Nitrogen trifluoride (NF3): Production and purificationT. Fiodorova, S. Igoumnov, V. Kornilov 1. Introduction NF3 is a toxic gas, inert under normal conditions , odorless in the absence of admixtures. At an elevated temperature it is a strong oxidizer comparable with fluorine. General constants:
Due to easy handling at room temperature, NF3 has advantages compared with fluorine and is used as a stable fluorinating agent and has a wide application in high-energy chemical lasers, at dry etching in production of semiconductors, as a filling gas in lamps to prolong their durability and increase brightness, as a detergent gas in CVD apparatuses ( chemical gas-phase precipitation) (3,4,77). 2. Methods of NF3 production For the first time, NF3 was produced by O.Ruff et al. in 1928 by electrolysis of melt anhydrous NH4* HF at 125oC, voltage of 7-9 V, current of 10 A in a copper electrolytic cell on a carbon anode (5). This method was assumed as a basis for modern technologies to produce NF3, the second industrial method is direct fluorination of ammonia. Paper (6) presents a review based on publications right up to 1984 including data on methods of NF3 production, purification, reactions with elements, organic and inorganic compounds and also its molecular properties, spectra, thermodynamic constants etc. That is why the present review considers only publications from 1983 y. 2.1. Electrolytic methods to produce NF3 General data (6) Electrolysis of NH4F*(1,1-1,8)HF melt is a main method to produce NF3 on an industrial scale. Carbon materials and nickel are used as anode materials each of them has own merits and demerits. Carbon materials contrary to nickel are not liable to anodic dissolution, but during their use there are observed anode polarization ( anodic effect), anodic corrosion and contamination of the goal product with carbon tetrafluoride. The process is described by the following equation: NH4F*2HF or 2NH4F*HF When carbon-containing materials of the anode are used, N2, N2F2, N2F4, NHF2, nitrogen oxides, oxygen and CF4 are formed together with NF3 and H2. The optimal process parameters are as follows: Temperature 100-120oC Current density 0.12-0.15 A/cm2 NF3 current efficiency ~70%. The electrolysis process is dangerously explosive due to the formation of a mixture of NF3 with hydrogen and other components of electrolysis gas in the cell. To decrease the danger , the anodic and cathode spaces are separated with a diaphragm or the NF3 concentration is kept beyond the bounds of explosiveness by nitrogen introduction into the cell. Improvement of the electrolysis process Within the period under review, a number of problems can be chosen which attracted intensive efforts of Japanese companies, particularly of Mitsui Chem. Inc.
This problem can be solved by the following measures: a) electrolytic cell modernization including
-with the resistance to bending of over 1000kg/cm2 (21); -with the resistance to bending of over 50 MPa, current density of 4-30 mA/dm2 and the ratio of HF/NF4F in the electrolyte of 1-3 (22). The anode is made of porous carbon; -with the resistance to bending of over 50 MPa, current density of 50 mA/cm2 max, potential between the cathode and neighboring anode of 6V max, quantity of electricity per 1 cm2 of the anode surface of over 500 coulomb (23); -with the resistance to bending of over 50MPa, porous anode containing fluorides of Li, Na, Al, Mg, Ca, Ni. The process is carried out at a current density of 4-30 mA/dm2 and a ratio of HF/NH4F equal to 1-3 (24). To increase the NF3 yield it was proposed: -To increase the purity of NF3 , it is necessary to use the starting material of high purity produced from gaseous NF3 of over 99.5 wt. % purity and HF of over 99.8 wt.% purity, nickel electrodes should have purity of 98.5 % min (29,30) or 99.5% min (31). -The starting NH4F contains 50 ppm of carbon maximum (32) 2.2. Investigation of possibility to produce NF3 using binary systems of electrolyte. German researchers made attempts to produce NF3 in a higher yields using compounds with a higher valence as an additional nitrogen-containing component , for example hydrazine difluoride in NH4F*HF melt (33). This attempt was not crowned with success since the following reactions take place on the anode in case of hydrazine use: 3N2H4 N2H4 Another direction of the investigation was possibility to use additives to NH4F*HF system with the purpose of simultaneous production of NF3 in a higher yield and a different process product. Sulfamoil fluoride H2NSO2F was taken as the additive (34,35). H2NSO2F*NH4F8*HF system behaved as a binary electrolyte and they failed to increase the current yield. Simultaneously it was possible to obtain NF3 from NH4+ and SO2F2 from H2NSO2F, at the same time increase in the H2NSO2F quantity resulted in an increase in SO2F2 yield but not NF3. Similar behavior of NH4F*HF systems with additives of NH4N(SO2F)2 and (NH2)2SO2 was observed (36). 2.3. Doshisha University (Kyoto) investigations. In the middle 80-s experts of Doshisha University continued a work started in the 60-s by N.Watanabe et al. devoted to the reactions taking place during electrolysis of salt melts with use of carbon-containing anodes and platinum reference electrodes. The purpose of the work was to determine the optimal conditions of NF3 formation. Further study of electrolysis of KH2F2- RNH2 systems, where CH3CONH2 and HCONH2 were used as RNH2, on an amorphous carbon electrode at 120oC has shown that the anodic effect was observed at a current density of 3-20mA/cm2 and a film of carbon polyfluoride (CxF)n was formed on the anode. 1% LiF addition prevented the anodic effect but reduced the yield of NF3 and the water content in the melt. The maximum NF3 yield of 50.9% was obtained at a current density of 5.3 mA/cm2 and 4 mol% of HCONH2 additive (37,38). The same yield under similar conditions was obtained when a glossy anode was used (39). An addition of urea to a system of KH2F3 was studied earlier (40). An addition of urea to a different system of NH4F-HF-KF gives the same results, i.e. an increase in the NF3 current efficiency due to removal of water which is consumed for urea hydrolysis and COF2 forming in the process. The maximum content of NF3in the anode gas is 87% (41). Later on investigations were carried out using NH4F*2HF melt and a nickel anode. A possibility to improve the process parameters due to introduction of such additives as NH4NiF3,(NH4)3FeF6 and AlF3 to the electrolyte has been tested. If the maximum NF3 yield attained 66% without additives at the anodic dissolution of 3%, then an addition of 0.1mol% of the most effective complex of NH4NiF3 reduced the anodic dissolution to 1%, but the NF3 yield reduced to 60% also (42,43,44). The process with the use of NH4NiF3 has been patented (45). CsF (46,47) and LiF (48,49) were used as additives. The additives of these salts reduce the anodic dissolution and increase the NF3 yield. The processes using CsF and LiF have been patented (47,49). Further studies on the effect of fluorides such as KF,MgF2,NaF, AlF3, CsF and LiF confirmed the effectiveness of CsF and LiF additives and showed that KF reduced the NF3 yield and stimulated the anodic dissolution (50,51). A search for metal workable in NH4F*HF*CsF melt has found that Al, Fe, Mo, Nb, Sn, Ta and Ti are subjected to vigorous dissolution. Ag, Co, Cu, and Zn are also unusable because of passivation preventing from current passage. Nickel was found the most acceptable metal for the anode, and also Monel , one of its alloys (52). Electrodes made of Ni-NiO1-x composite produced by HIP ( hot isotactic pressing) method were also studied. The anode made of 10% composite was covered with a thick layer of oxides with infrequent porouness that prevented the anode from dissolution and increased the NF3 yield. An anode made of 20% composite has these parameters much worse. The anodes operated in NH4F*2HF melt (53). An anode made of a different material, carbon impregnated with lithium fluoride, was studied in systems of NH4F-HF, KF-NH4F-HF, CsF-NH4F-HF. The yield of NF3 on these anodes is lower than that on nickel ones (53). Further study of the anodic reactions on the nickel anode in melt of CsF-NH4F*HF at 50-80oC allowed to conclude that an increasing the potential resulted in change of the anodic reactions and four zones were separated in dependence on the voltage: 0-1V ( compared with H2 standard electrode): nickel anodic dissolution 1-3V :deposition of Ni (II) compounds containing oxides (passivation) 3-5V : Ni(II) oxidation to Ni(III) and/or Ni(IV) over 5V: proper NF3 fluorination The anode gas is composed of NF3 and N2 with a small additive of O2,N2O, N2F2, N2F4 (55). A similar picture is observed in melts of (CH3)4NFm*HF and (CH3)4NFm*HF + 50% CsF*2HF, where m=3,5+0.5n (n=1-4). Here also 4 zones are seen. An oxidized layer is formed at a potential above 4V and is composed of NiF2, fluorides with high oxidation level and a small quantity of oxides of different valence (56). Paper (57) describes a mechanism of fluoride films formation on the anode. This phenomena and possible effect of fluoride layers on the fluorination process have been reviewed in paper (54). 3. Ammonia direct fluorination Ammonia fluorination in NH4F*HF melt is far more practically feasible process than gas-phase fluorination . That allowed Air Products and Chem. to implement this process on an industrial scale and produce the product of 99.5% purity (6). Later on research in this field was directed to
A processing line with circulation of a melt containing ammonia, metal fluoride and HF has been patented. The processing line includes the following equipment through which the melt circulates: an electrolizer to produce fluorine and hydrogen, a reactor for NF3 production and 2 apparatuses- solubilizers located between them, one apparatus at the output of the melt from the electrolyzer and the second one at the input. Gas-carrier together with ammonia and fluorine respectively are charged into these apparatuses to keep the ammonia and HF concentration constant in the melt. Fluorine mixed with the gas-carrier comes to the reactor where it reacts with the melt to form NF3 (63). Paper (64) suggests to carry out interaction of elemental fluorine with ammonia complex NH4F*HF, (NH4)1-4MF2-8*nHF ( where M is an element of I A-V A group, I B-VII B group and VIII group) at mixing of the melt complex with a turbine-type mixer. 4. NF3 purification methods Modern electronic industry makes high demands for NF3 purity right up to 99.999% in which connection a great attention is paid to NF3 purification. Methods for NF3 treatment for removal of acid and hydrolyzable admixtures and difluorodiazine, N2F2, have been described in (6). N2F2 was removed by decomposition over a nickel-containing catalyst at 240oC, for example in a nickel pipe filled with nickel balls or other nickel packing. In this case the content of N2F2is reduced from over 0.1% to less than 0.01%. NF3 catalytic purification from N2F2 with nickel-containing compounds was further developed in the work of Mitsui Toatsu Chem. Inc. . They used heating NF3 raw at 150-600oC in a vessel covered with nickel fluorides. The vessel coating was made by its treatment with fluorine (66, 67, 68, 69, 70, 71). To increase the purification level, the vessel may be filled with fluorides (69-72), for example with fluorides of nickel or calcium (74). To purify NF3 from N2F2 and/or OF2 it is suggested to use other fluorides:
Sorbents are widely used for NF3 purification. A process with their use is carried out in adsorption or chromatographic regime.
Purification with use of natural and synthetic zeolites (81) is also wide spread. Before use, zeolites are preheated in a nitrogen flow at a temperature of 270-700oC (81,85,86). For their reuse, the heating is conducted step-by-step, first at 50-200oC, then at 250-700oC (87,88). To ensure the uniform heating of zeolites, metal perfluorinated disc-shaped plates are inserted in the zeolites (89). A purification temperature with use of zeolites is 125-110oC. The following admixtures are removed by means of zeolites:
To separate NF3 and CF4 a chromatography method is applied using zeolites of 3A type either containing 1-10% of water (84) or preliminarily wetted with water and heated for 16 hours at 300oC in inert medium of chabazite or zeolite of 5A type with the pore surface of 4.4-4.8 A. The charge of the flow to be separated is executed in a pulse regime. Purification of the separated gases reaches 99.99% (95). SiO2 adsorbents, carbon molecular sieves, Al2O3, macro-cross-linked polymers, kieselgur, active carbon, aluminosilicates and zeolites containing also chemosorbents for decomposition of admixtures may be used for sorption storage of gases in a special adsorption-desorption apparatus. A gas to be desorbed is charged directly to a CVD apparatus or a laser plant (107-109). Low-temperature rectification in the presence of a third component (He, Ar, Ne etc.) is used for purification of NF3 from N2F2, CO2, O2,N2 (96-98). A complex of procedures for NF3 purification including sorption purification and rectification in columns of middle and low pressure has been patented (99). A method to purify NF3 from CF4 by azeotropic and extractive low-temperature rectification to the content of admixtures of 10 ppm was described in (111). The following methods of NF3 purification were also described:
Usually the purified NF3 is stored in cryogenic tanks (91,92) including tanks joined to a heat exchanger for NF3 condensation (106). Gases containing NF3 and admixtures are fractionated by gas chromatography method and analyzed by IR-spectroscopy (46), gas chromatography identification of NF3 traces is also known (110) The water content in NF3 less 50 ppm may be determined by laser spectroscopy (112).
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NF3 + 3H2