Laser-induced Gas-phase Reaction Research Articles

The formation of nanoparticles in laser-induced SiH4 gas reactions

Ultrafine powders with a mean particle diameter (20–30) nm were synthesized from high purity SiH4 by laser-induced gas phase reactions and studied by transmission electron microscopy. Results showed that the powders consisted of Si crystals with an amorphous shell, or spherical Si crystals without the shell. Twinning of crystals often took place. Both types of crystals: poly and mono were observed inside an amorphous shell. The mechanism of the formation of all types of nanoparticles is proposed.

Microwave absorbing property and complex permittivity of nano SiC particles doped with nitrogen

Abstract Microwave absorbing property and complex permittivity of the nano SiC particles doped with nitrogen within the frequency range of 8.2–18 GHz were investigated. The nano SiC particles doped with nitrogen was synthesized from hexamethyldisilazane ((Me 3 Si) 2 NH) (Me:CH 3 ) by a laser-induced gas-phase reaction. The complex permittivities of the composites can be tailored by the contents of the nano SiC particles. The real part ( ɛ ′) and imaginary part ( ɛ ″) of the complex permittivity, and the dielectric dissipation factor (tg δ = ɛ ″/ ɛ ′) of the composites increase with the volume filling factor ( v ) of the nano SiC particles doped with nitrogen. The ɛ ′ and ɛ ″ of the composites can be effectively modeled using second-order polynomials ( ɛ ′, ɛ ″ = A v 2 + B v + C ). The ɛ ′ and ɛ ″ of the nano SiC particles doped with nitrogen decrease with frequency. The high ɛ ″ and tg δ of the nano SiC particles doped with nitrogen are believed to be caused by the substitution of nitrogen for carbon in the nanocrystals of SiC. The single layer composites of 7 wt% nano SiC particles doped with nitrogen with a thickness of 2.96 mm achieved a reflection loss below −10 dB (90% absorption) at 9.8–15.8 GHz, and the minimum value was −63.41 dB at 12.17 GHz. The reflection loss calculations show that the prepared nano SiC particles doped with nitrogen are good electromagnetic wave absorbers in the microwave range.

Magnesium-based nanocomposites

The Mg micropowder was generated by gas atomisation of a magnesium melt with argon. The ceramic nanoparticles (SiC, ZrO2, Al2O3) and carbon nanoparticles were generated by laser-induced gas phase reaction in a flow reactor. The Mg materials were mixed and ball milled with nanoscaled ceramic powder and then was hot extruded. The mechanical properties of new materials were investigated by tensile test at temperatures between room temperature and 300°C at a constant initial strain rate. The flow stress is significantly influenced by temperature. The nanocomposites exhibit high-damping characteristics. The damping was measured as the logarithmic decrement of the free decay of the vibrating nanocomposite beam. At lower strains, the logarithmic decrement is practically independent of the strain amplitude, whereas at higher strains the decrement increases with increasing strain amplitude. The volume fraction and the kind of reinforcement influence the value of the logarithmic decrement and its temperature dependence.

High-yield laser-synthesis of Si-based nanopowders

Amorphous Si 3N 4, Si/C/N and crystalline SiC nanopowders of 5–50 nm in mean diameter were synthesized via laser-induced gas-phase reactions with yields of 250 g h −1 using low cost hexamethyldisilazane (HMDS) and NH 3 (or H 2). Reaction flame temperatures, which were controlled by the total gas flow rate, greatly influenced powder composition and crystallinity. Amorphous Si 3N 4 powders (96 wt.%) were obtained at 1000 °C, with HMDS+NH 3; β-SiC (93 wt.%) was formed at 1600 °C with HMDS+H 2. Amorphous Si/N/C powders with various N/C ratios (0.1–6) were synthesized at various temperatures. The main impurities in the powders were the organo-ligands such as NH X and CH X ( X=1–3), whose existence also resulted in a strong surface absorption of oxygen. Key techniques for commercial production of the abovementioned powders were also stated.

Dispersion behavior of laser-synthesized Si 3N 4 nanopowders in N-N-dimethylformamide

The dispersion behavior of Si 3N 4 nanopowders prepared by laser-induced gas-phase reaction in a non-aqueous solvent, N-N-dimethylformamide (DMF) was studied, and triethanolamine (TEA) was selected as a dispersant. One hundred and sixty-six-day sedimentation tests were used to study the stability of the suspensions. Photon correlation spectroscopy was employed to measure the sizes of aggregates and coagulation rates. TEA was proved an effective dispersant for nano-Si 3N 4 in DMF, presoaking treatment could enhance the effect of the dispersant. TEA 1.0 wt.% referred to Si 3N 4 and a 1-month-presoaking gave the best dispersion, the dispersion stability was improved from several minutes to several months, and the average size of the aggregates was reduced to about a twenty-fifth. The dispersion ability of TEA was believed being related to its strong affinities both to the powder surface and to the solvent.

Dielectric properties of nano Si/C/N composite powder and nano SiC powder at high frequencies

The dielectric properties of nano Si/C/N composite powder and nano SiC powder at high frequencies have been studied. The nano Si/C/N composite powder and nano SiC powder were synthesized from hexamethyldisilazane ((Me 3Si) 2NH) (Me:CH 3) and SiH 4–C 2H 2, respectively, by a laser-induced gas-phase reaction. The complex permittivities of the nano Si/C/N composite powder and nano SiC powder were measured at a frequency range of 8.2–12.4 GHz. The real part ( ε′) and imaginary part ( ε″) of the complex permittivity, and dissipation factor ( tg δ=ε″/ε′) of nano Si/C/N composite powder are much higher than those of nano SiC powder and bulk SiC, Si 3N 4, SiO 2, and Si, especially the tg δ . The promising features of nano Si/C/N composite powder would be due to more complicated Si, C, and N atomic chemical environment than in a mixture of pure SiC and Si 3N 4 phase. The charged defects and quasi-free electrons moved in response to the electric field, diffusion or polarization current resulted from the field propagation. Because there exists graphite in the nano Si/C/N composite powder, some charge carries are related to the sp 3 dangling bonds (of silicon and carbon) and unsaturated sp 2 carbons. The high ε″ and tg δ of nano Si/C/N composite powder were due to the dielectric relaxation. The nano Si/C/N composite powder would be a good candidate for electromagnetic interface shielding material.

MICROWAVE PERMITTIVITY OF NANO Si/C/N OMPOSITE POWDERS

The microwave permittivity of nano Si/C/N composite powders suspended in paraffin wax has been studied at the frequency range of 8.2—18GHz. The nano Si/C/N composite powders were synthesized from hexamethyldisilazane ((Me3Si)2NH) (Me:CH3) by a laser induced gas phase reaction. The dissipation factors of the nano Si/C/N composite powders are high at the microwave frequencies. The microwave permittivity of the mixture of nano Si/C/N composite powders and paraffin wax (or other dielectric materials) can be tailored by the content of the composite powders. And ε′, ε″ and tan δ increase with the volume filling factor ( v ) of nano Si/C/N composite powders. The ε′ and ε″ can be effectively modeled using second order polynomials ( ε′, ε″=Av2+Bv+C ). The ε′ and ε″ of the nano Si/C/N composite powders decrease with frequency at the frequency range of 8.2—18GHz. The difference being the microwave resonance is not sharply peaked but rather smeared out over a large frequency range. The promising features of nano Si/C/N composite powders would be due to more complicated Si, C, and N atomic chemical environment than in a mixture of pure SiC and Si3N4 phase. The SiC microcrystallines in the nano Si/C/N composite powders dissolve a great deal of nitrogen. The local structure around Si atoms changes by introducing N into SiC. Carbon atoms around Si are substituted by N atoms. So there exist a large number of charged defects and dangling bonds in the nano Si/C/N composite powders. Thus charged defects and quasi free electrons move in response to the electric field, diffusion or polarization current resulting from the field propagation. The high ε″ and dissipation factor tan δ(ε″/ε′) of Si/C/N composite powders are due to the dielectric relaxation.

Magnesium strengthened by SiC nanoparticles

Mg was reinforced with SiC nanoparticles by powder metallurgical technique. The SiC nanoparticles were generated by laser-induced gas phase reaction in a flow reactor and had a median particle diameter of 30 nm. In order to distribute the nanoparticles in the Mg matrix, Mg micropowder with a median particle diameter of 40 μm was mixed or ball milled with the nanoscaled ceramic powder followed by hot extrusion. The mechanical properties of the new material were investigated by tensile tests and creep measurements and the microstructure of the composites were examined by light and transmission electron microscopy (TEM).

Laser synthesis and crystallization of nanocomposite Si/C/N powder

Ultrafine preceramic Si/C/N composite powders have been prepared from hexamethyldisilazane (HMDS) by laser-induced gas phase reaction, using a new kind of tworeaction-zone reactor which could efficiently increase laser efficiency and production yield compared with a one-reaction-zone reactor. The as-formed products were nanosized (50–80 nm), amorphous powders containing Si–C and Si–N bonds homogeneously mixed with some excess carbon. The production yield was in the range of 88–120 g/h. Changes of chemical composition and crystallization of the powders during heat treatment at 1350 and 1550 °C under nitrogen for 1 h were studied.

Synthesis and thermal stability of SiC-Si 3N 4 composite ultrafine particles by laser-induced gas-phase reaction

SiC-Si 3N 4 composite ultrafine particles (UFPs) with various compositions were prepared by laser-induced gas-phase reaction. These particles were heat-treated in Ar or Ar + N 2 atmosphere at a temperature range from 300 K to 1773 K. Structural changes such as crystalline phase, particle shape, particle size distribution and composition after heat treatment were investigated by X-ray diffraction, transmission electron microscopy, specific surface area analysis and chemical analysis. SiC-Si 3N 4 composite UFPs have the composite SiC, metallic Si and Si 3N 4 phases depending on the nitrogen and carbon contents. They are thermally stable up to 1573 K in Ar without significant changes in crystalline phase and microstructures, but the nitrogen containing SiC phase decomposes into SiC, Si and Si 3N 4 phases accompanied by a remarkable increase in particle size above 1773 K. However, a small amount of nitrogen in the atmosphere could suppress the phase transformation and particle size growth.

Preparation of carbon nitride fine powder by laser induced gas-phase reactions

We present the possibility of carbon nitride fine powder synthesis by sensitized laser pyrolysis of acethylene/nitrous oxide/ammonia mixtures. The powders were analyzed using X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and IR transmission measurements. It was found that nitrogen was incorporated in powders and that in the carbon-nitrogen phases formed, the presence of the triple bonded C≡N was not detected. The majority of X-ray diffraction data suggests the presence of a mixture of the predicted α- and β-C3N4 structure, with an α-C3N4-like form being prevalent. The powders were found to be slightly contaminated by SF6 sensitizer products. Our results suggest that by improving the experimental parameters this contamination might be reduced and that the laser pyrolysis method offers possibilities for production of CxNy materials, with controlled composition.

Characterization of ultrafine SiC [sbnd]Si 3N 4 composite powder after heat-treatment in Ar + N 2

Ultrafine SiC Si 3N 4 composite powder synthesized by laser-induced gas-phase reaction was heat-treated at a temperature range of 295–1773 K in a mixture gas flow of Ar + N 2. The changes in crystalline structure, shape, particle size, crystalline size, chemical composition, specific surface area and defect state were investigated by X-ray diffraction (XRD), transition electron microscope, chemical analysis, specific surface area analysis and electron spin resonance. No significant changes in the structure are observed after the heat treatment below 1673 K in Ar + N 2, similar to the result of the heat-treatment in Ar. At or above 1673 K, the characteristic changes occur, but these depend on the heating atmosphere. The precipitation of metallic silicon, which is observed in the heat-treatment in At, is suppressed by introducing only 5% N 2 to the atmosphere. The composite powder with a desired composition of SiC and Si 3N 4 can be obtained by adjustment of the nitrogen partial pressure.

EFFECT OF HEAT TREATMENT ON THE STRUCTURE OF SiC−Si3N4 COMPOSITE ULTRAFINE POWDER

Effect of heat treatment on the structure of SiC−Si3N4 composite ultrafine powder was studied. SiC-Si3N4 composite ultrafine powder synthesized by laser-induced gas-phase reaction was heat-treated at various temperatures below 1773 K in 99% Ar +1% H2 gas mixture. The change of the structure was studied by chemical analysis, x-ray diffraction (XRD), and transmission electron microscope (TEM). The structure of the powder did not change significantly in lattice constant, particle size, and composition by the treatment up to 1573 K . The structure of the powder changed drastically by the treatment above 1673 K . The broad XRD pattern due to β- Si for the original powder changed to the four phases of β- SiC , Si and α, β- Si 3N4 by the treatment at 1773 K , accompanied by large increases in particle size and lattice parameter, and decreases in nitrogen content and specific surface area.

Deposition and analysis of silicon clusters generated by laser-induced gas phase reaction

Silicon clusters have been generated by CO2-laser-induced decomposition of SiH4 in a flow reactor. By introducing a conical nozzle into the reaction zone, they are extracted into a molecular beam apparatus and analyzed with a time-of-flight mass spectrometer. The mass spectra show that the cluster source emits, besides small clusters, also nanosized species containing around 103 atoms. These clusters were deposited on silicon and sapphire targets at room temperature. The deposited films have been analyzed with a Raman spectrometer and with a field emission scanning electron microscope (FE-SEM). The Raman spectra reveal a broad amorphouslike band and a relatively sharp peak at 518.1 cm−1. Interpretation of the sharp Raman feature, based on the phonon confinement model, suggests the presence of silicon nanocrystallites in the deposited films with a particle size of about 3–3.6 nm in diameter. The FE-SEM micrographs show an agglomerate of spherical particles of 3–12 nm in diameter, with a pronounced maximum in the size distribution at around 3.5 nm. The various methods of characterization allow us to conclude that the size of the nanoclusters is largely preserved if they are deposited on the substrate. Therefore, the technique presented here might be an efficient means to produce silicon quantum dots of about 3 nm in diameter.

Production of boron carbides by IR laser-induced gas phase reactions

Abstract The irradiation of gaseous mixtures of nido-decaborane (B10H14) and ethylene by a TEA CO2 laser results in the formation of solid deposits which can be identified, on the basis of X-ray photoelectron spectroscopy (XPS) and Fourier transform IR (FTIR), to consist of boron-carbon compounds. Deposits with a wide range of B C stoichiometries can be prepared by varying the ratio of decaborane to ethylene partial pressures.

Low-cost high-yield synthesis of nanosize powders of Si-based nitrides and carbides by CO 2 laser induced gas phase reactions

Amorphous Si 3N 4, Si/N/C and crystalline SiC nano-powders, 5–50 nm in mean diameter, with yields of 40–120 g/h were synthesized via laser induced gas phase reactions of low cost hexamethyldisilazane (HMDS) and NH 3 (or H 2). Reaction flame temperatures, which were controlled by the total gas flow rate, greatly influenced powder composition and crystallinity. Amorphous Si 3N 4 powders (96 wt%) were obtained at 1000 °C from HMDS+NH 3; β-SiC (93 wt%) was formed at 1600 °C with H 2. Amorphous Si/N/C powders with various N C ratios (0.1–6) were synthesized at various temperatures. The main impurities in the powders were the organo-ligands such as NH x and CH x ( x = 1–3) whose existence also resulted in a strong surface absorption of oxygen.

Synthesis and characterization of ultrafine Si powders from laser induced SiH4 gas reactions

Ultrafine silicon powders with a mean particle diameter of 20–200 nm and a mean crystallite diameter of 18–37 nm were synthesized from high purity SiH4 by laser induced gas phase reactions. The Si powders have been characterized by TEM, XPS, FTIR, XRD and BET techniques, and the effects of processing variables including SiH4 flow rate, laser power, cell pressure and the ArSiH4 ratio on the formation of the Si powders have been studied. Results showed that the powders consisted of monodispersed spherical particles, and agglomerated particles; both types of particles have polycrystalline structure, which resulted from the interparticle collision growth during the formation of the powders. The nonagglomerated spherical particles with a small size werefavored to form with a lower laser power and higher ArSiH4 ratios. The Si powders have O-Si-H surface configuration due to oxidation when exposed to air.

Laser synthesis of ultrafine Si 3N 4SiC powders from hexamethyldisilazane

Ultrafine Si 3Ni 4SiC powders have been successfully prepared from hexamethyldisilazane ((Me 3Si) 2NH) (Me  CH 3) and NH 3 by a laser-induced gas phase reaction. An optimization experiment aimed at increasing the Si 3N 4 content of the powders has been done with an orthogonal design of the main processing parameters, including the laser power and the gas flow rates of (Me 3Si) 2NH and NH 3. The Si 3N 4 content of the powders has been raised to 70 wt.% and the corresponding processing parameters determined. The powders produced have been characterized by chemical analysis, transmission electron microscopy, IR spectroscopy and X-ray diffraction. They are white-grey in colour, free of hard agglomerates, of average particle size 20 nm and their chemical composition changed with the processing parameters.

A model for the formation of nanosized SiC powders by laser-induced gas-phase reaction

Abstract Powder refinement is one approach to reducing the frequency of microstructural inhomogeneities in ceramic parts. Laser-induced pyrolysis of gaseous precursors is a reproducible and reliable process to fabricate nanosized particles. In the thermal decomposition of SiH 4 and C 2 H 2 by a continuous wave CO 2 laser, the size of the silicon carbide particles is adjusted according to their residence time in the reaction zone. The collision and coalescence model provides a satisfactory agreement with experimental results, whereas the nucleation theory is inappropriate, due to the small dimensions of the critical nuclei.

Nanocomposite ceramic powder production by laser-induced gas-phase reactions

A comparative study of laser-produced nanocomposite ternary SiCN powders, obtained from different SiH 4, amines and NH 3 mixtures, is presented. It is shown that the final composition of powders is correlated with and may be controlled by the experimental parameters. Preliminary results on laser irradiation of SiH 4, NH 3 and (CH 3) 3Al mixtures demonstrate the possibility of producing nanosized powders composed of a matrix of Si 3N 4 with a different content of Al.