Reactions with Halogens
For reactions occurring in a temperature gradient and resulting in the transport of metallic tungsten, see Chapter 2.5, pp. 34/110.
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Reactions with Fluorine
diffraction lines for films prepared at 600 to 650·C is alm ost the same as that for a W2 N powder sample [2]. Amorphous WN films are reported to be formed by CVD using a WF 6-NH 3 gas mixture [3]. deposition tempo in ·C
N content in wt%
X-ray. diffraction results
calculated amount W2 N in wt%
765
2.123
W strong W2 N strong
58
800
2.167
W strong W2 N strong
59
885
0.421
W strong W2 N weak
11
For details on the heat treatment for the decomposition, see the paper [1]. References:
[1] Landingham, R. L.; Austin, J. H. (J. Less-Common Met. 18 [1969] 229/43). [2] Nakajima, T.; Watanabe, K.; Watanabe, N. (J. Electrochem. Soc. 134 [1987] 3175/8). [3] Ikeda, Y. (Jpn. 63002319 [1988] 1/4 from C.A. 109 [1988] No. 46547).
2 Reactions with Halogens For reactions occurring in a temperature gradient and resulting in the transport of metallic tungsten, see Chapter 2.5, pp. 34/110.
2.1
Fluorine
The behavior of a variety of materials (including tungsten) with F2 and HF is reviewed [1]. A list of W fluorides and oxide fluorides reported in the literature has been published [2]. Elemental F2 attacks tungsten severely, even at room temperature [3, p. 60], [4], [5, pp. 220/3], see also [26]. The attack of W by F2 is more intense than that by other halogens [5]. According to gravimetric studies in the range 200 to 500·C, the reaction of solid W with fluorine gas starts at 200·C with W powder and at 300·C with solid tungsten rods [6]. A spontaneous reaction at room temperature did not occur when tungsten powder was exposed to F2 of 800 Torr. Even when a W fuse wire was used for ignition, the powder sample was not completely fluorinated even though the fuse wire was [7]. The re action products of W with F2 are volatile. Since there is no protective layer formed, the reaction at normal pressure is highly exothermic and violent [8, p. 24]. Quantitative measurements of the tungsten loss rate in flowing F2 or F were performed [9 to 12, 24]. The rate of W loss from the surface of hot tungsten filaments in an F2 stream at low pressure was independent of temperature and press ure in the ranges 2000 to 2400 K and (1 to 6) x1 0- 3 Torr. It had a rather high value of -10 16 atoms'cm- 2 's- 1 [9]. Even when taking into account the differences in experimental techniques and conditions, the magnitudes and trends of the fluorGmelin Handbook W Suppl. Vol. A Sb
H. Jehn et al., W Tungsten © Springer-Verlag Berlin Heidelberg 1993
Reactions with Fluorine
13
ination rate data of [9] could not be reconciled with the results of later studies performed in the temperature range from about 700 K to the W sublimation threshold with both molecular and atomic fluorine in an Ar carrier gas at -1 Torr total pressure and a flow velocity of -10 4 cm/s. The atomic F was generated in a microwave discharge. Fig. 6 shows the W atom removal probability E for p(F 2 ) = 3 x1 0- 2 and p(F)=1.6 x1 0- 2 Torr; E is defined as the ratio of the specific rate of W loss (in atoms' cm- 2 • S-l, regardless of the chemical state of aggreg
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