Patent GB-916316-A (GB)
Title: Vapour-phase purification process for metallic and semi-metallic substances
Inventor: BARRETT JACK WHEELER
IPC: C22B9/02
Filed: 1960-02-15
Abstract: <PICT:0916316/III/1> <PICT:0916316/III/2> Metallic or semi-metallic substances are purified by passing the vapour thereof into a gas chromatographic column at an elevated temperature and collecting the purified product.  The elements Li, Na, K, Ag, Cu, Be, Mg, Ca, Sr, Ba, Zn, Cd, Hg, Al, Ga, In, Tl, Si, Ge, Sn, Pb, As, Sb, Bi, Se, Te, Mg, Pd and Ce are referred to and may subsequent to purification be used as components of semi-conductors, e.g. gallium and indium arsenides, indium and aluminium antimonides, gallium, indium and boron phosphides, cadmium sulphide and selenide, bismuth and mercury tellurides, bismuth antimony telluride, silver antimony telluride and mercury cadmium telluride.  Thus gallium or indium arsenide may be prepared by passing a stream of purified Ga or In vapour into contact with a rod of the corresponding arsenide in an atmosphere of As vapour.  Intermetallic compounds are also referred to.  The process may be used to reduce the proportion of an impurity to a desired level in semi-conductor substances, e.g. to reduce the P content of Si to a level at which it is an N-type semi-conductor or the B content of Si to a level at which is a P-type semi-conductor. Specified temperatures are from 500 DEG -3000 DEG C. and the column may be made of stainless steel, carbon, ZrO2, Re, Ta, W, SiC, Mo, Nb, Ru or Ir; it is preferred, and may be essential, when using high temperatures to operate the column in an oxygen-free atmosphere. Specified solid stationary phases are SiC, Al2O3 and SiO2 aerogel; specified liquid stationary phases are Sn, Fe, Ni, Co, Cr, Hf, Pt, Rh, Th, Ti, V and Zr which may be supported on a packing of a material referred to above as a material of construction or on MgO or SiC: it is preferably in a form of large surface area, e.g. foamed or sintered.  Vapours of liquid stationary phase condensing with the products are removed conventionally.  A and He may be used as carrier gases, and if in the conditions they are inert, N2 and H2 may be used.  Superatmospheric or reduced pressure may be employed.  The following combinations are mentioned: (a) separation of Cd and Hg at 800 DEG C., (b) purification of Zn from Cd at 1000 DEG C., (c) purification of In from unspecified impurities at 1500 DEG C.; in each case using argon and SiC solid stationary phase. Fig. 1 shows an apparatus suitable for use in the invention: the carrier gas is passed in at 2 and after removing air, a sample is placed in chamber 3 between valves 4 and 7.  In an example a 2mg. sample was used.  The column 1 is then heated, as shown by tubular SiC radiant heaters 14, the temperature being indicated by thermocouple 15 (e.g. an Ir-Pt, Re-W or carbon thermocouple; optical pyrometers can also be used). Other suitable methods of heating the column are by means of electrical heating wires wrapped round it, induction heating (e.g. with a carbon column) and resistance heating.  A chamber 13 formed of insulating material, e.g. graphite, MgO, foamed SiC or vermiculite firebrick is provided; alternatively an enveloping reflector, e.g. of Zr can be used.  The chamber is connected at 16 to a supply of argon. Valves 4 and 6 are opened after the column has reached operating temperature and the sample drops into the column and is vaporized. The vapours pass through detector 8 which comprises an earthed container of stainless steel or tantalum having electrodes 9 supported on silicon nitride, boron nitride or alumina plugs 10: one electrode is connected to a 700 volt D.C. potential and the other to the grid of an electrometer amplifier connected to a potentiometric recorder.  If thermal ionization is insufficient, the vapour may be subjected to a stream of alpha particles or electrons.  Other detectors based on changes in dielectric constant or X-ray absorption may be employed.  The vapours then pass through outlet tube 11 having electric heating wiring and lagging to valve 12 by means of which it may be directed to alternative stainless steel condensers.  If desired the valve 12 may be within the chamber 13.  Fig. 2 shows an alternative device to valve 12 for use at higher temperatures.  The column outlet is connected to tube 20 and is directed either to outlet 21 or 22 connected to the condensers by means of adjustable streams of argon which are passed down tubes 23, 24.  If preferred an electrostatic precipitator may be employed to collect the products. It is stated that semi-continuous operation can be obtained by injecting a fresh sample when the preceding sample has passed through the column (e.g. automatically or semi-automatically) and that continuous separation may be achieved by arranging for the liquid or solid stationary phase continuously to move down the column.