shape of the powder was spherical with the average size in the range of 10–50 nm measured by SEM, TEM, BET and ZetaPALS compared with the typically larger than 100 nm for commercially available fine Al powders. In addition, the effects of the operating conditions such as Ar flow rate, precursor feed rate and reaction temperature on the properties of the product powder were investigated.

A Novel Approach for Chemical Vapor Synthesis of ZnO Nanocrystals: Optimization of Yield, Crystallinity

AbstractThe experimental yield of ZnO nanocrystals decreases drastically with increasing reactor temperature in a typical chemical vapor synthesis (CVS) of ZnO nanocrystals from diethylzinc. A novel CVS set‐up – a microwave plasma combined with a hot‐wall zone – is described to minimize the loss of particles at higher reactor temperatures. The powder samples have been characterized by transmission electron microscopy (TEM) and X‐ray diffraction (XRD). It is observed that the synthesis set‐up and reaction temperature have substantial influence not only on yield but also on crystallite size and crystallinity of the pure wurtzite‐type ZnO nanocrystals. The lattice constants of ZnO nanocrystals increase with decreasing crystallite size. Defect densities (twin and stacking faults), as well as microstrain, decrease with increasing reactor temperature, whereas crystallinity increases.

Effect of Gas Flow Rates on the Anatase–Rutile Transformation Temperature of Nanocrystalline TiO2 Synthesised by Chemical Vapour Synthesis

Of the three crystallographic allotropes of nanocrystalline titania (rutile, anatase and brookite), anatase exhibits the greatest potential for a variety of applications, especially in the area of catalysis and sensors. However, with rutile being thermodynamically the most stable phase, anatase tends to transform into rutile on heating to temperatures in the range of 500 degrees C to 700 degrees C. Efforts made to stabilize the anatase phase at higher temperatures by doping with metal oxides suffer from the problems of having a large amorphous content on synthesis as well as the formation of secondary impurity phases on doping. Recent studies have suggested that the as-synthesised phase composition, crystallite size, initial surface area and processing conditions greatly influence the anatase to rutile transformation temperature. In this study nanocrystalline titania was synthesised in the anatase form bya chemical vapour synthesis (CVS) method using titanium tetra iso-propoxide (TTIP) as a precursor under varying flow rates of oxygen and helium. The anatase to rutile transformation was studied using high temperature X-ray diffraction (HTXRD) and simultaneous thermogravimetric analysis (STA), followed by transmission electron microscopy (TEM). It was demonstrated that the anatase-rutile transformation temperatures were dependent on the oxygen to helium flow rate ratio during CVS and the results are presented and discussed.

Chemical vapor synthesis of Mg–Ti nanopowder mixture as a hydrogen storage material

Abstract Magnesium-based alloys are among the promising materials for hydrogen storage and fuel cell applications due to their high hydrogen contents. In this work, the hydrogen release and uptake properties of Mg/5%Ti nanopowder mixture prepared by a chemical vapor synthesis (CVS) process were investigated. Samples were made in a CVS reactor, in which reactant powders were fed into evaporator placed inside the reactor by means of specially designed powder feeders. The produced Mg/5%Ti was hydrogenated in an autoclave under 10.3 MPa pressure and 150 °C for 12 h. Thermogravimetric analysis (TGA) showed that 5.2 wt% of hydrogen began to be released from 190 °C while temperature was increased at a heating rate of 5 °C/min up to 350 °C under an argon flow. This onset temperature of Mg–Ti nanopowder dehydrogenation was much lower than that of MgH2 alone, which is 381 °C. In addition, the activation energy of dehydrogenation was 104 kJ/mol, which is much smaller than that of as-received MgH2 (153 kJ/mol).

Granular nanocrystalline zirconia electrolyte layers deposited on porous SOFC cathode substrates

Thin granular yttria-stabilized zirconia (YSZ) electrolyte layers were prepared by chemical vapor synthesis and deposition (CVD/CVS) on a porous substoichiometric lanthanum–strontium–manganite (ULSM) solid oxide fuel cell cathode substrate. The substrate porosity was optimized with a screen printed fine porous buffer layer. Structural analysis by scanning electron microscopy showed a homogeneous, granular nanocrystalline layer with a microstructure that was controlled via reactor settings. The CVD/CVS gas-phase process enabled the deposition of crack-free granular YSZ films on porous ULSM substrates. The electrolyte layers characterized with impedance spectroscopy exhibited enhanced grain boundary conductivity.