Note: Descriptions are shown in the official language in which they were submitted.
33
BACKGRO[~ND OF THE INVENTION
l. Field of the Invention
This invention relates to a process for the purifi-
cation of a solid material. More particularly,
this process relates to a process for purifying a
solid material such as a metal, a metalloid, or a
metal compound which ha.q been heated to a tempera-
ture approaching the melting point of the material
to be purified while contacting the heated material
with a purifying agent which is substantially non-
reactive with the solifl material.
2. Description of the Prior ArtThere is an increasing demand for high purity ma-
terials such as silicon, titanium, boron, gallium
arsenide, silicon carbide, etc. for diverse appli-
cations such as solar cells, rocket fuel, highpurity alloys, semiconfluctors, and nuclear fuel
applications.
For example, an increasing demand for silicon of
sufficiently high purity to be suitable for use in
the semiconductor and solar cell industries nas
lead to investigation of many processes to achieve
such purity levels. Such processes typically in-
volve some sort of treatment of molten silicon.
Purification of a material such as silicon in a
- . . .
.
~39~33
--3--
molten state is, however, not new. For exa~ple,
Allen u.S~ Patent 1,~37,713 descrihes the purifica-
tion of silicon by treating molten silicon with
metals, such as alkali metals and alkaline earth
metals including magnesium.
Brockbank U.S. Patent 1,180,968 describes melting
silicon under a slag of natural or artificial sili-
ca to eliminate impurities while Pacz U.S~ Patent
1,518,872 describes silicon as a valuable byproduct
of a reaction between aluminum powder and a metal-
lic fluorosilicate, such as magnesium fluorosili-
cate.
Pruvot et al U.S. Patent 3,034,886 describes the
purification of silicon or ferrosilicons by the
injection of silicon fluoride gas into the liquid
bath to react with aluminum and calcium impurities
to form aluminum and calcium fluorides.
The use of molten metal fluorides for purification
of silicon at a temperature of lnOn-1600C has been
proposed by Coursier et al U.S. Patent 3,143,131.
~he patentees, however, propose the use of metal
fluorides which, in the main, either represent
costly materials or materials known to react with
silicon to form silicon fluoride and inject impur-
ities in the silicon that are detrimental to its
electronic properties.
Boulos U.S. Patent 4,379,777 teaches passing pow-
dered silicon through a plasma which apparently
causes migration of the impurities to the surface
of the molten silicon particles. After quenching,
,
~ ~9;~;~3
the particles are acid-leached to remove the sur-
face impurities.
Kapur et al U.S. Patent 4,388,286 combines vacuum
refining of silicon with mixing the silicon with an
effective fluxing agent, such as a fluoride of an
alkali metal or an alkaline earth metal, to form a
molten silicon phase and a slag phase.
One of us has also authored or coauthored papers
which refer to the purification of molten silicon
in contact with NaF in "Silicon Sheet for Solar
Cells", by A. Sanjurjo published in the Journal of
the Electrochemical Society, Volume 128, pp. 2244-
2247 (1981) and "Fluxing Action of NaF on Oxidized
Silicon", by L. Nanis, A. Sanjurjo, and S. Westphal
published in Metallurgical Transactions ~, Volume
12~, pp. 535-573 of the American Society for Metals
and the Metallurgical Society of AIME (1981).
Not all prior silicon purification proces.ses, how-
ever, involve the melting of silicon. Ingle U.S.
Patent No. 4,172,883 discloses a process for puri-
fying metallurgical grade silicon by heating it to
800 to 1350C and contacting it with silicon fluor-
ide gas which is said to react with the impurities
causing them to deposit out. The aforementioned
Coursier et al patent also speaks of purification
temperatures below the melting point of silicon.
It is also known to purify materials such as sili-
con by acid-leaching of the material in powder form
as well as by unidirectional solidification of the
material. In the case of silicon, some of these
~89333
--5--
processes may be less expensive than the conven-
tional method for obtaininq hi~h purity silicon
from chlorosilane reduced - pyrolyzed in H2 to
produce pure polycrystalline silicon which can cost
as much as 70 times the metallurgical ~rade silicon
starting material. ~owever, most of the other
methods proposed either involve high costs or are
of limited value in producing a very high purity
silicon, such as needed for solar applications,
i.e., a purity of 99.999 to 99.9999~0.
In the unidirectional solidification method, such
as æone melting purification or Czochralski crystal
growth, the material is melted and then s]owly
cooled down in such a way that the heat loss and
the solidification occur mostly in one direction.
The chemical potential of an impurity in the solid
material (,ul) i5 higher than in the liquid mater-
ial. As a consequence, the impurity will migrate
toward the area of minimum chemical potential thus
establishing a segregation between the solid and
the molten phase. This segregation, in turn, re-
sults in the purification of the phase in which the
impurity has the more positive (higher) chemical
potential.
In the slagging method, a molten material to be
purified (such as iron) is put in contact with
another molten material (such as CaSiO3) called
"slag". The slag wets the material to be purified,
but is substantially non-reactive to this material.
The chemical potential of an impurity in the mater-
ial to be purified is typically higher than the
corresponding chemical potential of the same
'
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~39~33
--6--
impurity, or its corresponding ion, in the slag.
As a consequence, the impurity will migrate from
the molten material to be purified to the slag,
thus resulting in purification of the material,
e.g., the iron. The degree of purification can be
estimated from the difference in the chemical
potentials.
SUMMARY OF THE INVENTION
In studying the thermodynamics of these purifica-
tion reactions, we have discovered that reactionsbetween impurities in solid material and certain
purifying agents such as molten slags are more fa-
vorable than reactions between the same impurities
in the same material when the material is in molten
form and contacted with the same purifying agent.
Furthermore, we have found that the relative puri-
fication power of such purifying agents for a solid
material, with respect to same material in liquid
form, can be even greater, if the solid material is
crushed into smaller particles.
It is, therefore, an object of the invention to
provide a process for the purification of a mater-
ial in solid form in which a purity of 99.999 or
better may be achieved in some case~s.
It is another object of the invention to provide a
process for the purification of a material in solid
form to a possible purity of 99.999 or better by
heating the solid material to a temperature ap-
proaching the melting point of the material and
contacting the heated solid material with a
~.~89333
purifying agent which is substantially nonreactive
with the material to be purified.
It is yet another object of the invention to pro-
vide a process for the purification of a material
in solid form to a possible purity of 99.999 or
better by heating the solid material to a tempera-
ture approaching the me]ting point of the material
and contactinq the heated material with a purifying
agent which is substantially nonreactive with sili-
con and such that the impurities in the solidmaterial migrate to this purifying agent.
These and other objects of the invention will be
apparent from the description which follows.
In accordance with the invention, a process for
purifying a material in solid form to a purity as
high as 99.9999% comprises contacting the solid
material at a temperature approaching the melting
point of the material with a purifying agent which
is substantia]ly non-reactive with the material to
cause the impurities in the solid material to enter
the purifying agent.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a flow sheet illustrating the process
of the invention.
Figure 2 is a diagram illustrating the thermodynam-
ics of the process in contrast to prior art pro-
cesses.
-
DESCRIPTIOM OF THE PREFERRF.D EM~ODIMEMTS
The process of the invention provides for the puri-
fication of a solid material when heated to just
below the melting point of the material in the
presence of a purifying agent which is non-
reactive with the material to be purified.
~s discussed earlier, the chemical potential of an
impurity in a solid material (~1) is higher than
the chemical potential of the same impurity in the
same material when the material is in a liquid or
molten form. ~ecause of this, the impurity will
migrate toward the area of minimum chemical poten-
tial thus estahlishing a segregation hetween the
solid and the molten phase resulting in the purifi-
cation of the phase in which the impurity has themore positive thigher) chemical potential.
When the material to he purified is in molten form
in contact with a purifying agent, i.e., the prior
art slagging process, the chemical potential of an
2~ impurity in the molten material to be purified is
also typically hiqher than the corresponding chemi-
cal potential of the same impurity in the slag.
The impurity then migrates from the material to be
purified to the slag, thus resulting in purifica-
tion of the molten material~
Because the chemical potential is a function ofstate (its value does not depend on the path by
which the system reaches a determined state), the
thermodynamic principles involved in both of the
prior art purification processes (unidirectional
.
1~89;~33
solidification and slagging) are utilized in the
process of the invention as shown in Figure 2~
As shown in Figure 2, the chemical potential levels
of an impurity are shown for the process of the
invention (labeled as AS for "advanced slagging")
and two prior art purification processes which are
labeled in the figure, respectively, as crystalli-
zation, which equals purification during solidifi-
cation and slagging, which is the purification
during metallurgical slagging of a molten material.
In the conventional unidirectional solidification
(labeled "crystallization" in Figure 2), the im-
purities accumulate into the liquid material driven
by the difference in chemical potential between the
two phases (UIlM(~ uI~M(s))-
Likewise, during the conventional slagqing purifi-
cation process, some impurities accumulate in the
slag driven by the difference in chemical potential
YIFx(slag) ~I,M(l)
In the process of the invention, a solid material
to be purified is put in contact with a purifying
material such as a slag which may be another solid
or a liquid. Some impurities will now accumulate
in the purifying agent (slag) driven by the
difference in chemical potential of the impurity in
the purifying agent and in the material to be
purified (,UIFx(slag)~~I~M(s))-
It will be noted that this last difference is equalto the sum of the two previous ones. Therefore,
,
--10--
the purification power of the process of the inven-
tion will be the sum of the conventional solidifi-
cation plus the conventional slaqqinq processes.
In Fiqure 2, the relative maqnitude of the driving
forces for purification (chemical potentials) of
the conventional prior art processes and the
process of the invention are shown. In the figure,
~I M(s) is the chemical potential of impurity I in
material M in a solid state. ~I M(l) is the
chemical potential of impurity I in material M in a
q state ~IFx(slag) is the chemical potential
of impurity I (in the original material) reacted
with the slag to form a compound IFx.
For example, if one assumes that the purifying
agent is NaF, an impurity such as iron in the
material to be purified will form FeF2 in the NaF.
Under conventional methods, one would show two
separate equilibrium: a) solid versus liquid mater-
ial representing unidirectional solidification: and
b) liquid material versus slag.
The solid material to be purified may comprise any
one of a number of materials, including metals,
metalloids, and metal compounds. Examples of metals
include titanium, vanadium, chromium, manganese,
iron, cobalt, nickel, copper, gallium, sodium, and
zirconium. Examples of metalloids include boron,
germanium, and silicon. Examples of metal corn-
pounds include gallium arsenide, gallium aluminum
arsenide, indium phosphide, copper indium selenide,
boron nitride, silicon nitride, tungsten carbide,
and silicon carbide.
,
33;~
In each case, however, the particular material to
be purifie~ must, in accordance with the invention,
be contacted in solid form, at a temperature ap-
proaching the melting point of the material to be
purified, with a purifying agent in which the im-
purity to be re~oved from the solid material has a
lower chemical potential than the impurity has in
the solid material.
The expression "approaching the melting point of
the material to be purified" is intended to mean
that the temperature at which the purification
process is carried out should be a temperature
within at least about 300C of the melting point of
the solid material to be purified. Preferably, the
temperature should be within about 200, more pre-
ferably within about 150C, and most preferably
within ahout 100C of the melting point of the
material to be purified, but always below the melt-
ing point.
For example when ~silicon, which melts at a tempera-
ture of 1420C, is to be purified, the temperature
of the silicon should be at least ahout 1120C or
higher, preferably 1220C, and more preferably at
least 1270C. ~hile it is theoretically possible
to purify the material at a lower temperature, the
excess time necessary to obtain sufficient migra-
tion of the impurities from the solid material to
the surface and then into the purifying agent would
not be very practical.
The solid material is preferably comminuted into
fine particle form, e.g., to a particle size range
1~39~.~33
of equal to or smaller than 0.1 to 1 millimeter in
at least one direction, i.e., preferably smaller
than the grain size in the solid material, e.g.,
metallurgical grade sllicon, to thereby expose most
of the grain boundaries in which much of the impur-
itie~ in the qolid material will be concentrated.
This increases the surface area of the material to
be purified and reduces the time needed for impuri-
ties in the bulk of the material to diffuse to the
surface where they can react and/or become dis-
solved in the purifying agent. The purification
process may also be enhanced by the use of the
solid material in particulate form due to the ex-
posure of the grain boundaries of adjacent grains
in the polycrystalline material and the tendency of
the impurities to concentrate along the grain
boundaries of adjacent crystals during solidifica-
tion of the material.
Although the use of comminuted particles of the
solid material is preferred, it will be understood
that, alternatively, thin sheets or a ribbon of the
solid material, or even sponge-type high porosity
materials can be used instead of crushed particles
to provide a high surface area exposed to the
molten purifying agent. Whatever form is used, the
solid material should, preferably be in a form
wherein the ratio of surface area to weight is at
least about 1 cm2 per gram or higher.
The purifying agent may comprise a reagent grade of
any material, which is substantially non-reactive
with the material to be purified and, preferably,
having a melting point below the melting point of
.
12~3933~3
-13-
the material to be purified. In one embodiment,
the purifying agent is in molten form. sy the term
"substantially non-reactive" i9 meant less than 1
percent, preferably less than 0.1 percent, more
preferably less than 0.01 percent, and most prefer-
ably less than 0.001 percent of the solid material
will react with the purifying agent at the purifi-
cation temperature used.
To be effective, however, the particular purifying
agent must be further selected to be one to which
the impurity or impurities to be removed from the
solid material will preferably migrate, i.e., the
chemical potential, in the purifying agent, of the
impurity which is to be removed, must be lower
(more negative) than the chemical potential of the
impurity in the solid material.
Thus, it will be noted that the process of the
invention, while very effective in removing certain
undesired impurities from a material to be puri-
fied, may be ineffective in removing other impuri-
ties when the chemical potential of the impurity in
the purifying agent is higher than in the material
to be purified. However, it should be noted that,
in such cases, the process of the invention may be
used ln conjunction with other prior art purifica-
tion processes to achieve the desired level of
purity.
In other instances, where it is desired to remove a
mixture of impurities present in a material, this
may involve a multiple purification process in
which different purifying agents are selectively
~ .
1~89333
-14-
useA to remove certain impurities and this should
be deemed to be within the scope of the invention.
The purifying agent must have a boiling point suf-
ficiently high to permit operation of the process
at a temperature approaching the melting point of
the solid material to be purified without loss of
the purifying agent by volatilization. Preferably,
the purifying agent will have a boiling point above
the me]ting point of the solid material to be
purified. The use of a purifying agent with a low
boiling point will be found to be unduly restric-
tive with respect to the temperature at which the
purification process may be conducted.
Examples of purifying agents which may be used in
the purification of silicon, for example, there-
fore, include those purifying agents having boiling
points at least above 150C below the melting point
of the solid material to be purified selected form
the class consisting of silica, silicates of alkali
metals and alkaline earth metals, and hali~es of
alkali metals and alkaline earth metals.
Further, by way of example, in the purification of
silicon (which melts at 1420C) the purifying agent
should have a boiling point above 1270C. Thus,
for example, cesium fluoride, which has a boiling
point of 1250C, would be excluded from such a list
of useful purifying agents. Preferably, however,
the purifying agent will have a boiling point above
the melting point of silicon to permit the maximum
flexibility in processing conditions.
39333
Thus, for example, in a preferred example, the
purification of silicon involves the initial pro-
vision of a source of silicon which i.s preferably
crushed into particulate form, heated to a tempera-
ture approaching the melting point of the silicon,and then contacted at this temperature with a puri-
fying agent, preferably in molten form, which will
extract the impurities without substantially react-
ing with the silicon, cooling the mixture, and then
separating the impurity-rich purifying agent leav-
ing silicon having a purity of 99.999 or better.
The material used in the process may be an already
partially purified material or may be a reagent
grade material. For example, when silicon is to he
purified, preferably, metallurgical grade silicon
or better is used as the starting material. Pre-
ferably, the starting material should be at least
98-99% pure.
Prior to the purification step, the particulate
material may be optionally prepurified in an acid
leaching step, for example, in the case of silicon,
with an acid mixture, such as HNO3 - HF or M202 -
HF for a period of 10-60 minutes. The temperature
may vary from room temperature to slightly under
the boiling point of the acid mixture. If higher
temperatures are used, the reaction time should be
correspondingly shortened.
The solid material is mixed with the purifying
agent in a ratio of from 0.1:2 to 2:1, preferably
about 1:1. As previously discussed, the purifying
agent is selected to be a material which will react
' ' ~
~.
' ' ~ .
1~8~;~33
with the typical impurities found in the solid
material but will be substantially non-reactive
with the solid material. For example, when solid
silicon is the material to be purified, the puri-
fying agent will be selected to extract impuritiessuch as boron, phosphorus, iron, an~ aluminum and,
to a lesser extent, calcium, chromium, and nickel
from the solid silicon, but will be substantially
nonreactive with solid silicon.
When solid silicon is to be purified, the purifying
agent may comprise an oxide of silicon, including
silicon oxide or a silicate or halide of lithium,
sodium, potassium, magnesium, strontium, calcium,
or barium which ~substantially is nonreactive with
silicon. Especially preferred material~s are sio2,
NaF, and Na2SiO3 both because of their effective-
nes~s and the material cost.
As previously stated, the purifying agent should be
at least of reagent grade purity. However, in one
embodiment, when purifying silicon, the purifying
agent may be preselected to contain a doping agent
for the silicon in an amount to permit tailoring of
the resistivity of the final purified silicon pro-
duct.
The purifying agent is preferably ground to a par-
ticle size range approximating the particulate
solid material to be purified to facilitate thor-
ough mixing of the materials. The particulate
solid material and purifying agent are then heated
to a temperature approaching the melting point of
the solid material to be purified. In the case of
-
~28g333
-17-
silicon, this temperature will more preferably be
at least about 1270C and, most preferably, frorn
1320C to ~ust under 1420C.
The mixture is placed in a containment vessel which
will not react with the solid material or cause
impurities therein to migrate into the solid mater-
ial at the purification temperature. When silicon
is to be purified, for example, such a vessel may
comprise a SiO2, SiC or Si3N4 material or at least
a vessel lined with one of these materials. A
graphite ves~el may also be used under certain
circumstances where the carbon will not react with
the silicon, e.g., in the presence of a NaF puri-
fying agent.
The materials are held at the purification tempera-
ture for a period of time which may range from as
short as 15 minutes to as long as 3 hours depending
upon the reactivity of the molten purification
agent. For example, when using SiO2, such as sili-
ca gel, a time period of 3 hour~ may be used whileNaF can be used for 15 minutes or Na2SiO3 for 30
minutes.
The purifying agent may be added and removed
continuously until the desired purification has
been achieved. In general, the mixture is then
cooled to room temperature, and the now purified
material may be recovered by leaching the ~olidi-
fied mixture with a reagent which is a solvent for
the impurity-rich purifying agent but which will
not appreciably attack the purified material.
12~933~
Concentrated, or at least 5%, ~F has been found to
be a satisfactory leaching agent when purifyin~
silicon. The leaching agent should preferahly be
at least as pure as the final desired purity of the
silicon to avoid introduction of contamination
after the high temperature purification step. An
electronic grade HF is satisfactory in a final
leach although less pure, i.e., commercial grade,
~F can be used in a first leach. After leaching,
the purified silicon is recovered by separating the
solid silicon from the now solubilized purification
agent, such as by decantation or filtration.
In a preferred embodiment, when silicon is to be
purified, sodium fluoride is used as the molten
purifying agent. However, when a sodium-containing
liquid purifying agent is used, it will be neces-
sary to separately remove sodium in a subsequent
purifying step. In such instances, after extrac-
tion of the purified material, e.g., after leach-
ing, the purified silicon may be sub~ect to afurther purification step to remove sodium. This
may be accomplished by heating the silicon in vac-
uum to a temperature of 800C or higher for about 5
to 60 minutes minutes. Alternatively, the sodium
may be oxidized and then the oxide layer removed by
leaching in HF.
While we do not wish to be bound by any theories of
operation, the success of our process appears to be
related to the more negative or lower partial chem-
ical potential of impurities in a molten purifyingagent such as sodium fluoride than in a solid
material being purified such as solid silicon, thus
128g~33
--19--
making the impurities more stable in the molten
purifying agent.
The process of the invention may also be operated
on a continuous basis if desired wherein at least
5 the solid material and, preferahly the purifyin~
agent as well, are continuously fed into a reactor
and the treated materials continuously removed from
the reactor.
For example, solid silicon can be continuously fed
through a pool of molten purifying agent as a sheet
thus permitting direct use of the purified silicon
product in the manufacture of electronic devices,
such as solar cells.
The following examples will serve to illustrate the
process of the invention:
Example I
Metallurgical grade silicon with a particle size of
less than 1 mm was heated with an equal amount of
spectrographic grade SiO2 gel in a graphite cru-
cible to 1360C for 1~0 minutes under an Ar at-
mosphere. After cooling to room temperature, the
si-sio2 mixture was leached with concentrated HF.
The impurities (in parts per million) of the ini-
tial metallurgical grade silicon and the purified
silicon are shown in Table 1.
1289~33
-20-
TABLE I
Silicon Silicon
Si2 Ge~8efore Treatment After SiO2
Treatment
Mg20 35 ~6
Ca15 40 7
BaclO 17.5 10
TiC12 250 12
Zr~ 35 75 ~35
10 V~ 25 50 ~ 25
Cr~ 7 800 ~ 7
Moc 35 __ __
Mn~ 8 300 ~ R
Fe~ 20 2800 ~ 20
15 Nic 8 30 8
Cu' 4 80 50
8~ 30 ' 30 --
Al~ 10 1300 100
P~ 4500~4500 ~ 4500
(Impurities in parts hy million by weight)
Example II
The same silicon as in Example I was mixed with an
equal amount of reagent grade Na2SiO3. The mixture
was heated in a graphite crucible at 1360C for 30
minutes under an Ar atmosphere. After cooling, the
mixture of Si-Na2SiO3 was leached with concentrated
HF. The impurities (in parts per million) of the
initial metallurgical grade silicon and the puri-
fied Si are shown in Table 2.
.~ :
': . '
.
.
33;~:3
TABLR 2
Silicon Silicon
sefore Treatment After Ma2SiO3
Treatment
Mq 35 17.5
Ca 40 15
sa 17.5 ~ 10
Ti 250 ~ 12
Zr 75 ~ 35
V 50 ~5
Cr 800 ~ 7
Mn 300 ~ 8
Fe 2800 ~ 20
Ni 30 8
15 Cu no ~o
B ~ 30 ~ 30
Al 13nO 150
P ~4~00 '4500
(Impurities in parts by million by weight)
To illustrate that even higher purity levels can be
achieved using this process when the starting mate-
rials are of higher purity, and that this process
results in higher purification than conventional
processing of liquid silicon, we performed the
following experiments.
Example III
Twenty grams of silicon powder (semiconductor grade
from Ventron) were ground to a particle size of
less than 1 mm and mixed with Ultrapure NaF (CERAC)
of similar particle size in a weight ratio of 1:1.
" "': . ' ' ' :
12~9;~33
-22-
The mixture was placed in a graphite crucible and
heated in a RF induction furnace for 15 minutes at
1300C so that the solid silicon was in contact
with molten NaF. A second sample was heated at
1450C for lO minutes to melt hoth the NaF and the
silicon. The samples were then cooled to room
temperature, and the silicon was separated from the
NaF in each sample by aqueous leaching f ollowed by
decantation and filtering. The resulting purified
silicon, as well as the original silicon and NaF
and the final NaF product were analyzed by Spark
Source Spectrography. The results, in parts per
million, are listed in Table 3 below:
:
~Z89333
-23-
TABLF: 3
REACTANTS PRODUCTS
Si NaF Si Si NaF
(Melted) (Solid) (After Leach)
~ .
B 0.~ 0.1 1 0.02 2
P 0.3 1 0.2 0.05 0.1
Al 3 0.07 1 0.5 0.7
As G 0.05 0.08 0.05 ~ 0.05 0.0
Ti ~ 0.1fi ~ 0.15 0.08 ~ 0.16 ~ 0.1
Zr ~ 0.24 ~ 0.14 ~ 0.1 ~ 0.24 0.05
V 0.04 -- ~ 0.04 ~ 0.04 --
Cr 0.2 0.08 1 ~ 0.2 0.4
Mn 0.4 0.06 2 0.04
Fe 40* 0.6 40* 3 4
Ni -- ~0.2 -- -- 0.2
Cu ~ 0.1 0.06 -- 0.07 0.5
Na C 0.1 -- 820 6 --
K -- 4 ~ 0.04 ~0.04 0.2
Ca 7 0.5 0.4 4
Mg 6 7 5 2 2
* Heterogeneous
(Impurities in parts by million by weight)
Example IV
Germanium may also be purified by the process of
the invention by exposing it at an elevated temper-
ature of just under 937C (the melting point of
germanium) to a purifying agent such as NaBr, Kbr,
BAI2, or CsI which are all liquid at 937C to
remove many impurities, except silicon and boron,
as illustrated in the table below. Natively grown
GeO2 or SiO2 may also be used as purifying agents
.
' :''
. . .
~2893~3
-24-
for germanium. The approximate segregation coef-
ficients for common impurities for common impuri-
ties in germanium are listed in the table below.
TABLE 4
5 Element Segregation Coefficient
Bi 5 x 10 5
Pb 1 x 10-4
In 1 x 10 2
As 1 x 10 1
Sn 1 x 10~1
Ga 1 x 10 1
Al 1 x 10 1
P 0.2
Si 10
15 B 30
Titanium may be purified similarly to silicon using
purifying agents such MgF2 or BaF2. the use of
fluorides also permits decreasing the amount of
oxygen in titanium by the formula:
TiO2(in Ti) + MgF2(1)----~TiF2(g) + 2MgO(in MgF2)
The reaction qoes through an intermediate formation
of fluorooxytitanates and fluorotitanates which at
the end decompose to give TiF2 vapors. The net
effect is that oxygen is pumped out of the titanium
into the purifying agent giving the purified titan-
ium superior mechanical characteristics.
Gallium may be purified by the process of the
~B~3
-25-
invention to remove impurities such as Ag, ~g, In,
Pb, and Sn which may have more negative chemical
potentials in liguid rather than solid phase mater-
ials.
Gallium arsenide may he purified in accordance with
the practice of the invention to remove most impur-
ities with the exception of beryllium by heating
small particles of gallium arsenide in the presence
of a purifying agent. Purifying agents useful in
the purification of gallium arsenide may include
B203, Ga2S3 (M-P-=1255C), Na2Ga2S4 (M.P.=952C),
gallates such as MaGaO2, or complex gallium solids
such as Ba3(GaF6)2, LiGaF6, or CsGaC14
(M.P.=385C), or mixtures thereof.
~hus, the invention provides a novel process for
the purification of a solid material by contacting
it at an elevated temperature approaching the melt-
ing point of the solid material with a purifyinq
agent which is substantially unreactive with the
solid material and into which the impurities in the
solid material will preferentially migrate.
Having thus described the invention, what is
claimed is:
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