U.S. patent application number 11/479735 was filed with the patent office on 2007-01-18 for conversion of high purity silicon powder to densified compacts.
Invention is credited to Mohan Chandra, Alleppey V. Hariharan, Jagannathan Ravi.
Application Number | 20070014682 11/479735 |
Document ID | / |
Family ID | 37605089 |
Filed Date | 2007-01-18 |
United States Patent
Application |
20070014682 |
Kind Code |
A1 |
Hariharan; Alleppey V. ; et
al. |
January 18, 2007 |
Conversion of high purity silicon powder to densified compacts
Abstract
This invention describes methods of compacting and densifying
high purity silicon powder to defined geometric forms and shapes.
High purity silicon powder is first mixed with binder from a select
group of binders and pressed into desired shapes in a mechanical
equipment. The binder is removed either in a separate step or
combined with a subsequent sintering operation. The binders and
process conditions are chosen to make negligible change to the
purity of the silicon in the end product. When high purity silicon
powder is utilized in the process, the end use for the densified
silicon compacts is primarily as feedstock for silicon-based
photovoltaic manufacturing industries.
Inventors: |
Hariharan; Alleppey V.;
(Austin, TX) ; Chandra; Mohan; (Merrimack, NH)
; Ravi; Jagannathan; (Bedford, MA) |
Correspondence
Address: |
Alleppey V. Hariharan
3225 Summer Canyon Dr
Austin
TX
78732
US
|
Family ID: |
37605089 |
Appl. No.: |
11/479735 |
Filed: |
June 30, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60696235 |
Jul 1, 2005 |
|
|
|
Current U.S.
Class: |
419/36 |
Current CPC
Class: |
Y02P 70/50 20151101;
H01L 31/1804 20130101; Y02P 70/521 20151101; Y02E 10/547 20130101;
C04B 33/02 20130101; C01B 33/02 20130101; C04B 35/584 20130101 |
Class at
Publication: |
419/036 |
International
Class: |
B22F 3/12 20070101
B22F003/12 |
Claims
1. A process to form compacted densified geometric shapes from
silicon powder and the resultant products thereof, which is robust
for industrial manufacturing application. The densified silicon
compacts may have such shapes as cylinders, cuboids, discs, wafers,
etc.
2. The compacted densified form of silicon according to claim 1
that derives from high purity silicon powder, and which can be used
as feedstock in photovoltaics materials industry.
3. The compacted densified form of silicon according to claim 1
that derives from nominal purity silicon powder and which can be
used as feedstock in ferrous and non-ferrous alloy industry.
4. The compacted densified form of silicon according to claim 2
where the purity of the silicon powder is to be >99% and
preferably >99.99%, with particle size in the range of 0.01-200
microns (preferably 0.1-40 microns) and mean size of 1-20 microns
(preferably 5 microns).
5. A method for making a compacted densified silicon material
comprising the steps: Providing an agglomerate-free source of high
purity silicon powder, Blending the said silicon powder with
additives including selected high purity binder. Pre-drying the
said blend, Feeding a controlled amount of said dried blend of
silicon powder and binder from said blend into a shape forming die,
Compacting with pressure said controlled amount of said blend in
said die thereby forming a compact of defined shape of the blend of
high purity silicon and binder Discharging said compact from said
die, and Repeating the previous five steps thereby producing a
quantity of said compacts. All these operations to be performed at
ambient temperature on a shape forming machine that uses mechanical
pressure to compact the feed material, Removing the binder in a
de-binder operation by heating the pressed compact in a furnace
environment of flowing inert gas or reducing gas such as hydrogen
in inert gas or vacuum and at temperatures of 100-500 C., and
Sintering the de-bindered compact in a furnace environment of inert
gas or reducing gas such as hydrogen in inert gas or vacuum and
temperatures of 1000-1350 C. to provide for complete removal of
binder materials, and to provide for silicon particle bonding,
densification and compact strength.
6. A method for making a compact of silicon material according to
claim 5, wherein the silicon powder is blended with a binder from
select groups of binders.
7. The select groups of binders according to claim 6 are derived
from select silicon-based and carbon-based high purity chemicals
that have specific advantages for application to silicon powder
compaction.
8. The select binders according to claim 7 consists of high purity
fumed silica, high purity colloidal silica, polyalkoxysilanes
(typically ethyl silicate with 10-60% effective silica content),
polyalkylene carbonate (typically polypropylene carbonate), stearic
acid and zinc stearate.
9. The select binders according to claim 7 which cannot be used
directly is suspended or dissolved in inorganic carrier such as
water or organic carrier solvents of the type acetone, isopropyl
alcohol, methyl ethyl ketone, etc.
10. The fumed silica binder content of the blend with silicon
powder according to claim 6 is in the range 0.01-5 weight percent
of silicon powder, and preferably in the range 0.05-0.2 weight
percent.
11. The colloidal silica binder content of the blend with silicon
powder according to claim 6 is in the range 0.01-5 weight percent
of silicon powder, and preferably in the range 0.05-0.2 weight
percent.
12. The ethyl silicate binder content of the blend with silicon
powder according to claim 6 is in the range 0.01-5 weight percent
of silicon powder, and preferably in the range 0.05-0.5 weight
percent.
13. The polypropylene carbonate binder content of the blend with
silicon powder according to claim 6 is in the range 0.01-5 weight
percent of silicon powder, and preferably in the range 0.05-1.0
weight percent.
14. The stearic acid or zinc stearate binder content of the blend
with silicon powder according to claim 6 is in the range 0.01-5
weight percent of silicon powder, and preferably in the range
0.05-0.2 weight percent.
15. A method for making a compact of silicon material according to
claim 5 wherein the binder chosen from the group according to claim
7 is thoroughly blended with the silicon powder to provide a
uniform blend.
16. A method for making a compact of silicon material according to
claim 5 wherein the blend of silicon powder and binder with carrier
solvent according to claim 9 is dried at temperatures up to 150 C.,
and preferably at 100 C.
17. A method for making a compact of silicon material according to
claim 5, said dried blend is compacted at ambient temperature by
progressively compressing a controlled weight or volume of said
powder to a pressure calculated to achieve a desired compact
density.
18. A method for making a compact of silicon material according to
claim 5, said dry compact is de-bindered in a furnace environment
of inert gas or reducing gas such as hydrogen in inert gas or
vacuum at temperatures of 100-500 C. to remove the binder and its
decomposition products.
19. A method for making a compact of silicon material according to
claim 5, said dry compact, after de-binding, is sintered in a
furnace environment of inert gas or reducing gas such as hydrogen
in inert gas or vacuum at temperatures of 1000-1350 C. to remove
all traces of the binder and its decomposition products and provide
silicon particle adhesion, bonding, densification and compact
strength.
Description
RELATED DOCUMENTS
[0001] This Utility Patent Application claims priority of a
Provisional Patent Application No. US60/696,235 dated Jul. 1, 2005,
and titled "Powder metallurgical conversion of high purity silicon
to densified compacts".
FIELD OF INVENTION
[0002] The present invention is directed towards conversion of fine
silicon powder into densified silicon compacts for use in silicon
melting and alloy industries. This conversion process is achieved
by the use of selective binders to aid in compacting the powder
towards subsequent sintering and densification. When adapted to
high purity silicon powder, the end use for the densified silicon
compacts is primarily as feedstock for silicon-based photovoltaic
manufacturing industries.
BACKGROUND OF THE INVENTION
[0003] Compacting of powders is well known in metallurgical and
ceramic process industries and is a highly developed method of
manufacturing various parts and shapes. In these processes powder
metals, ceramics or a mixture of ceramics and metals are compacted
into various shapes by operations of cold isostatic pressing, hot
isostatic pressing, extrusion, injection molding and such other
arts. In all such processes some binder or additive of an inorganic
or organic nature is added to effect particle binding and
compaction. In some instances sintering aids are purposely added in
the compaction process to aid in subsequent sintering of the
compacted body. Under controlled process conditions the binders
and/or additives are essentially removed leaving only very trace
amounts of such residues. The final sintering operation is usually
performed at high temperatures in controlled-atmosphere or
air-atmosphere furnace to provide for essentially complete removal
of the binders and additives, bond the particles metallurgically
and impart strength to the compacted body.
[0004] Silicon powder is industrially produced by various
processes. Nominal purity silicon powder is formed as reaction
residues from preparation of organochlorosilanes or chlorosilanes
from the reaction of elemental silicon with chlorinated
hydrocarbons or hydrogen chloride. The powder is used as alloy feed
in ferrous and non-ferrous industries, for manufacture of silicon
nitride, and so forth. For such applications the powder is
agglomerated with a binding agent to form granules of 250-500
microns. The binders are typically organic materials such as
starch, and lignin. Other agglomeration methods include microwave
heating of the powder to 1200-1500 C. Where it is necessary to
stabilize silicon dust and powder and make them into a more stable
form for transportation and disposal (deactivation of silicon) the
silicon dust is milled in an aqueous solution of pH>5 to form
colloidal silica. This helps to agglomerate the dust.
[0005] Ultra fine silicon is a by-product of the Fluid Bed process
to manufacture high purity electronic grade polysilicon. In this
process silicon is deposited by thermal decomposition of silane
(SiH.sub.4) or chlorosilane (SiCl.sub.xH.sub.y, where y=4-x) gas on
granules of silicon seed particles. The granules grow in size from
an initial seed size of .about.0.2 mm to .about.3 mm in diameter.
The granules are utilized in silicon melting and crystal growth
applications. The Fluid Bed process, however, also results in a
large quantity of ultra fine silicon dust. This is tapped out of
the reactor outlet and remains as a process waste. This powder is
of high purity, but cannot be recycled or used in silicon melting
and crystal growth applications.
[0006] High-pressure hot pressing of silicon powder with sintering
aids and subsequent high temperature sintering of pressed silicon
bodies are known in the literature. High-pressure hot pressing of
silicon powders is described in the art, such as in "The Effects of
Processing Conditions on the Density and Microstructure of
Hot-Pressed Si Powder", by C. J. Santana and K. S. Jones, J.
Materials Sci. 31 (18), 4985-4990 (1996); and "High Pressure
Hot-Pressing of Si Powders", by K. Takatori, M. Shimada and M.
Koizumi, J. Jap. Soc. Powder Metal. 28 (1) 15-19 (1981). In one
such application silicon powder was hot pressed into
polycrystalline wafers 1.5'' diameter using various process
conditions, typically hot pressing at 1300 C./2000 psi in hydrogen
gas ambience. The wafers were contaminated with iron, aluminum,
carbon and oxygen. There are also several studies of sintering
silicon compacts at high temperatures, ranging from 1250 C. to
close to the melting point of Si (1412 C.), in an inert atmosphere.
Silicon sintering with addition of sintering aids such as Boron, or
retardants such as Tin, is described in the art, for example "The
effect of small amounts of B and Sn on Sintering of Si" by C.
Greskovich, J. Mater. Sci 16 (3), 613-619 (1981). Making silicon
articles by sintering and densification is also described by
Greskovich and J. H. Rosolowski in two U.S. Pat. Nos. 4,040,848 and
4,040,849. Granulation and augmentation of the silicon powder
particle size by electron beam melting is described in a Japanese
Patent 11199382JP. Such uses are mainly for making silicon nitride
and other silicon compounds.
[0007] There is no published prior art that purports to utilize a
process for effective use of otherwise unusable silicon dust and
powder, i.e., there is no robust, industrially practical and
cost-effective methodology to convert silicon powders to forms that
keep the purity of the product close to the initial powder quality,
can be produced in a manufacturing environment, and, more
importantly, be transported without form failure to subsequent
product users. There is also no known industrially useful and
practical methodology to convert high purity silicon dust and
powder to high purity densified silicon compacts that can be used
as significant polysilicon feedstock for photovoltaic
industries.
SUMMARY OF THE INVENTION
[0008] It is an object of the invention to provide a viable and
practical process and technology to convert silicon powder into a
form, typically compacted densified shapes, that can be
manufactured, transported and utilized to produce silicon feed
stock for other applications. It is a further object to provide a
process and technology that will maintain the purity of the silicon
to nearly the same level as the starting silicon powder.
[0009] It is another object of the invention to provide a system
and facility for conducting a powder-to-compact conversion on a
commercially useful production rate, such as high speed compacting
and densification, and processing upwards of 25 kg or more of
silicon powder per hour.
[0010] The most important aspect of this invention is the
development of a process that provides silicon feed stock material
to user industries while maintaining the product purity very close
to that of the starting material. In particular, when used with the
high purity silicon powder from the electronic industry it adds
significant value to the material and provides a feedstock to the
photovoltaic industry. In the present invention the silicon
compacts can be produced in regular geometric shapes. Typically,
the silicon used for crystal growth comes in irregular chunks or
granules. The advantages of regular shaped silicon of this
invention provide for better packing possible, whether for
transportation purposes or in the crucible used for melting silicon
prior to growing crystalline silicon ingots.
[0011] The process of this invention uses selective binders to the
silicon powder to aid in powder flow, provide material binding and
lubricity in mechanical operations in the process of converting the
powders to formed shapes. An effective binder will hold dry powders
or aggregate together with exceptional green strength during
compacting, burning out cleanly and uniformly and provide
sufficient strength during subsequent sintering to densify the
parts.
[0012] While the binders are selected for their functionality, the
focus is the purity of the formed compacted silicon body after
subsequent process steps and near complete elimination of all
byproducts and subproducts from the binder. The compacting step
itself is performed at ambient temperature to prevent in-process
reaction of such binders and die/punch material with the silicon,
as occurs in hot pressing operations.
[0013] It is the combination of the ability to convert silicon
powder into compacted form by a selective binder technique and
subsequent process steps to provide densification and compact
strength to the silicon compact while removing the extraneous
binder material and components and sintering the compact that
enables subsequent value-added use of the silicon powder,
especially high purity silicon powder, for example to critical uses
such as feedstock materials for photovoltaic applications. The
purity level of the silicon material feedstock for photovoltaic
applications should be 99.99% or better.
[0014] A similar application of selective binder is in the
manufacture of nuclear fuel oxide pellets by the MOX process. 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.
[0015] As explained above, compacting of powders is well known in
metallurgical and ceramic process industries. All such processes
utilize some binder or additive to effect compaction. In some
instances sintering aids are purposely added in the compacting
process. Notably, the binders/additives/aids leave a residue of
organic or inorganic nature during subsequent operations that
render those methods unuseful in this instance. In addition,
compacted bodies are sintered at high temperatures to provide
compact strength and densification. Although a simple binder-less
process is optimum, if practical, to convert high purity silicon
powder to compacted shapes, such a method by itself will hardly be
robust in industrial handling and transport simply due to lack of
compacted body strength with such silicon.
[0016] The process of this invention utilizes either silicon-based
or carbon-based types of binders, each with its specific advantages
for application to silicon powder compaction.
Silicon-Based Binders are the Following Types
[0017] (a). High purity Fumed silica [0018] (b). High purity
colloidal silica, which is a suspension of tiny silica particles in
an organic medium. [0019] (c). Polyalkoxysilanes with typically
10-60% effective SiO.sub.2 are operationally viewed as liquid
sources of silicon dioxide, and possess material binding
properties. Polydiethoxysiloxane with 40% SiO.sub.2 content (ethyl
silicate 40) is the most widely used polyalkoxysilane with use as a
binder in such processes as investment casting. Carbon-Based
Binders are the Following Types [0020] (d). Polyalkylene carbonate
(dissolved in selective solvents) possesses a number of unique
characteristics which make it ideal for use as binders with
refractory materials: high purity, good binding, imparts higher
green strength to compacted body, and clean burning at low
de-binder temperatures. Among these, polypropylene carbonate of the
type with trade name of QPAC-40 and polyethylene carbonate of the
type with trade name of QPAC-25 are the most widely used binders in
ceramic and powder metallurgical processing. [0021] (e). Stearic
acid or zinc stearate has binding and lubricating properties with
powder compaction processes.
[0022] The binders (a), (b) and (c) belong to the specific group
that contains silica (SiO.sub.2) either as added or as the product
of binder removal. Both forms of silica, fumed and colloidal, and
ethyl silicate have unique properties particularly attractive to
silicon powder processing. Apart from their binder properties,
their cation silicon is the same as the material processed, its
anion oxygen helps to form Si-O-Si type of bonds in the process and
also reacts with the silicon at high temperature to form volatile
SiO, and thus be removed. Because the cation content of these
binders is the same as the element silicon that is intended to be
processed, these are the most ideal and preferred binding
additive.
[0023] The additive materials (d) and (e) are used because such
binders are easy to remove, leave no or very little residues in the
completed process and provide a basis to conserve the purity of the
processed silicon compact.
[0024] Variations of these described binders are recognized in
terms of the chemical family and form of such additives, and
appropriate changes in the complete process of making the silicon
compacts. Such variations should be apparent to those skilled in
the art of powder metallurgical and ceramic materials
processing.
[0025] In an embodiment of the method of producing densified and
robust silicon compacts the ultra fine silicon powder is
transferred into a clean feed hopper attached to a blending system
where it is blended with the appropriate binder. After an optional
drying step, depending upon the binder used, the blended and dried
mix is conveyed to a batch compacting machine, such as pellet press
or tablet press. By design such machines are to be of high quality
to handle high purity materials. Controlled quantities of the
powder are fed into the die by use of an appropriate powder feeder.
Special high purity powder feeder may be required. The powder is
pressed by the punch with a press force of several tons. The
pressed compact in the form of pellet or tablet is ejected into a
clean collection bin and/or transferred into a conveyor system to
transport to the next stage. The latter itself may be a sintering
furnace if the binder is either fumed silica or zinc stearate, or
to a de-binder furnace if the binder is one of the following:
colloidal silica, ethyl silicate, polypropylene carbonate or
stearic acid. The product from the de-binder furnace is transported
to the sintering furnace. The sintered compacts are transferred to
a lined storage or shipping container.
[0026] The powder compacting machinery can be semiautomatic or
automated for control of operation. The compacting process
machinery 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.
[0027] The de-binding and sintering furnaces are of the
conventional type suitable for the temperature and thermal
requirements and with provisions for operation in inert gases, such
as argon or helium, or in reducing gas such as hydrogen or in
vacuum. The process load carriers are to be high purity silica
boats and trays or such refractory containers lined with silicon
sheets.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a general flow sheet for compaction of silicon
powder.
[0029] FIG. 2 gives some example shapes of the compacted silicon
product.
[0030] FIG. 3 is a process flow sheet for silicon powder compaction
with combined de-binder and sinter operation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0031] A process flow sheet of converting silicon powder to
compacted and densified silicon shapes is described in FIG. 1.
[0032] The invention is amenable to many embodiments. In a
preferred embodiment, we utilize fine silicon powder of median
particle size about 5 micrometers, bulk density 0.5 g/cc (grams per
cubic centimeter) and convert them into cylindrical compact shapes
or pellets or tablets of nominal sizes, for example 10 mm diameter
by 20 mm length. The compacting or pellet/tablet pressing may be
done on a clean multi-station press machine with compression force
capacity of up to 25 tons.
[0033] The actual shape and size of the compact are not critical.
In the process, preferably, a precise quantity by weight of the
blended silicon powder and binder is fed into the compacting die as
a unit charge, and compressed by a matching punch to the required
force to achieve the predesigned dimensions. Alternatively, the
process may be operated on the basis of compressing a precise
charge by volume of powder. The compaction of the precise charge
may be performed, to a pre-determined final pressure, whether by
calculation or trial and testing to achieve the desired result.
[0034] The compressed compact is ejected from the machine through
the take-off system. The silicon compact provide a bulk material
form of silicon for further operations of de-binding and sintering
that provide for binder removal and at the same time add
densification and strength to the compacted form.
[0035] The term "compact" is herein inclusive of any form factor
and a descriptive term that implies a compacted small volume of the
raw powder material. Its shape may include cylindrical or
square/rectangular block, rods, disks, flats, slabs, wafers, etc.
and sizes that are practical for process machinery and handling
(FIG. 2).
[0036] The invention covers the utilization of the compacted and
densified dry silicon as feed material by different industries.
Silicon compacts of high purity are intended as feedstock to
photovoltaic materials industry to make high purity silicon
crystals by various means. Silicon compacts of nominal purity are
intended for auxiliary ferrosilicon, aluminosilicon and other alloy
manufacturing operations.
[0037] The basic steps of a preferred method for making high purity
silicon compacts is as follows: providing a source of high purity
silicon powder, feeding the powder into a blender, and mixing with
appropriate binder, providing an in-situ drying if desired,
discharging the powder into a hopper, feeding a controlled amount
by weight or volume of the powder into a die, compacting the powder
with pressure, exclusive of any local additive or lubricating
agents, and then discharging the dry compact from the die. The
machinery may be configured to operate multiple lines of multiple
dies, to meet high volume requirements. Additionally, the parts of
the machinery that come into contact with the high purity silicon
powder and compact may be provided with protective coating to
eliminate contamination from the machinery.
[0038] Additional steps of processing the compacted shapes are:
providing an inert flowing gas environment and temperature of
250-500 C. to de-binder the formed compact and providing an inert,
reducing or vacuum environment and temperatures of 1000-1350 C. to
effect densification and strength to the compact and further remove
any binder-related residues.
[0039] The further steps of especially making high purity silicon
ingots from the sintered silicon compacts is conventional and known
to those familiar with the art of crystal growth. The crystal
growth processes include methods such as Czochralski (CZ), Edge
defined Film Growth (EFG), Heat Exchanger Method (HEM), or
other.
EXAMPLES
Example 1
[0040] High purity Silicon powder is mixed with high purity fumed
silica as a binder. Typically, the fumed silica is in the range
0.01-5 weight percent of the silicon powder, preferably in the
range 0.05-0.2 weight percent. When added to the silicon powders,
fumed silica aids powder flow, by forming a layer on the silicon
surface and acts like a lubricant, aiding flow and compression. Due
to the hydrophilic nature of the fumed silica it absorbs water off
the surface of the particles and prevents caking. The mix is well
blended, then formed into compacts or pellets/tablets of required
shape. The compacted shape is then sintered in an inert gas or
reducing gas such as hydrogen in inert gas or vacuum environment at
1000-1350 C. to produce the compacted densified final product.
[0041] During the sintering operation the fumed silica binder
reacts with the silicon matrix to form SiO gas, which vaporizes
from the compact. The residual oxygen in the sintered silicon
compact is expected to be only the saturation solubility of oxygen
in solid silicon (=20 ppm).
Example 2
[0042] High purity Silicon powder is mixed with high purity
colloidal silica as a binder. The high purity colloidal silica is
nominally 40-50% by weight SiO.sub.2 in isopropyl alcohol or
toluene. Typically, the colloidal silica is in the range 0.01-5
weight percent of the silicon powder, preferably in the range
0.05-0.2 weight percent. When added to the silicon powders,
colloidal silica aids powder agglomeration and particle bonding.
The mix is well blended, then dried to remove essentially all
carrier solvent, then formed into compacts or pellets/tablets of
required shape. The compacted shape is then sintered in an inert
gas or reducing gas such as hydrogen in inert gas or vacuum
environment at 1000-1350 C. to produce the compacted densified
final product.
[0043] During the run up to the sintering temperature any remaining
carrier solvent is removed from the compact (FIG. 3). During
sintering the silica content of the binder reacts with the silicon
matrix to form SiO gas, which vaporizes from the compact. The
residual oxygen in the sintered silicon compact is expected to be
only the saturation solubility of oxygen in solid silicon (=20
ppm).
Example 3
[0044] High purity Silicon powder is mixed with high purity ethyl
silicate 40 (polydiethoxysiloxane with 40% SiO.sub.2) as a binder.
Typically, the ethyl silicate 40 is in the range 0.01-5 weight
percent of the silicon powder, preferably in the range 0.05-0.5
weight percent. The mix is well blended, then formed into compacts
or pellets/tablets of required shape. The use of ethyl silicate 40
binder requires a de-binder step prior to sintering. Ethyl silicate
40 decomposes completely at >300 C. to silica and ethyl alcohol.
The latter boils off the compacted body without any significant
reaction with silicon.
[0045] After binder removal the compacted shape is then sintered in
an inert gas or reducing gas such as hydrogen in inert gas or
vacuum environment at 1000-1350 C. to produce the compacted
densified final product. During the sintering step all volatile
decomposition products of ethyl silicate 40 will be released
completely from the compact. The silica will react with silicon to
form silicon monoxide, SiO, which volatilizes off from the compact.
The sintered silicon compact may have only very low levels of
carbon and oxygen from the binder incorporated in it (of the order
of 20 ppm each).
Example 4
[0046] High purity Silicon powder is mixed with high purity
polypropylene carbonate (QPAC-40) as a binder. Typically, the
polypropylene carbonate is in the range 0.01-5 weight percent of
the silicon powder, preferably in the range 0.05-1 weight percent.
The polypropylene carbonate itself is used as a solution dissolved
in solvents of the type acetone, methyl ethyl ketone, etc. The
concentration of polypropylene carbonate in the solution is in the
range 1-25% based on weight, and preferably 10-20%.
[0047] The mix is well blended, dried and then formed into compacts
or pellets/tablets of required shape. The use of polypropylene
carbonate binders usually results in higher green strength in
compacted bodies. Use of such a binder requires a de-binder step
prior to sintering. Polypropylene carbonate binders decompose
completely in air below 250 C., at temperatures at least 100 C.
less than conventional binders. Complete burnout in nitrogen and
argon and reducing atmospheres that contain hydrogen is possible at
temperatures as low as 300 C., and under vacuum, Polypropylene
carbonate burns out as carbon dioxide and water vapor. At the low
temperatures of binder removal these products do not react at all
significantly with silicon.
[0048] After binder removal the compacted shape is then sintered in
an inert gas or reducing gas such as hydrogen in inert gas or
vacuum environment at 1000-1350 C. to produce the compacted
densified final product. During the sintering step all
decomposition products of polypropylene carbonate will be released
completely from the compact. The sintered silicon compact may have
only very low levels of carbon and oxygen from the binder
incorporated in it (of the order of 20 ppm each).
Example 5
[0049] High purity Silicon powder is mixed with high purity stearic
acid or zinc stearate as a binder. Typically, the stearic acid or
zinc stearate is in the range 0.01-5 weight percent of the silicon
powder, preferably in the range 0.05-0.2 weight percent. When added
to the silicon powders, stearic acid or zinc stearate acts as a
binder and like a lubricant in the subsequent compacting
process.
[0050] The mix is well blended, then formed into compacts or
pellets/tablets of required shape. The compacted shape is then
sintered in an inert gas or reducing gas such as hydrogen in inert
gas or vacuum environment at 1000-1350 C. to produce the compacted
densified final product.
[0051] Use of stearic acid as a binder requires a de-binder step
prior to sintering. Use of zinc stearate as a binder may avoid a
separate de-binding step. During the sintering operation zinc
stearate decomposes to zinc oxide and organic byproducts. The
latter decomposes to volatile products at temperatures <500 C.
The zinc oxide vaporizes at the sintering temperatures. Any
residual zinc oxide will be reduced to zinc by silicon at the high
temperature of sintering. Residual zinc will also be removed in
subsequent melting processes, if used. The solubility of zinc in
silicon is estimated to be .about.6 ppm by weight at 1300 C. Zinc
has also a decontamination factor of 100,000
(C.sub.melt/C.sub.solid) in the melting and crystallization
process.
[0052] Other and various embodiments will be evident to those
skilled in the art, from the specification, abstract, and claims
that follow.
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