U.S. patent application number 12/909353 was filed with the patent office on 2011-02-24 for dry conversion of high purity ultrafine silicon powder to densified pellet form for silicon melting applications.
This patent application is currently assigned to GT SOLAR INCORPORATED. Invention is credited to Kedar P. Gupta, Chandra Khattak, Santhana Raghavan Parthasarathy, Yuepeng Wan.
Application Number | 20110044842 12/909353 |
Document ID | / |
Family ID | 46327160 |
Filed Date | 2011-02-24 |
United States Patent
Application |
20110044842 |
Kind Code |
A1 |
Gupta; Kedar P. ; et
al. |
February 24, 2011 |
DRY CONVERSION OF HIGH PURITY ULTRAFINE SILICON POWDER TO DENSIFIED
PELLET FORM FOR SILICON MELTING APPLICATIONS
Abstract
A method for making bulk silicon material consisting of silicon
pellets for making silicon ingots from an agglomerate-free source
of high purity, ultra fine silicon powder includes feeding a
controlled amount of silicon powder into a pellet die and dry
compacting the powder at ambient temperature with pressure to
produce a pellet that has a density of about 50-85% of the
theoretical density of elemental silicon, a weight within a range
of about 1.0 gram to about 3.0 grams, a diameter in the range of 10
mm to 20 mm and preferably of about 14 mm, and a height in the
range of 5 mm to 15 mm and preferably of about 10 mm.
Inventors: |
Gupta; Kedar P.; (Merrimack,
NH) ; Wan; Yuepeng; (Nashua, NH) ;
Parthasarathy; Santhana Raghavan; (Nashua, NH) ;
Khattak; Chandra; (Danvers, MA) |
Correspondence
Address: |
Vern Maine & Associates
547 AMHERST STREET, 3RD FLOOR
NASHUA
NH
03063-4000
US
|
Assignee: |
GT SOLAR INCORPORATED
Merrimack
NH
|
Family ID: |
46327160 |
Appl. No.: |
12/909353 |
Filed: |
October 21, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11668488 |
Jan 30, 2007 |
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12909353 |
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|
10413774 |
Apr 15, 2003 |
7175685 |
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11668488 |
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60372980 |
Apr 15, 2002 |
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Current U.S.
Class: |
419/66 |
Current CPC
Class: |
C30B 15/00 20130101;
C30B 15/02 20130101; C30B 11/00 20130101; C01B 33/02 20130101; C22B
1/24 20130101; C30B 29/06 20130101 |
Class at
Publication: |
419/66 |
International
Class: |
B22F 3/02 20060101
B22F003/02 |
Claims
1. A method for making a silicon pellet comprising: providing a
source of high purity silicon powder having a median particle size
within a range of up to about 100 micrometers and a range of
particle size of not greater than about 1000 micrometers; feeding a
controlled amount of said powder from said source into a pellet
die; compacting with pressure said controlled amount of said powder
in said die thereby forming a dry pellet of high purity silicon;
and discharging said pellet from said die.
2. The method for making a silicon pellet according to claim 1,
said compacting being done at about ambient temperature.
3. The method for making a silicon pellet according to claim 1,
said powder being free of intentional additives and binders.
4. The method for making a silicon pellet according to claim 1,
said powder comprising an organic binder, and said compacting with
pressure comprising adding sufficient heat whereby said binder is
evaporated.
5. The method for making a silicon pellet according to claim 1,
wherein said step of dry compressing comprises applying a force
greater than or equal to about approximately 10,000 Newton's to
said powder.
Description
RELATED APPLICATIONS
[0001] This application is a divisional application to pending
application Ser. No. 11,668,488 filed Jan. 30, 2007. Application
Ser. No. 11/668,488 is a continuation-in-part to application Ser.
No. 10/413,774, filed Apr. 15, 2003. Application Ser. No.
10/413,774 claims the benefit of U.S. Provisional Patent
Application Ser. No. 60/372,980, filed Apr. 15, 2002, under 35
U.S.C. .sctn.119(e).
FIELD OF INVENTION
[0002] This invention relates to raw materials used for melting to
make silicon ingots, in particular to a bulk silicon material
consisting of silicon pellets make by compacting silicon powder
without additives or binders under pressure at ambient temperatures
into pellet form.
BACKGROUND OF THE INVENTION
[0003] The crystalline (mono-crystal or multi-crystal) silicon
materials used for semiconductor as well as for photovoltaic
devices manufacturing are produced by crystal growth from melt. The
feedstock for the silicon crystal growth process is high purity
silicon fabricated or produced by high temperature decomposition of
silicon containing chloride (such as trichlorosilane or
mono-silane).
[0004] The forms of the silicon feedstock that come directly from
silicon manufacturers are generally chunks and chips (from the
breaking down of large silicon rods) and silicon beads (size ranges
from several hundred micrometers to millimeters). Such feedstock
can be packed well into the crucibles that are used for melting the
silicon charges with a packing factor of more than 50%.
[0005] The silicon beads are generally made by a fluidized bed
process. This process also produces ultra-fine silicon powder (size
ranges from sub-micron to several hundred micrometers) as a
byproduct in addition to the useful silicon beads. The ultra-fine
silicon powder is normally referred as Cyclone powder or filter
powder depending on where it is deposited; the range of sizes for
the two powders is also different. So far there is no effective way
to utilize this ultra-fine powder hence the effective silicon
conversion yield of the fluidized bed reactors is low.
[0006] Such ultra-fine silicon power has a purity as high as that
used for the crystal growth. However, because of its submicron
size, it has a bulk density of about 0.25-1 gm/cc which is
significantly low when compared to the silicon solid density of
2.33 gm/cc. Because of the loosely bound nature of the powder the
crucible cannot be loaded with more powder. For example, a
69.times.69.times.42 cm crucible can hold up to 300 Kg of solid
silicon, while the powder can be charged only up to a maximum of
150 Kg. Conventional melt replenishment by continuous feeding of
chips or beads is also not possible with the above said powder
because of the loosely bound nature. Lack of proper way to utilize
the ultra-fine silicon powder renders such by-products of much less
value. Furthermore, the low bulk density presents storage problems
due to requirements of large space.
[0007] Such ultra-fine silicon powder can also be formed by
homogenous thermal decomposition of silicon containing gases (such
as mono-silane). This homogenous decomposition is a much cheaper
process comparing to the heterogeneous deposition process used in
the Siemens and the fluidized bed reactors. Therefore, a practical
method of charging and feeding the ultra-fine silicon powder would
have a significant impact on the cost of manufacturing the silicon
feedstock. This is especially important for photovoltaic
applications where cost reductions will make this renewable energy
source viable for terrestrial applications.
[0008] Another problem associated with ultra-fine powders is the
very large surface area; this results in an oxide coating on the
powders. When such powders are heated to melt temperatures in ingot
growth furnaces this presents problems in melting, reactions with
the hot zone and degradation in performance of the silicon when
used for solar cells. Therefore, ultra-fine powder additions to
ingot growth charge have been limited to approximately five percent
(5%).
[0009] The rapid growth of photovoltaic industry and current severe
shortage of silicon feedstock has forced the manufacturers of
crystalline silicon to use all sorts of silicon feedstock in all
kinds of forms. This includes the mixing of silicon powder,
generally of larger than 100 micrometers, with other forms such as
chunks of silicon. However, the use of the ultra-fine silicon
powder (sub-micron size) remains a challenge due to its very low
bulk density. Moreover, the ultra-fine silicon can easily flow with
gases, which make it very hard to handle. Typically, the first
operation in an ingot growth furnace after loading the charge is
evacuation of the chamber; this can result in sucking the
ultra-fine powders out of the crucible.
[0010] Compacting of silicon powder has been mentioned in several
publications, as listed in the references. Both Moller's paper,
Sintering of Ultrafine Silicon Powder, and Takatori's paper, High
Pressure Hot-Pressing of Silicon Powders, were specific sintering,
i.e., with elevated temperature to achieve full densification. Both
papers are focused on the grain boundaries formed after the high
temperature sintering due to their interests in the mechanical
properties of the formed bulk silicon materials.
[0011] In Moller's paper, the starting silicon powder has sub
micron particle sizes in the order of 0.02 to 0.1 .mu.m. A
pre-shaped compaction of silicon powder, of little or no
description, was made and subjected to high temperature sintering
without pressing. The densification and microstructure development
(grain size, grain boundary transformation, etc.) were investigated
under different sintering conditions for purposes other than those
of this invention.
[0012] In Takatori's paper, the compaction of silicon power was
achieved by applying pressure and elevated temperature
simultaneously. As known to the materials researchers, high
temperature helps bulk diffusion as well as the grain-boundary
diffusion, and thus helps the densification of the sintered
material. In fact, as reported in this paper, sintered density of
close to 100% of theoretical was achieved at temperature above
677.degree. C. when combined with high pressing pressure (>1
GPa).
[0013] What is needed is a cost effective method and product for
reclaiming ultra fine silicon powder of high purity for uses
requiring silicon of a high degree of purity.
SUMMARY OF THE INVENTION
[0014] It is an object of the invention to provide a viable and
practical process and machinery to convert otherwise unusable ultra
fine silicon powder into a form factor, typically pressed pellets,
that can be utilized in silicon melting processes. It is a further
object to provide a process and machinery that will maintain the
purity of the silicon to nearly the same level as the starting
powder, either in the pellet form directly or in the consumption of
the pellet for its intended purpose.
[0015] It is another object of the invention to provide a system
and facility for conducting a powder-to-pellet conversion on a
commercially useful production rate, such as high speed pellet
pressing of up to 600 pellets or more per minute, and processing of
up to 50 kg or more of silicon powder per hour.
[0016] It is an object of the compacting of ultra-fine silicon
powder to enhance the packing factor of the silicon feed material
in a crucible through the use of compacted silicon pellets.
[0017] Another objective of compacting the ultra-fine silicon
powder is to enhance the thermal conductivity of the powder so that
the silicon feed stock can be easily heated and melted.
[0018] It is an object of the present invention to provide a
process for the production of pellets having the structural
integrity to withstand packaging, handling and further processing
by compressing silicon powder at an effective force, according to
one embodiment, about approximately 10,000 N (1.12 US Tons) to
achieve adequate adhesion between silicon particles.
[0019] One requirement for the resulting silicon pellets (compacts)
is purity. The purity of the silicon in pellet form has to be
maintained to nearly the same level as the starting powder in order
for the pellets to be useful for making high purity silicon
crystals. The silicon crystals are generally used for making
semiconductor devices and solar cells. In some applications, a
small amount of binder may be tolerable. In other application,
small amounts of binder may be removed during consumption of the
pellet.
[0020] One aspect of this invention is the development of a process
that adds significant value to the silicon dust (ultra-fine
powders) byproduct from the Fluid Bed and other processes.
[0021] The universal method of treating fine powders is by addition
of organic and inorganic binders to convert the powder to granules
and pellets, as are practiced in ceramics industry, certain powder
metallurgical industry, and pharmaceutical industry. These
processes add difficult-to-remove ingredients to the compact, and
which in some processes are just contaminants to the material.
[0022] However, in one aspect of the process of this invention,
silicon is pressed dry without addition of any outside material and
at ambient temperature. It thereby keeps the purity of the pressed
pellet very close to that of the starting material. It is the
combination of the ability to convert silicon dust into compressed
form by a dry no-binder technique that enables subsequent
value-added use of the by-product silicon powder, for example in
the silicon melting processes of photovoltaic and electronic
applications.
[0023] The only other application of dry pressing of powders is in
the manufacture of nuclear fuel oxide pellets by the MOX process.
Even in this process small quantities of zinc stearate are utilized
as an additive to provide for initial agglomeration and pellet
strength while also serving as a lubricant in the pressing
operation. It is removed in the subsequent high temperature
sintering step.
[0024] In another aspect of the invention, a binder may be
introduced into the powder to facilitate the forming of the
pellets. Some or all of the binder may be removed during the pellet
forming process, as by the heat of compression or the addition of a
small amount supplemental heat.
[0025] Compacting fine powders at an elevated temperature
(sintering) is another general practice for obtaining densified
components. Sintering at high pressure (hot press) can achieve
close to 100% density material and the fine grains obtained from
hot pressure provide the needed structural strength. However, the
application of the disclosed process has minimal structural
requirements and the grain boundary is not important for the
resulting pellets. Additionally, in an elevated temperature
environment, impurities from the die can diffuse into the formed
silicon pellets, and thus contaminate the feed stock. During heat
up to high temperatures the oxide layer on ultra-fine powders can
lead to reactions leading to silicon carbide formation, thereby
resulting in contamination and degradation in performance.
[0026] One process of this invention utilizes binder-less cold
pressing of silicon powder without additives to form pellets that
can be utilized in subsequent silicon processes. However, an
organic binder may be used in some cases, and be all or partially
evaporated during or after forming. The exact pellet shape and
size, or uniformity of shape and size, is not of particular
importance except as may be dictated by the available pressing
machinery. The structural integrity of the pellets needs only be
sufficient to tolerate packaging and handling. There are no
published references known to the Applicants that purport to
utilize a process for effective use of otherwise unusable silicon
dust.
[0027] One aspect of the invention is that the material purity may
be maintained in the powder-to-pellet conversion operation, so long
as the process is a binder-less dry method. In another aspect of
the invention, any remaining binder may be removed in the
consumption of the pellets, so that the resulting silicon product
is of high purity not withstanding the use of binder in the feed
stock pellets.
[0028] The form factor of the pellet is important, because the
pellets have significantly less surface area as compared to the
ultra-fine powders and, therefore, do not cause significant
reactions during heat up. This minimizes contamination and
degradation in performance and allows larger proportions (up to
100%) in the charge for ingot growth.
[0029] This invention is important because it provides a method to
convert high purity, but otherwise wasted, ultra fine silicon
powder into a form that is transportable in bulk, pure, and usable
as feed material to silicon melting processes, and in particular
those processes requiring high purity silicon.
[0030] In one aspect of the invention, the ultra fine silicon
powder is transferred into a clean feed hopper attached to a pellet
press machine such as a high quality Courtoy-type rotary indexing
die and punch machine. Controlled quantities of the powder are fed
into the die by use of an appropriate powder feeder. Since the
powder is ultra fine, a special powder feeder may be required. The
powder is pressed by the punch with a press force of several tons.
No binder or additive is necessary to the process, although for
some applications an organic binder may be used in the initial
forming stage, and subsequently be evaporated during or after the
pellet is formed.
[0031] The pressed pellet is ejected into a clean collection bin
and transferred into a lined shipping container. The pelleting
machinery is equipped for automated and controlled operation. In
addition, the entire process zone is located inside a controlled
enclosure to maintain process and environment quality. The process
facility also provides controlled ingress and filtered egress for
environmental safety.
[0032] Since no binder or additive is required in converting the
ultra fine silicon powder to pressed pellet form, only nominal
pellet strength, satisfactory for the purpose of compaction and
transfer to secondary operations, is necessary. Some surface
dusting and occasional breakage of pellets still provides an
acceptable yield of the high-value end product, the dry, high
purity silicon pellets.
[0033] In other and various aspects of the invention, there is no
minimum particle size for the powder. A small amount of binder may
be added to the powder to facilitate forming of the pellets. A
small amount of supplemental heat may be applied during the pellet
forming process, during which some or all of the binder may be
evaporated. What binder remains may be partially or fully
evaporated during heating of the pellets for consumption, so that
the purity of the silicon product is affected only slightly if at
all.
[0034] The present invention is a method for making a silicon
pellet. The method includes providing a source of high purity
silicon powder having a median particle size within a range of up
to about 100 micrometers and a range of particle size of not
greater than about 1000 micrometers, feeding a controlled amount of
said powder from said source into a pellet die, compacting with
pressure said controlled amount of said powder in said die thereby
forming a dry pellet of high purity silicon, and discharging said
pellet from said die.
[0035] In some embodiments, the compacting is done at about ambient
temperature. In some embodiments, the powder is free of intentional
additives and binders. In other embodiments the powder includes an
organic binder, and the compacting with pressure includes adding
sufficient heat whereby the binder is evaporated.
[0036] And in certain embodiments the step of dry compressing
includes applying a force greater than or equal to about
approximately 10,000 Newton's to the powder.
[0037] The features and advantages described herein are not
all-inclusive and, in particular, many additional features and
advantages will be apparent to one of ordinary skill in the art in
view of the drawings, specification, and claims. Moreover, it
should be noted that the language used in the specification has
been principally selected for readability and instructional
purposes, and not to limit the scope of the inventive subject
matter.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0038] The invention is susceptible of many embodiments. As listed
in Table 1, the results of one of several tests conducted by the
Applicant, show that an ultra fine silicon powder of median
particle size of about 13 micrometers, bulk density 0.56 g/cc
(grams per cubic centimeter) and tap density about 0.68 g/cc, is
converted into pellets of size 14 mm (millimeter) diameter.times.10
mm height, with a pellet weight of 2.3.+-.0.05 g. The pelleting
data for Table 1 was taken from pellets made with a clean Courtoy R
53 rotary multi-station pellet press with compression force
capacity of up to 14 tons. The tool set is made out of Tungsten
Carbide.
TABLE-US-00001 TABLE 1 Pellets Pellets Pellets Pellets 1 2 3 4
Starting powder 13.5 .mu.m 3.97 .mu.m 4.97 .mu.m 11.5 .mu.m median
size Powder bulk density 0.56 0.58 0.65 0.34-0.58 (g/cc) Pellet
diameter 14 18 18 12 (mm) Pellet height 9.5-10.0 5.3 5.3 4.1 (mm)
Pellet density 1.49-1.58 1.41 1.41 1.72 (g/cc) Pellet density (% of
64-68% 60% 60% 74% bulk Si) Compression force 10,000 10,000 10,000
10,000 (N) Pellet purity NA NA NA GDMS, Close to powder
[0039] Table 1 illustrates silicon pellets made with different
powder sizes and pellet sizes. The actual pellet size is not
critical, and uniformity in size is not critical. In one
embodiment, a precise quantity by weight of the silicon powder is
fed into the die as a unit charge, and progressively compressed by
a matching punch to the required force to achieve the
pre-calculated dimension that represents the desired final pellet
density based on the weight of the unit charge of powder fed into
the die. Alternatively, the process may be operated on the basis of
compressing a unit charge that is sized as a pre-calculated volume
of powder, and/or compacting the unit charge to a pre-calculated
final pressure or volume. The process criteria may be selected by
calculation and/or by trial and testing of a useful range of the
listed variables in Table 1 to achieve the desired pellet
product.
[0040] To produce silicon pellets having the mechanical integrity
to withstand packaging, handling, storage and further processing,
the silicon powder must be exposed to an effective pressure to
insure that there is adequate adhesion between the silicon power
particles. By compressing the powder, the silicon atoms of
different particles are in sufficiently close proximity to permit
bonding or attraction between the atoms and as a result, the
particles. The compression packs the particles closer together,
eliminating voids and holding the particles, and the atoms of which
they are comprised, in close proximity. While this packing need not
be the highly ordered packing of crystalline silicon, higher
degrees of order and of bonding produce more mechanically stable
and purer pellets. According to one embodiment a force of about
approximately 10,000 N (1.12 US Tons) was used. Those skilled in
the art will readily appreciate that the forces in excess of 10 KN
would likewise produce pellets of sufficient structural integrity,
as would weaker forces so long as the particles are compacted
sufficiently to adhere without additives.
[0041] For example, the starting power particle size may be
expected to range up to 20 .mu.m and more. The power density may
range from less than 0.60 g/cc to more than 0.75 g/cc. The
compressive force, exact pellet geometry and volume, and final
pellet density may be a function of the available machinery, but
testing suggests that a good pellet can be produced from the range
of powder specified, with 10,000-20,000 Newton's in a volume range
of about 0.5 to 2.6 cubic centimeters, weight range of about
1.0-3.0 grams, and a density range of about 50-75% of the
theoretical density of elemental silicon. Extrapolating our test
results, it is expected that an average pellet density of at least
50% will be necessary to survive bulk packaging and handling.
Densities as high as 85% are expected to be attainable with smaller
particle sizes and higher compression forces. Our test results
demonstrated that the smaller the size of the powder, the better
integrity of a formed pellet, or the easier for compacting.
[0042] As another example, For example, the starting power medium
particle size may be expected to range up to 20 .mu.m and more. The
power density may range from less than 0.60 g/cc to more than 0.75
g/cc. To the silicon powder there may be added an organic binder.
Organic binders such as Acetone, or Alcohol (methyl, ethyl or iso
propyl) or isobutyl methacrylate mixed with ethylene dichloride and
carbon tetra chloride, and others, may be used to facilitate
forming.
[0043] The organic binders such as acetone or alcohol provide a
good binding force between the silicon particles and easily get
evaporated at room temperature without leaving any significant
residues.
[0044] Other binders such as cellulose ethers (solid binder or
added with alcohol solvent) need a high temperature bakeout to burn
off the binder. The time for weight loss upon heating can be
determined by differential thermal analysis. For cellulose ethers
the complete burn off occurs at 400 degrees centigrade, cleanly and
predictably with first order kinetics. All binders added should be
of electronic grade.
[0045] The compressive force, exact pellet geometry and volume, and
final pellet density may be a function of the available machinery,
but testing suggests that a good pellet can be produced from the
range of powder specified, with 10,000-20,000 Newton's in a volume
range of about 0.5 to 2.6 cubic centimeters, weight range of about
1.0-3.0 grams, and a density range of about 50-75% of the
theoretical density of elemental silicon. Extrapolating our test
results, it is expected that an average pellet density of at least
50% will be necessary to survive bulk packaging and handling.
Densities as high as 85% are expected to be attainable with smaller
particle sizes and higher compression forces. Our test results
demonstrated that the smaller the size of the powder, the better
integrity of a formed pellet, or the easier for compacting.
[0046] According to one embodiment of the present invention dry
silicon powder is used. The presence of moisture might result in
clumping, or impurities that might have a deleterious effect on the
purity of the final product, the integrity of the pellet, or the
operation of the die. Those skilled in the art would, however,
readily appreciate that the use of wet silicon powder would be
within the scope of the invention.
[0047] The compressed pellets are ejected from the machine through
the take-off system. The pellets provide a loose bulk material form
of silicon for melting for high purity silicon requirements. The
powder-to-pellet conversion is accomplished dry, with no added
ingredient in the process. This is required to maintain the purity
of the silicon for subsequent use.
[0048] The term "pellet" is herein inclusive of any form factor and
descriptive term that implies a compacted small volume of the raw
material, the pellets produced in quantity in the nature of a loose
granular bulk material that facilitates easy handling methods and
ready conformance to container shapes.
[0049] The invention is further extended by the utilization of the
pelletized dry silicon in the making of high purity silicon ingots.
The suitability of the high-purity dry-compacted pellets for
melting into high purity silicon ingots is effectively demonstrated
by the method of one embodiment of containing the pellets in a
fused quartz crucible, baking in vacuum at 1350.degree. C., and
then melting in an inductively heated graphite susceptor system as
is well known in the art. The melt is taken up to 1600.degree. C.,
and then cooled. The resulting ingot is very bright and shiny, with
no inclusions in it. There may be a trace of residual oxide
material as slag on top center of the melted ingot. Dry silicon
pellets, pressed from silicon powder, were first melted to form an
ingot of high purity silicon on Jan. 28, 2002.
[0050] The basic steps of another embodiment method for making high
purity silicon pellets is as follows: providing a source of high
purity silicon powder, feeding the powder into a blender, operating
the blender to remove agglomerates, discharging the powder into a
hopper, feeding a controlled amount by weight or volume of the
powder into a die, dry compacting the powder with pressure,
exclusive of any additives or wetting agents, and then discharging
the dry pellet from the die. The machinery may be configured to
operate multiple lines of multiple dies, to meet high volume
requirements.
[0051] The further steps of this embodiment of making high purity
silicon ingots from the dry silicon pellets is conventional except
for the use of the dry silicon pellets and the resulting purity of
the ingots: containing a suitable number of the silicon pellets in
a fused quartz crucible, baking the crucible with pellets in vacuum
at about 1350 degrees C., melting the pellets at up to about 1600
degrees Centigrade in an inductively or resistively heated graphite
susceptor system, and cooling the melt so as to produce an
ingot.
[0052] In the disclosed invention, in distinction to the work of
Moller and Takatori discussed previously, the grain boundaries of
the pellets is of less importance because the formed silicon shapes
are to be used as feed stock for melting. Therefore, no sintering
is necessary as the disclosed process can obtain reasonably
aggregated silicon pellets. High temperature sintering may
introduce impurity into the silicon pellets, which is highly
prohibited in the application of the disclosed process. Also, the
invention disclosed does not require elevated temperatures,
although supplemental heat may be added in some cases. The achieved
densification is in the range of 60-75% of theoretical, which has
been shown to be dense enough for the designated applications.
[0053] The foregoing description of the embodiments of the
invention has been presented for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed. Many modifications and
variations are possible in light of this disclosure. It is intended
that the scope of the invention be limited not by this detailed
description, but rather by the claims appended hereto.
* * * * *