Improvements in the manufacture or Freparation of Crysta Silicon.
Year: 1899

page_text: Date of Application, 15th Sept., 1899
Complete Specification Left, 14th June, 1900-Accepted, 28th July, 1900
Improvements in the manufacture or Freparation of Crysta
Silicon.
5+
1, BERNHARD SCHEID, Ph. D., of Brockenheim, Frankfort-on-Main, in the
German Empire, do hereby declare the nature of this invention to be as follows:-
Contrary to the assertion of Moissan (compare H. Moissan, Der elektrische
Ofen" pp. 327 & 329) that on heating silica with carbon at the temperature of
the electric arc, carborundum (SiC) is obtained as the most stable product, accord-
ing to the equation
SIO,+C
=
SiC + 200,
I have ascertained that by altering the proportions of the mixture, metallic
silicon may be produced in accordance with the equation
Si0, + 20
=
Si + 200
If, for example, 60 grams of silica are heated with 24 grams of carbon while
submitted to an electric current of about 24 volts and about 500-600 ampères
under favourable conditions as regards strength of current, and precautions
against volatilisation of the silica produced, metallic silicon can be obtained in
considerable amount. This is very pure, if purified materials have been employed,
and contains only a small quantity of carbon, or graphite. The method, however,
is neither economical, nor remunerative, the application of charcoal on a large
scale introducing considerable difficulties. At the temperature of the electric
arc, the components of the mixture become separated, the carbon burning partly
away, or being shot out when more of the mixture is added, so that in the
neiglibourhood of the are the mixture does not preserve the composition indicated
by the above equation.
-
i
I have completely overcome the disadvantages of this method by a newly devised
process whereby not only is a higher potential maintained, but separation of the
components is avoided, so that on interrupting an operation, the mixture remain-
ing in the electric furnace can be used on the next occasion without the necessity
for augmenting either ingredient.
I use a mixture with carborundum and by observing certain precautions I
obtain metallic silicon from it according to the equation
2SIC + SiO,
==
3Si + 200
If, for example, 80 parts (by weight) of carborundum be heated with 60 parts
of pure silica, while submitted to an electric current of about 38-45 volts and
about 500-600 ampères, at the temperature of the electric arc metallic silicon
is obtained in lumps, or nodules; which when freed from mechanical impurities,
and a small quantity of carbon, or graphite, consist of pure metallic silicon,
provided that the materials employed were free from iron.

page_text: By this process 20-25 grams of metallic silicon per 1 kilo-watt-hour of electric
energy can be obtained. The employment of silicates reduces the strength of
the current.
The conditions of the above example are not unalterable, as a somewhat lower,
or higher, potential can be used, also a much higher current can be used without
seriously impairing the yield, always provided that suitable precautions such as
proper arrangement of electrodes, and construction of the furnace are adopted
to prevent loss of silicon by volatilization. In this connection the current density
is an important factor, and is best maintained at about one ampère per square
millimetre section of the electrodes. If however carbons of large cross-section
be employed, a much smaller current density must be maintained than when
smaller carbons are being used, in which case a current of about 2 ampères per
square millimetre cross section can be applied. It is of course immaterial whether
an alternating or a continuous current be employed and the operation is equally
successful whether the source of heat be the arc, or be produced on resistance
principles.
In the latter case the mixture of carborundum and silica is exposed to heat in
contact with an electrically heated resistance; for instance, a carbon resistance
of rectangular form can be arranged concentrically about the mixture.
For the attainment of the high temperature necessary to the reaction, some
suitable source of heat other than electricity can be used.
In the place of pure silica, silicates of the alkalis, alkaline earths, and the
earths, either alone or in combination with silica can be used according to the
equation
4SiC + Na2Si,0,- Na,Si0, +4CO + 6Si
The production of the silicates of the alkalis alkaline earths and the earths,
may itself be allowed to take place in the electric furnace, by adding the quantities
necessary for the formation of acid, or neutral, silicates for example, 43 parts
(by weight) of calcined sodium carbonate and 75 parts of silica for the pro-
duction of an acid silicate of the formula
Na₂Si₂O,
I have found it most advantageous, in using pure silica, to mix with it an
acid water glass, of the formula Na,Si,O,, for instance thus avoiding the formation
of lumps which occurs when silica is employed alone. If, for example, instead
of using 100 parts of silica, only 80 parts be employed, along with the quantity
of a water-glass of the above formula which corresponds to 20 grams of silica
(viz.about 40 parts) the formation of a sodium bi-silicate is thus rendered
possible and the production of lumps is avoided. Other mixtures of silica
and silicates can also be used with good results and the above description is
merely an example and I do not limit myself thereto. If silicates alone be
used, without the addition of silica, the evaporation of the alkalis, alkaline earths,
or their oxides, exerts a deteriorating influence on the reaction.
The form of the crystallised metallic silicon obtained by this process has been
hitherto unknown to science and technology, consisting of compact nodules, or
lumps which, when the oven employed is of large dimensions can be obtained
in the form of cast amorphous pigs, whereas, hitherto, the substance has always
been placed on the market as a powder or in small crystals.
Dotad the 15th day of
1900
JOHNSONS & WILLCOX,
47, Lincoln's Inn Fields, London, W.C., Agents.

page_text: The production of the silicates of the alkalis, alkaline earths or carths
may itself be allowed to take place in the electric furnace, by adding the quantities
necessary for the formation of acid, neutral or basic silicates, for example 43 parts
(by weight) of calcined sodium carbonate and 75 parts of silica for the pro-
duction of an acid silicate of the formula
Na Si₂O,
I have found it most advantageous in using pure silica, to mix with it an
acid water glass, of the formula Na Si,O, for instance thus avoiding the formation
of lumps which occurs when silica is employed alone. If for example instead
of using the full number of parts of silica only four-fifths of those parts be employed
along with the quantity of a water glass of the above formula which corresponds
to the remaining one-fifth of silica (ciz:-about 40 parts to each 80 parts of silica)
the formation of a sodium bisilicate is thus rendered possible and the production
of lumps is avoided. Other mixtures of silica and silicates can also be used with
good results and the above description is merely an example and I do not limit
myself thereto. If silicates alone be used without the addition of silica the evapora-
tion of the alkalis, alkaline earths or their oxides, exerts a deteriorating influence
on the reaction.
The form of the crystalline silicon obtained by this process, has been
hitherto unknown to science and technology, consisting of compact nodules or
lumps which when the oven employed is of large dimensions can be obtained
in the form of cast amorphous pigs, whereas, hitherto, the substance has always
been placed on the market as a powder or in small crystals.
Having now particularly described and ascertained the nature of the said
invention and in what manner the same is to be performed I declare that what
I claim is:-
1. The manufacture or preparation of crystalline silicon by subjecting a
mixture of silicon carbide (carborundum) and silica to a high temperature sub-
stantially as hereinbefore described.
2. The manufacture or preparation of crystalline silicon by subjecting a
mixture of carborundum with an acid, neutral or basic silicate of the alkalis,
alkaline earths, or earths, to a high temperature substantially. as hereinbefore
described.
3. The manufacture or preparation of crystalline silicon by subjecting car-

page_text: borundum together with a mixture of silica and an acid, neutral or basic silicate
of the alkalis, alkaline earths, or earths, to a high temperature substantially as
hereinbefore described.
4. The manufacture or preparation of crystalline silicon by subjecting a mix-
ture of carbon and silica to a high temperature the proportion of carbon to silica
being less than is necessary for the production of carborundum substantially as
herein before described.
"
5. The manufacture or preparation of crystalline silicon by subjecting a mixture
of carbon and an acid, neutral or basic silicate of the alkalis, alkaline earths,
or earths to a high temperature the proportion of carbon to the silicate being
less than is necessary for the production of carborundum substantially as berein-
before described.
6. The manufacture or preparation of crystalline silicon by subjecting carbon
together with a mixture of silica and an acid, neutral or basic silicate of the
alkalis, or alkaline earths, or earths to a high temperature the proportion of
carbon to the silica and silicate being less than is necessary for the production
of carborundum substantially as hereinbefore described.
Dated this 14th day of June 1900.
JOHNSONS & WILLCOX,
47, Lincoln's Inn Fields, London, W.C., Agents.