HEOPTAHHHECKHE MATEPHAttbl, 2009, moM 45, № 12, c. 1501-1505
UDC 546.28
DECOMPOSITION OF SILICON TETRACHLORIDE BY MICROWAVE PLASMA JET AT ATMOSPHERIC PRESSURE
© 2009 Lifeng Wu, Zhibin Ma, Aihua He, Jianhua Wang
Province Key Lab of Plasma Chemistry and Advanced Materials, School of Material Science and Engineering, Wuhan Institute of Technology, Wuhan, China e-mail: mazb@mail.wit.edu.cn linwlf28@yahoo.com.cn Received 02.11.2008
A new method for destroying silicon tetrachloride has been developed, which based on a microwave plasma jet that operates at atmospheric pressure using hydrogen as work gas. Experiments showed that the silicon tetrachloride was dissociated into silicon and hydrogen chloride under the effect of the plasma jet. The silicon was deposited on the molybdenum substrate of the plasma reactor, which was yellow and typical nano-sized particles. These solid samples have been analyzed by SEM, eDx, XRD, FT-IR and the active particle in the plasma is detected by atomic emission spectroscopy. The results show that the silicon tetrachloride is mainly transformed into nano-silicon with size of 54nm. The dissociation efficiency reaches 50%.
INTRODUCTION
The semiconductor industry in the large-scale use of si-lane produces a large number of halogenated silane in flue gases. Such undesirable waste is usually disposed of by dumping in depots or discharge into the aquatic medium or the atmosphere. This has resulted in increasing contamination of ground water and degradation of tropospheric ozone [1]. Silicon tetrachloride, one of volatile compounds in flue gases, easily brings health hazards in the air. Recently, a number of technologies for destroying such environmental hazards ranging from air pollutants to solid wastes have so far been tested including thermal decomposition [2], catalytic oxidation [3], biofiltration, carbon adsorption, membrane separation, UV oxidation [4], condensation and plasma-based procedures [5].
As we known, non-thermal plasmas provide an alternative method for generating highly reactive species; in which electrons are accelerated by the applied electrical fields and transfer their energy via elastic and non-elastic collisions with neutral molecules. The reactions occurring under these conditions are usually far from thermodynamic equilibrium and result in destruction associated with higher electron temperatures of 10000-20000 K, by contrast, the neutral gas remains at much lower temperatures. This mechanism is useful for the applied energy to generate radicals and excited atomic and molecular species, thereby facilitating the decomposition of contaminant molecules. Although the temperature of a non-thermal plasma is not so highly influential on the destruction efficiency, an increased temperature facilitates the process. In addition to enhanced destruction and removal, plasma-based chemical processes provide some advantages such as low implementation, operating and capital costs, and modest facility
size requirements, in relation to conventional volatile compounds destruction techniques [1].
In this letter, we report a system for destroying SiCl4 based on a microwave plasma jet (MPJ) operating at atmospheric pressure. MPJ at atmospheric pressure has gained huge potential from industry in recent years as a clean, high-temperature intense energy source for various applications of material processing [6].
EXPERIMENTAL
The experimental set-up for SiCl4 destruction and byproduct analysis consists of two distinct parts, namely: (a) the destruction zone, which includes a microwave plasma jet operating at atmospheric pressure; (b) the analytical systems of atomic emission spectroscopy.
Fig. 1 depicts the proposed SiCl4 destroying system. The former comprises a magnetron connected to a rectangular waveguide (WR340). They are operated at a frequency of 2.45 GHz and their power is approximately 800 W. The power supply, consisting of a full-wave voltage double circuit, provides the electrical power to the magnetron, which generates the microwave radiation and is cooled by a water-cooled matched load. The generated microwave radiation from the magnetron is guided through the waveguide, passes through the three-stub tuner, and enters the discharge part [7]. The resonant microwave is adjusted by sliding short circuits and is induced into a copper nozzle located at the centre of the reaction section. The center axis of nozzle is located one-quarter wavelength from the shorted end and is perpendicular to the wide waveguide walls.
For the 2.45 GHz cavity, the modeling results gave the following optimum condition. The nozzle position from the waveguide cavity short circuit wall is 45 mm, nozzle tip
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Cool-water
Molybdenum substrate
Carrier
Cooper nozzle
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20 mm
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Exhaust Gas
3-Stub Tuner.
-n _ flfla _ 2.45 GHz Magnetron
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Waveguide
Photomultiplier
Flow Meters
Spectrometer
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H2
SiCl4
Fig. 1. Schematic diagram of the proposed SiCl4 destruction set-up.
Fig. 2. SEM pictures of Si particles extracted at different regions. (a): Si nanopowder as collected at the centre of substrate and at the edge for (b) with EDX spectra of the powder deposited on the molybdenum substrate. Plasma conditions: input power 800 W, 100 kPa, flow rate 500 ml/min.
diameter is 2 mm, nozzle hole is 0.5 mm and cavity hole is 20 mm in diameter. This diameter has been optimized to prevent the electrical field from breaking down inside the cavity and also to stop the jet reflected back into the cavity during the material processing [8]. Hydrogen was used as a plasma gas and carrier gas with 100 kPa pressure and a 0.4~0.6 l/min flow rate. The powder was collected by quenching from molybdenum substrate.
Surface morphology of the powder was studied by a scanning electron microscopy (SEM) with JSM 5510LV microscope equipped with an EDX analyzer. Crystal structure was detected with a YB-XD-5A X-ray diffraction. Further information about the chemical structure was obtained by Impact 420 FT-IR. Atomic emission spectrosco-py was used for monitoring excited species in the discharge with WDS-8A grating spectrometer.
RESULTS AND DISCUSSION
We primarily considered H2 as a reaction gas, which was injected as the carrier gas of the microwave plasma jet. The desired reaction pathway is the following process.
H2 — 2H, (1)
H + SiCl4 -H + SiCl3 -H + SiCl2 H + SiCl -
SiCl3 + HCl, SiCl2 + HCl, - SiCl + HCl. Si + HCl.
(2)
H2 is to deoxidize the silicon in SiCl4 to Si. The powder deposited on the substrate was yellow in color.
The SEM images of as-produced Si nanoparticles by the microwave plasma jet are shown in Fig. 2. By comparing samples extracted at different regions, also the difference between the different agglomeration degrees can be
found. An agglomerate is equal to just one primary particle at the centre of substrate, whereas it is built up out of many primary particles at the edge. When the plasma has a high degree of particle loading (a high precursor feed rate and higher temperatures), the sample containing particles with a small degree of agglomeration will show a more open and fluffy structure (Fig. 2a), while the sample containing a high degree of agglomeration shows a dense structure (Fig. 2b). Quenching is the more effective at higher temperatures for centre, and much smaller particles in the agglomerates can be obtained and have a more open structure. In order to obtain the information of element species in the powder. EDX survey spectra were used to determine which elements were present in the powder. Si and oxygen species were detected with atomic fraction of 61.42% oxygen and 31.72% silicon consistently. The EDX spectra showed an additional line characteristic of the existence of Cl (due to the chlorine atoms of SiCl4). In addition, the Cu and Al signal is significantly detected due to a little erosion of copper nozzle and aluminum fixture around the molybdenum substrate.
Representative FT-IR spectra that show the transmit-tance against wavenumber for specimen is given in Fig. 3. Fig. 3 are the FT-IR spectra corresponding to SEM images in Fig. 2b, respectively. Strong absorbance peaks occur at 584, 1110 and 3440 cm-1. These features are associated with Si-H2 bonds that have a characteristic bend mode (584 cm-1) .The system was open to the atmosphere in the process of SiCl4 destruction, as shown in Fig. 1. Therefore, the constituents of the atmosphere may be involved in the destroying process, resulting in possible byproducts during the production of Si nanoparticles, and Si-OH bonds that have a bend mode (1620 cm-1) and a stretch mode (3440 cm-1) could be detected in FT-IR spectra .We also find a non-symmetric stretching vibration absorbance peak (1110 cm-1) corresponding to asymmetric stretching Si-O-Si bond. Moreover, for the sample the Si-O-Si bending bond peak situated at 826 cm-1 does not appear. So the as-collected powder is of a lower level of oxidation.
XRD is a powerful tool for determining the structure of as-produced material clearly. Therefore, an XRD pattern of the collected powder was taken, and is shown in Fig. 4. As shown in Fig. 4, black circles indicate the peaks corresponding to Si [space group: Fd3m(227)] with lattice constants a = 5.430 A, which are in good agreement with the reported data (JCPDS file № 65-1060) [9] and white circles correspond to CuCl2 • H2O (JCPDS file № 33-0451) [10]. And in the process of discharge, detected CuCl2 • H2O in the powder was caused by a little erosion of copper nozzle. In the role of the plasma jet, the high activity copper atoms through high-temperature evaporation move up with the chlorine atoms decomposed from silicon tetrachloride , and precipitate as CuCl2 • H2O crystal when touch the cold-Mo substrate. The Willamson-Hall method [11] can be used to determine the crystallite size and lattice strain when there are three more reflections available for measurement. Contributions of th
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