Process for the protection of carbon bodies

A process for protecting carbon bodies against attack by molten silicon by applying a separating protective layer is provided which is characterized in that a separating layer is applied to the surface of the carbon body to be protected in such a manner as to wet it. The separating layer consists of alkaline earth metal fluorides in admixture with 0 to 90 mole % of alkaline earth metal silicates having a melting point lower than that of silicon.

The present invention relates to a process for protecting carbon bodies 
against attack by molten silicon by applying a separating protective 
layer. 
Carbon mold members, and especially graphite mold members, are widely used 
in the art for high temperature processes because of their excellent heat 
durability. While graphite crucibles may be used, for example, for 
crucible pulling germanium according to the Czochralski technique, this is 
not possible in the crucible pulling of silicon. This is because at the 
high temperatures occurring in this process, the silicon melt attacks the 
graphite crucible to form silicon carbide. When manufacturing high-purity 
silicon for electrical components, direct contact between the silicon and 
the carbon should be avoided, because the carbon becomes concentrated in 
the silicon and this would lead to leakage currents in finished electrical 
components made from this type of material. For these reasons, when 
crucible pulling silicon, crucibles of quartz or quartz glass are used, 
which are considerably more expensive; but although they too can as a rule 
be used only once, they fracture only when the residual silicon in the 
crucible solidifies and not during the pulling process as is the case when 
using carbon or graphite crucibles. 
A process for the production of shaped silicon articles, in which silicon 
is melted down in a carbon body, is known from DE-OS No. 20 09 459. In 
this process, the hollow mold of carbon must be cooled. Furthermore, the 
surface coming into contact with the silicon must be covered with a mold 
member of sintered quartz wool and/or sintered quartz sand. The silicon 
itself is melted in the mold by means of inductive heating and 
subsequently cooled by lowering it out of the inductively heated area. 
However, here too, problems can arise in the case of larger shaped silicon 
articles. In particular, since the melted silicon must remain in the 
prepared melting mold for a long time until all the silicon has melted, 
the protective layer of sintered quartz wool and/or quartz sand is 
destroyed. Moreover, the manufacture of a liquid-cooled mold is rather 
expensive. Protecting such hollow molds of graphite, the surface of which 
is coated with sintered quartz wool, sintered quartz sand or sintered 
silicon nitride, is at best only satisfactory when liquid silicon is not 
melted in the mold but is simply poured into it and solidified immediately 
by corresponding cooling of the mold (see in this connection, for example, 
DE-PS No. 22 44 211). 
It is also known from U.S. Pat. No. 2,992,127 to render graphite crucibles 
resistant to oxidation processes, and especially to attack by molten 
silicon, by covering the surface of the graphite body with pulverulent 
silicon, which, when the graphite body is heated to above 1360.degree. C. 
in an inert gas atmosphere, reacts on the surface to form a cohesive layer 
of silicon carbide. In accordance with this process, there can be a second 
step in which, in addition, silicon nitride is also applied in pulverulent 
form and sintered together with the mold member at elevated temperature. 
The disadvantage of this process, however, is that curved carbon mold 
members cannot be covered with a uniformly thick layer of silicon carbide 
or silicon nitride. Another disadvantage is that these sintering processes 
produce only a surface layer having a very poor pore density. With time, 
the molten silicon penetrates into these pores and attacks the graphite 
body underneath and decomposes it. Moreover, carbon can conversely escape 
from the graphite body through pores in the covering layer and penetrate 
into the silicon melt as an undesirable impurity. Thus, it will also 
penetrate at least partially into the silicon rod that is pulled from this 
melt. 
Finally, it is known from U.S. Pat. No. 3,871,872 to protect graphite 
stirrers that are immersed in liquid metal baths, by coating them with 
alkaline earth metal alumosilicates. However, if, for example, mold 
members of carbon (which are not claimed in the U.S. patent) were to be 
coated with such a layer of alkaline earth metal alumosilicate, the shaped 
silicon article obtained by casting could not easily be released from the 
mold member after solidifying below its melting point, because the 
alkaline earth metal alumosilicates mentioned in this U.S. patent have a 
melting point above that of silicon. In this case, it would be possible to 
prevent contact between the molten silicon and the carbon, but not, 
however, the destruction of the crucible on cooling and separating the 
mold and the molded article. 
The problem underlying the invention was therefore to provide a protective 
layer for carbon bodies, which protects the surface against attack by 
molten silicon and makes it possible to remove solidified silicon from 
carbon mold members that are coated in this manner without the mold being 
destroyed, thus allowing the mold to be used several times. 
This problem is solved in accordance with the present invention in that a 
separating layer consisting of alkaline earth metal fluorides in admixture 
with 0 to 90 mole % of alkaline earth metal silicates having a melting 
point lower than that of silicon is applied to the surface of the carbon 
body to be protected in such a manner as to wet it. In accordance with the 
invention, the fluorides and, if applicable, the silicates of the alkaline 
earth metals magnesium, calcium, strontium and barium are used, wherein 
calcium and magnesium compounds are preferred solely because of their 
lower cost. The alkaline earth metal fluorides can be used individually, 
in admixture with other alkaline earth metal fluorides, or in admixture 
with alkaline earth metal silicates in the quantities given above. 
Mixtures of calcium fluoride and magnesium fluoride, in which the melting 
point can be reduced to 950.degree. C. (when there is 45 mole % of 
magnesium fluoride in the mixture), by altering the mixture proportions, 
are, for example, suitable. Also suitable are mixtures of magnesium 
fluoride with calcium silicate, or of magnesium fluoride with magnesium 
silicate, mixtures of calcium fluoride with magnesium silicate, and, 
preferably, mixtures of calcium fluoride with calcium silicate, especially 
because calcium fluoride has a considerably lower vapor pressure than does 
magnesium fluoride. 
Advantageously, a separating layer of the mentioned melts is applied to the 
carbon body to be protected in a thickness of 0.01 to 2 mm, preferably 0.1 
to 0.5 mm. In order to be able to apply a layer of this thickness, the 
carbon parts must be wetted by the protective melt. This wetting can be 
achieved in the case of mixed fluoride/silicate melts, for example, by an 
excess of silica (SiO.sub.2) of approximately 10% or more, calculated on 
the stoichiometric composition of the alkaline earth metal silicate 
(MO.SiO.sub.2, M.dbd.alkaline earth metal atom), the upper limit being 
determined by the fact that the addition of silica produces a layer with a 
higher melting point and, on the other hand, the separating layer should 
by definition still be liquid at the melting point of silicon. A further 
method of achieving the wetting of the separating layer and the carbon 
body, is by heating the mixture prepared for the separating layer 
comprising alkaline earth metal fluorides and alkaline earth metal 
silicates to a temperature above 1600.degree. C. when it is applied to the 
carbon bodies. 
The third and preferably used method is by adding elemental silicon to the 
protective melt in quantities of above 2% by weight, preferably 
approximately 2 to 5% by weight. This method may be used both in pure 
fluoride melts and in mixed fluoride/silicate melts. 
The carbon bodies may be coated by various methods, for example, by 
applying and melting pulverulent layers, wherein the individual 
constituents of the mixture to be used for the layer are mixed together in 
solid pulverulent form and scattered as uniformly as possible over the 
surface of the carbon body to be protected. A further possibility is to 
melt the mixture provided for the protective layer and to spray this onto 
the surface of the parts of the carbon mold member to be protected. 
Finally, a further technique which is preferably used, is to immerse the 
carbon mold members to be protected in the protective melt provided for 
the separating layer, or if the carbon mold members are to be protected 
inside, to fill them with protective melt and subsequently pour this out 
again. The carbon mold members to be protected are advantageously left in 
the melt for several minutes, approximately 5 to 15 minutes usually being 
sufficient, before removing them from the melt or, in the case of curved 
carbon mold members that are to be protected on the inside, before the 
melt is poured out again. An additional factor that must be taken into 
account here is whether the carbon mold members are cold or hot when they 
are immersed in, or filled with, the melt. The application time is 
required in order to allow the protective melt to attach itself to the 
surface of the carbon mold member to be protected; a uniform wetting of 
the protective layer and the carbon again being absolutely necessary. 
After the surplus protective melt has been drained off, a thin layer 
remains adhering to the carbon articles. The thickness of the layer is 
determined by the viscosity of the protective melt and this, in turn, can 
be regulated by the composition and temperature. For example, the 
viscosity of the mixed melt of calcium fluoride/calcium silicate may be 
reduced by increasing the proportion of calcium fluoride. For example, 
mixtures of calcium fluoride with calcium silicate containing 40 to 70 
mole % of calcium silicate are ideally suitable. 
Carbon bodies that may be protected in this manner are, for example, 
melting crucibles, casting molds, displacers, pipes, overflow channels, 
gas puddling lances, strirring and ladling tools, resistance heaters and 
slit-shaped nozzles for band-drawing. 
If a protective layer composition having a very low melting point is 
selected, that is to say, for example, solely fluoride melts or fluoride 
rich silicate melts, the layer remains as a liquid separating layer 
between the silicon crystal and the carbon body for a long time during the 
cooling process after the silicon has solidified. The correlation between 
the melting point and the proportion of the components in the mixture may 
be taken from the phase diagrams (see, for example, J. Mukerji "Phase 
Equilibrium Diagram", Journal of the American Ceramic Soc. 48, 210 (1965). 
Protective layers having a low melting point are accordingly, for example, 
mixtures of calcium silicate with approximately 40 mole % of calcium 
fluoride, the melting point of which is about 300.degree. C. below that of 
silicon. The melting point of, for example, a mixture of calcium fluoride 
with 45 mole % of magnesium fluoride is even lower (950.degree. C.). 
The liquid protective layer prevents the solidifying silicon from adhering 
to the carbon bodies, allows displacement and thus reduces mechanical 
stresses. Furthermore, this makes it possible to separate the solidified 
silicon from the carbon mold components without difficulty. Low-melting 
liquid separating layers of this type may be used, for example, in the 
crystallization of silicon in conical carbon crucibles. In this case, the 
crucible is no longer fractured by the expansion of the silicon as it 
solidifies. Also, the coarse-grained crystallization of silicon can be 
achieved in this manner, since the liquid separating layer prevents 
induced nucleation on the solid carbon wall and the crystallization can be 
initiated as slowly as desired without risk of the silicon reacting with 
the crucible. Also graphite casting molds, such as those used for the 
manufacture of polycrystalline base material for solar cells, may be 
advantageously used with a liquid separating layer of this type, as the 
silicon plates cast in them have a coarser crystalline structure that is 
favorable for the efficiency of the future solar cells. Moreover, after 
solidifying, while the separating layer is still liquid, the silicon 
plates can be removed from the casting mold so that the mold remains 
intact and can be prepared for a second casting operation. For the 
manufacture of more complicated silicon shaped articles, multi-component 
graphite molds may be used with the liquid separating layer according to 
the invention, which molds can be removed intact from the shaped silicon 
body after the silicon has solidified but before the liquid separating 
layer has hardened. 
The invention makes it possible to use carbon bodies protected in this 
manner several times, it being advantageous to renew the protective 
separating layer from time to time.

In the following, a number of examples are given which serve to illustrate 
the invention in further detail, but which are not given by way of 
limitation. 
EXAMPLE 1 
A separating layer according to the invention was applied to a graphite 
crucible externally shaped as a cylinder having a diameter of 90 mm and a 
height of 90 mm and provided with a bore having a semispherical base with 
a diameter of 70 mm, and tapering outwards slightly towards the top at an 
angle of approximately 3.degree., with a total depth of 75 mm. For this 
purpose, it was filled with a protective melt consisting of 30% by weight 
of calcium fluoride and approximately 70% by weight of magnesium silicate 
(30% by weight of CaF.sub.2 +35% by weight of MgO+35% by weight of 
SiO.sub.2) and 3% by weight of silicon powder, and left for 10 minutes at 
a temperature of approximately 1400.degree. C. After subsequently pouring 
out the protective melt, a separating layer approximately 0.1 mm thick 
remined on the surface to be protected. 
400 g of silicon were then melted by high-frequency heating in the crucible 
that had been coated in this manner and, by lowering the crucible out of 
the high frequency coil, crystallization proceeding from the bottom to the 
top was effected in the solidifying silicon. The silicon crystal 
solidified in a controlled manner was removed from the crucible at a 
temperature of approximately 1250.degree. C. by simply turning over the 
crucible. The separating layer was still liquid at this temperature and 
the graphite crucible remained undamaged and could be used without 
after-treatment for a fresh melting process of silicon with subsequent 
solidification. 
EXAMPLE 2 
A cubic two-piece casting mold of graphite with dimensions of 
100.times.100.times.100 mm and a wall thickness of 40 mm, divided in the 
middle and having a filling opening in one of the cube faces, was immersed 
for 10 minutes at approximately 1400.degree. C. in a melt consisting of 
40% by weight of calcium fluoride and approximately 60% by weight of 
calcium silicate (40% by weight of CaF.sub.2 +30% by weight of CaO+30% by 
weight of SiO.sub.2) to which 3% of silicon powder had been added in order 
to wet the graphite. After the casting mold has been taken out of the 
melt, a separating layer of approximately 0.1 mm remained on each face of 
the cube. 1500 g of silicon were melted in a graphite crucible that had 
been protected with the same separating layer by immersing in the same 
melt, and were then poured into the coated casting mold. The solidified 
cast silicon block was removed from the casting mold at a temperature of 
approximately 1200.degree. C. by simply separating the two parts of the 
mold. 
By coating on all sides, which was achieved by immersing the casting mold 
and the melting crucible in the calcium fluoride/calcium silicate melt, it 
was possible to operate in an oxidizing atmosphere (air) without an 
undesired burning off of the graphite. 
EXAMPLE 3 
Exactly the same procedure was carried out as in Example 2, except that a 
melt bath consisting of 55% by weight of calcium fluoride and 45% by 
weight of magnesium fluoride was used. The solidified cast silicon block 
was removed from the casting mold by separting the two parts of the mold 
at a temperature of approximately 1050.degree. C., that is to say, at a 
temperature at which the separating layer was still liquid. In this case 
too, the casting mold and the melting crucible remained undamaged and 
could be reused without after-treatment. 
Thus, while only several embodiments of the present invention have been 
described, it will be obvious to those persons of ordinary skill in the 
art, that many changes and modifications may be made thereunto, without 
departing from the spirit and scope of the invention.