doi:10.14311/ppt.2017.2.169Plasma Physics and Technology 4(2):169–172, 2017 © Department of Physics, FEE CTU in Prague, 2017

APPLICATION OF RF (13.56 MHZ) ARC DISCHARGE FOR PLASMACHEMICAL CONVERSION OF VOLATILE FLUORIDES

R.A. Korneva,∗, P.G. Sennikova, V.V. Nazarovb

a G.G. Devyatykh Institute of Chemistry of High-Purity Substances of RAS, Nizhny Novgorod, Russiab Institute of Applied Physics RAS, Nizhny Novgorod, Russia∗ kornev@ihps.nnov.ru

Abstract. The report discusses the use of a new type of plasma source of hydrogen plasma basedon a RF (13.56MHz) arc discharge of atmospheric pressure between two electrodes. This dischargewas used for hydrogen reduction of the tetrafluorides of silicon, boron and molybdenum. As a resultof the studies, the main regularities of the hydrogen reduction process were established and the mainsynthesized products were determined. Samples of carbides of silicon, boron and molybdenum wereprepared.

Keywords: RF arc discharge, boron fluoride, silicon fluoride, molybdenum fluoride.

1. IntroductionVolatile fluorides of boron (natBF3), silicon (natSiF4)and molybdenum (natMoF6) are the most suitable forthe physico-chemical characteristics of the starting ma-terials for preparing their isotopic varieties (10/11BF3,28/29/30SiF4, 92/94/95/96/97/98/100MoF6) by the cen-trifugal method [1]. In the future, already isotopicallyenriched fluorides can be used to prepare isotopicvarieties of B, Si and Mo. The listed isotopes and ma-terials on their basis find application in fundamentalscientific research, and also have considerable practi-cal significance. Among the seven stable isotopes ofmolybdenum, the most important isotopes are 98Moand 100Mo. Both are used to generate radioactive99Mo, which turns into 99mTc, used as a radiopharma-ceutical, and the 100Mo isotope can be used in nuclearphysics experiments to study double neutrinoless betadecay [2]. It was proposed in [3] to use as starting ma-terial molybdenum carbide of a natural compositionhaving a developed surface, which would allow theformed 99Mo atoms to be accumulated with a higherefficiency in comparison with the metal. 10B is mainlyused in the control rods of thermal neutron reactors.In order to reduce the size of the control rods whileincreasing their absorbing capacity with respect toneutrons, the most expedient is the use of highly con-centrated 10B in the form of metal ceramic of boroncarbide with aluminum oxide [4]. The highly enriched29Si silicon isotopes in 28Si could be used to developa quantum computer [5]. An important property ofmonoisotopic silicon, distinguishing it from a naturalanalogue, is a higher thermal conductivity which canbe used to create various electronic devices [6]. Forexample, it is known that SiC compound is used tocreate semiconductor devices of power electronics, andin [7] it is noted that the use of 28SiC significantlyimproves their performance characteristics. As it hasbeen already mentioned, it is expedient to use their flu-

orides for preparation of the above-mentioned isotopesand materials. The high cost of enriched fluorides re-quires minimization of return and non-return lossesof the substance at the processing stage. Therefore,the development of highly efficient methods for theisolation of isotopically modified silicon, boron andmolybdenum from their fluorides is an urgent task.Plasma chemical methods, in comparison with con-ventional chemical methods, are most preferable forisolation of simple substances and synthesis of com-pounds from their thermodynamically stable volatilefluorides. The processes of hydrogen reduction of thefluorides of silicon and molybdenum to elementarySi and Mo in the plasma of RF-IC, RFC dischargeand RFAD were studied in [8, 9]. The purpose of thiswork is to study the processes of plasma chemical syn-thesis of SiC, B4C and Mo3C2 carbides from volatilefluorides of silicon, boron, molybdenum and methanein RFAD discharge at a pressure close to atmosphericand to study the structure of the prepared samples.

2. Description of the dischargeIn our case, the discharge is formed between tungstenelectrodes in a hydrogen atmosphere in the pressurerange 200–760Torr and represents an electric arc ofalternating current with a frequency of ω = 13.56MHz(Fig. 1a). In this case, there is a strong heating ofthe end parts of the electrodes that come into con-tact with the plasma. The electric current in thegas under given conditions is determined by the char-acteristics of the external circuit. To determine thetotal current, two electric shunts of alternating current(r1 = 2500µΩ, r2 = 3750µΩ) were connected in seriesto the grounded electrical high-frequency circuit (Fig.1c) . The characteristic value of the total currentdetermined by this technique was I ∼ 5A. An esti-mate of the current was also carried out by estimatingthe current density of thermoionic emission, knowing

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Electrode ElectrodeRegion of plasma

R1R1 R

C1C1 C

RF-power

Wall of reactor

SiF + CH + H 4 4 2 SiF + CH + H 4 4 2

E

Region of plasmaNozzle

Electrode

SiC SiC

a

b

c

d

r1 r2

Figure 1. a - is a type of discharge; b - is the equivalentelectrical circuit; c - is the scheme of plasma-chemicalreactor; d - is the type of precipitated B4C.

the temperature of the electrodes. The temperaturewas determined by modeling the thermal processesin the reactor [10] and for the selected experimen-tal conditions was equal to 3500K. For the electrodematerial used (tungsten) and the known area of theheated surface, the current density is determined bythe formula:

j = A · T 2 · exp(−eϕ/kT ) (1)where A for tungsten lies in the interval 40–100, eϕ =4.54 eV [11]. Knowing the area of the heated electrodesurface, the estimated current value is I ∼ 5A. Itcan be seen that the experimentally obtained valueof the total current coincide with the current valueestimated by 1. A double electric layer (DEL) modelwas used to describe the RF discharge under consider-ation. Formation of DEL occurs in the area of contactof the metal surface of the electrodes and plasma [11].According to the adopted model, the equivalent circuitof our discharge is the circuit shown in Fig. 1b. R isthe active resistance of the plasma, C = εS/(4πL) isthe capacity of the space between the DEL filled withplasma, where S is the characteristic cross-sectionalarea of the plasma arc, L is the characteristic dimen-sion of the plasma, ε = −ωpe

2/ν2en is the dielectric

constant of the plasma), R1 is the active resistanceresulting in the heating of the electrodes, connected

with the flow of thermoelectrons and electrons thathave passed from the plasma through the DEL tothe electrodes, C1 = S1/(4πd) is the capacity of theDEL, where S1 is the area of the electrodes, d = 3rD

(the radius of the Debye) is the characteristic longi-tudinal size of DEL. The value of R is determined bythe formula R = L/σS, where L is the characteristicdimension of the plasma, S is the characteristic cross-sectional area of the plasma arc, and σ = ωpe

2/4πνenis the conductivity of the plasma [12]. The resistanceand impedances of this circuit are calculated accordingto this formula and are equal to R = 6 Ω, ZC = 10 Ω,ZC1 = 400 Ω, where ZC = 1/iωC is the impedancecapacitance C. Since R is comparable with ZC , thereare two components of the electric current in theplasma: the conduction current and the bias current.The value of R1 due to the current of thermoionicemission can not be calculated, but judging by thestrong heating of the electrodes, R1 < ZC1 and thecurrent in the region of DEL is the conduction current,which in our case is 5A. The role of the bias currentis reduced to the transfer of energy through the dis-charge gap without absorption. Thus, the major partof the energy is released in the near-electrode region,where the deposition of the target product takes placeand energy consumption is less as compared with thearc discharge.

3. ExperimentalThis type of discharge was used to study the processesof hydrogen reduction of fluorides of silicon, boronand molybdenum and synthesis of their carbides at atotal reactor pressure of 240 and 760Torr. Hydrogenwas used as the plasma-forming gas. The molar ratioof H2/SiF4/CH4 and H2/BF3/CH4 was maintained at9/1.5/1 and the molar ratio of H2/MoF6/CH4 equalto 45/1.5/1. The plasma-chemical reactor was a silicaglass tube, inside which the rod electrodes were placedequipped with gas flow generators and connected toRF generator (Fig. 1c). During the synthesis of SiCand B4C, deposition was carried out at the ends of theelectrodes in contact with the plasma (Fig. 1d), andin the synthesis of Mo3C2, its finely dispersed fractionprecipitated on a porous nickel filter. The processof conversion of the fluorides of silicon, boron andmolybdenum into their carbides and other possible by-products of the reaction was studied experimentally.The total conversion of fluorides and the presenceof gas phase reaction products were determined byIR spectroscopy. The samples were analyzed by X-ray phase analysis, electron microscopy, laser massspectroscopy, and laser diffraction.

4. Results and DiscussionDuring the research, in the synthesis of silicon carbide,in gas-phase products at the outlet of the plasma chem-ical reactor, SiHF3, CF4, C2H4, C2H2 and CO2 weredetected. In the synthesis of boron carbide, C2H4,

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Figure 2. X-ray phase analysis of the samples of siliconcarbide (a),boron carbide (b) and molybdenum carbide(c).

C2H2 and CO2 were also found in the gas phase. Inaddition, an oily liquid condensed on the walls of theplasma chemical reactor, which represented the com-plex [CH3BF]−H+. It should be noted that both dur-ing the synthesis of silicon carbide and the synthesisof boron carbide, the hydrogen fluoride (HF) was notobserved in gas phase products. The intensive growthof silicon carbide and boron carbide was observed onelectrodes in the form of cylindrical polycrystal, andfinely dispersed soot precipitates on the walls of theplasma chemical reactor. According to X-ray phaseanalysis (Fig. 2a,b), samples of silicon carbides andboron contain up to 40% of carbon. In the processof synthesis of molybdenum carbide in the exhaustgas mixture, due to small concentration of methanein the initial mixture entering the plasma chemicalreactor, the ethylene and acetylene were not observed.The formation of solid-phase products, which includemolybdenum carbide, molybdenum and MoF3, is ob-served in the volume of the plasma chemical reactor inthe form of intense dust formation (Fig. 2c). On thebasis of the experimental data obtained, it is possibleto assume the basic chemical reactions responsiblefor the formation of carbides of silicon, boron andmolybdenum:

SiF4 + 2CH4 → SiC + CF4 + 4H2 (2)

4BF3 + 4CH4 → B4C + 3CF4 + 8H2 (3)

6MoF6 + 13CH4 → 2Mo3C2 + 9CF4 + 26H2 (4)

According to morphological studies, Mo3C2 is formedas a nano-powder with a particle size of 50–70 nm

0.5 1 1.5 2 keV

SiC

O

C

0.2 0.4 0.6 0.8 1 keV

OB

0.8 1.6 2.4 3.2 keV

Mo

W

CD

F

E

A

B

C

1000 nm

500 nm

1000 nm

Figure 3. A - Morphology of samples Mo3C2, B - B4C,C - SiC, and their element composition (D), (E) and(F), respectively.

(Fig. 3a). Samples of boron carbide and silicon havea layered structure (Fig. 3b,c). It is possible to saywith a high degree of probability that the growth ofB4C and SiC occurs layer by layer. A prerequisite forgrowth is the initial deposition of a graphite layer onthe electrode surface. Graphite precipitates on thesurface of the electrode heated up to 3000–3500K asa result of pyrolysis of methane, forming dendrides.The electric field strength and its gradient on thegenerated dendrides are high, which allows the SiF4or BF3 molecule to be polarized, and the Si or B layeris deposited. Then, graphite precipitation is requiredagain. The samples grown under these conditionsshould contain a significant amount of free carbon,which is confirmed by X-ray analysis data.

5. ConclusionsAs a result of these studies, the main gas-phase andsolid-phase products in the synthesis of the carbides ofsilicon, boron and molybdenum were determined. Themain chemical reactions responsible for the formationof target products – carbides – are proposed. Theassumptions are proposed for the mechanism of growthof polycrystalline carbides of silicon and boron onthe surface of the electrodes. Samples of carbides ofsilicon, boron and molybdenum were prepared and

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their phase, elemental and impurity composition werestudied.

AcknowledgementsThis work was financially supported by Russian ScientificFoundation Grant 17-13-01027.

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