chemical physics of discharges - Argonne National Laboratory

156
The Dissociation of Metal Halides in Electrical Discharges
F o KO MCTaggart
Mdsion of Mineral Chemistry, C.S.I.R.0. Box 124, Port
Melbourne, Victoria, Australia.
I
Introduction.
in electrical discharges in dissociating gae molecules is well known,
and is the basis of the chemical reactions that a H observed in plasnrs
systems. Since gases such a8 02, H2, B2dC1 9 88 well a8 vapours of
organic and other compounds have been diesociazed to atoms ad/m
radicals, it appears Logical to suppose that many other molecules would
behave similarly. This paper describes the diesociation of the halides
of the Croup I, I1 and 111, and rare earth metals.
&m-imental. Ideally, the halides should be introduced Into
the discharge tube in the form of vapoure at suitable pressures. Due
to the wide range of volatilities occurring in the above compounds and
to other erperimental difficulties it proved more convenient to we
carrier gmee, and to volatilize the halides from small boats contained
wlthin the reaction tube by heating these boats to suitable temperatures.
It was usually possible to make use of the heat generated in the plasma
itself for this purpose. Samples were placed upstream from,or within
the central plasma region,ln positione where heat sufficient to give the
desired rate of volatilization was produced.
The effect of the energetic electrons produced
Since heating of solib
due to atomic recombination ie largeu a surface effect it was also
possible to verg the sublimation rate of samples by changing the Lmrfece
area exposed to the plasma.
Both inert gases (in particular He and H ) and hydrogen have been
used and certain reactions which are discussed befow appear to involve E
atoms aa well a dissociative effects. These gases were of coomerciel
purity, the He and 5 being purified by passing them over zirconim ponder
heated to 800°C; although no essential differences between the cylinder
gases and the purified gases were observed. Pressures of the carrier
gaaes were varied between 0.5 ad 2.3 mm. Halides wed were of A.B. grade,
but in certain cases impure compounds were deliberately used in order to
determine the effects of impurities on the dissociation and on the purity
of the metal obtained.
Ertenslve use was made of a 2450 mC magnetron generator (Mullard
m2/205A) which although capable of an output in exce88 of 2 M continuow
wave power, was seldom operated at inputs higher than 600 watts. B&P
frequency energy from the magnetron was fed via a 50 ohm air dielectric
coaxial line, to a section of waveguide operated ea a resonant cavity by
means of a sliding plunger inserted into the open end. The discharge
tube passed tranevereely through the guide at a point of m a r i m electrostatic
field. h e investigatlons, Involving the chlorides of Li, Be,
and a, which dissociate very readily, were made using au 9-f" generator
consintlng of a 4-125A vacuum tube at a frequency of 30 mC, the power of
which could be varied from close to zero to about 400 watts. In these