U.S. patent application number 15/899692 was filed with the patent office on 2019-03-07 for b2f4 manufacturing process.
The applicant listed for this patent is Entegris, Inc.. Invention is credited to Oleg Byl, Edward E. Jones, Chiranjeevi Pydi, Joseph D. Sweeney.
Application Number | 20190071313 15/899692 |
Document ID | / |
Family ID | 48082344 |
Filed Date | 2019-03-07 |
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United States Patent
Application |
20190071313 |
Kind Code |
A1 |
Byl; Oleg ; et al. |
March 7, 2019 |
B2F4 MANUFACTURING PROCESS
Abstract
A reaction system and method for preparing compounds or
intermediates from solid reactant materials is provided. In a
specific aspect, a reaction system and methods are provided for
preparation of boron-containing precursor compounds useful as
precursors for ion implantation of boron in substrates. In another
specific aspect, a reactor system and methods are provided for
manufacture of boron precursors such as B.sub.2F.sub.4.
Inventors: |
Byl; Oleg; (Southbury,
CT) ; Jones; Edward E.; (Woodbury, CT) ; Pydi;
Chiranjeevi; (Danbury, CT) ; Sweeney; Joseph D.;
(New Milford, CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Entegris, Inc. |
Billerica |
MA |
US |
|
|
Family ID: |
48082344 |
Appl. No.: |
15/899692 |
Filed: |
February 20, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14350543 |
Apr 8, 2014 |
9938156 |
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PCT/US2012/059357 |
Oct 9, 2012 |
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15899692 |
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61545546 |
Oct 10, 2011 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01J 19/129 20130101;
B01J 2219/00148 20130101; B01J 19/0073 20130101; B01J 15/00
20130101; B01J 2219/0254 20130101; B01J 2219/0886 20130101; B01J
2219/0277 20130101; B01J 7/00 20130101; B01J 2219/00094 20130101;
B01J 3/03 20130101; Y02P 20/149 20151101; C01B 35/061 20130101;
Y02P 20/141 20151101; B01J 2219/0272 20130101 |
International
Class: |
C01B 35/06 20060101
C01B035/06; B01J 7/00 20060101 B01J007/00; B01J 3/03 20060101
B01J003/03; B01J 19/12 20060101 B01J019/12; B01J 19/00 20060101
B01J019/00; B01J 15/00 20060101 B01J015/00 |
Claims
1.-31. (canceled)
32. A method of forming B.sub.2F.sub.4 comprising: reacting
BF.sub.3 gas and boron-containing solids to form a first gaseous
mixture comprising BF and unreacted BF.sub.3 gas; cooling the first
gaseous mixture under temperature and pressure conditions effective
to condense the first gaseous mixture to form a first condensed
product; volatilizing the first condensed product to form a second
gaseous mixture comprising B.sub.2F.sub.4 and BF.sub.3; cooling the
second gaseous mixture under temperature and pressure conditions
effective to condense the second gaseous mixture to form a second
condensed product and a third gaseous mixture comprising
B.sub.2F.sub.4 and BF.sub.3; filtering the third gaseous mixture to
remove particulates; and recovering B.sub.2F.sub.4 from the third
gaseous mixture.
33. The method of claim 32, further comprising, when the second
condensed product comprises unwanted B.sub.xF.sub.y species,
removing the B.sub.xF.sub.y species from the second condensed
product by reacting the B.sub.xF.sub.y species with an active
compound selected from XeF.sub.2, F.sub.2, NF.sub.3, O.sub.2 or
O.sub.3, to form a residue, and mechanically removing the
residue.
34. The method of claim 32, further comprising, when the second
condensed product comprises B.sub.xF.sub.y species, removing the
B.sub.xF.sub.y species from the second condensed product by
reacting the B.sub.xF.sub.y species with an active compound
selected from XeF.sub.2, F.sub.2, NF.sub.3, O.sub.2 or O.sub.3, to
form a residue, and reacting the residue with a halogen-containing
agent to volatilize the residue.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] The benefit of U.S. Provisional Patent Application No.
61/545,546 filed on Oct. 10, 2011 is hereby claimed under the
provisions of 35 USC 119. The disclosure of U.S. Provisional Patent
Application No. 61/545,546 is hereby incorporated herein by
reference, in its entirety, for all purposes.
FIELD
[0002] The present disclosure relates to a reaction system and
method for preparing compounds or intermediates from solid reactant
materials. In a specific aspect, the disclosure relates to a
reaction system and methods of such type for preparation of
boron-containing precursor compounds useful as precursors for ion
implantation of boron in substrates.
DESCRIPTION OF THE RELATED ART
[0003] Ion implantation is utilized extensively in the manufacture
of microelectronic device products and in other industrial
applications. In the ion implantation process, a chemical species
is deposited in a substrate by impingement of energetic ions on the
substrate. To produce the desired ions, a precursor is required
that is susceptible to ionization resulting in an ionized medium
that may variously include precursor fragments, positive ions,
negative ions, and recombinant ionic and non-ionic species. This
ionized medium is processed by extraction, magnetic filtering,
acceleration/deceleration, analyzer magnet processing, collimation,
scanning and magnetic correction to produce the final ion beam of
the desired type of ions that is impinged on the substrate.
[0004] Precursors of widely varying type are utilized to form
correspondingly varied implanted materials and devices.
Illustrative precursors include argon, oxygen, hydrogen, and
hydrides and halides of dopant elements such as arsenic,
phosphorus, germanium, boron, silicon, etc. Boron in particular is
a very widely used dopant element, and in recent years attention
has been focused on increasing the efficiency and utilization of
existing boron precursors and developing new ones.
[0005] One of the main steps in manufacturing of many integrated
circuits involves implantation of boron into silicon wafers. Since
elemental boron exhibits very low vapor pressure even at high
temperatures, utilization of volatile boron-containing precursor
compounds is necessary. Currently, boron trifluoride (BF.sub.3) is
widely used as a precursor for boron implantation. In 2007,
worldwide consumption of BF.sub.3 for ion implantation was
estimated to be on the order of .about.3000 kg, and this volume has
continued to grow.
[0006] Despite its widespread utilization, BF.sub.3 does have
disadvantages. The BF.sub.3 molecule is very difficult to ionize
and only about 15% of all BF.sub.3 flowed into the ion source
chamber of conventional ionizers can be fragmented. The rest is
discarded. Further, only about 30% of the ionized BF.sub.3 is
converted into B.sup.+ ions that can be used for implantation. This
results in low B.sup.+ beam current that severely limits
implantation process throughput.
[0007] Some increase of B.sup.+ beam current can be achieved by
varying the process parameters, such as by raising the extraction
current, and by increasing the BF.sub.3 flow rate. These measures,
however, result in reduced life time of the ion source, high
voltage arcing that in turn leads to tool instability, poor vacuum
and beam energy contamination.
[0008] Throughput limitations associated with low B.sup.+ beam
current in the use of BF.sub.3 have become more important in the
semiconductor manufacturing industry in recent years due to the
general trend in such industry to utilization of lower implantation
energies. At lower implantation energies, the B.sup.+ beam
experiences a greater blow-out effect due to space charge.
[0009] A high-volume manufacturing capability for alternative boron
precursors that are reliable and cost-effective in character would
therefore provide a major contribution to the art of semiconductor
manufacturing as well as other ion implantation applications in
which boron doping is employed.
SUMMARY
[0010] The present disclosure relates to a reactor system and
methods for manufacture of boron precursors such as
B.sub.2F.sub.4.
[0011] In one aspect, the disclosure relates to a reactor system
comprising a reaction zone for contacting a gaseous reagent with a
solid material under temperature and pressure conditions effective
to form an intermediate species; an opening for allowing an
unreacted portion of the gaseous reagent and the intermediate
species to exit the reaction zone into a condensation zone; and a
seal surrounding the opening, the seal sealingly connecting the
reaction zone to the condensation zone.
[0012] In another aspect, the disclosure relates to a reactor
system comprising a reaction zone for contacting B.sub.3 gas with
boron-containing solids under temperature and pressure conditions
effective to form an intermediate species; an opening for allowing
an unreacted portion of BF.sub.3 gas and the intermediate species
to exit the reaction zone into a condensation zone for effecting
reaction between the intermediate species and the unreacted portion
of the BF.sub.3 gas to form a reaction product comprising
B.sub.2F.sub.4; a recovery zone for recovering the reaction product
and unreacted BF.sub.3 gas; and a recycling zone for recycling the
recovered unreacted BF.sub.3 gas to the reaction zone.
[0013] In a further aspect, the disclosure relates to a method of
forming B.sub.2F.sub.4 comprising reacting BF.sub.3 gas and
boron-containing solids to form a first gaseous mixture comprising
BF and unreacted BF.sub.3 gas; cooling the first gaseous mixture
under temperature and pressure conditions effective to condense the
first gaseous mixture to form a condensed product; volatilizing the
condensed product to form a second gaseous mixture comprising
B.sub.2F.sub.4 and BF.sub.3; filtering the second gaseous mixture
to remove particulates; and recovering B.sub.2F.sub.4 from the
second gaseous mixture.
[0014] In a further aspect, the disclosure relates to a method of
forming B.sub.2F.sub.4 comprising reacting BF.sub.3 gas and
boron-containing solids to form a first gaseous mixture comprising
BF and unreacted BF.sub.3 gas; cooling the first gaseous mixture
under temperature and pressure conditions effective to condense the
first gaseous mixture to form a first condensed product;
[0015] volatilizing the first condensed product to form a second
gaseous mixture comprising B.sub.2F.sub.4 and BF.sub.3; cooling the
second gaseous mixture under temperature and pressure conditions
effective to condense the second gaseous mixture to form a second
condensed product and a third gaseous mixture comprising
B.sub.2F.sub.4 and BF.sub.3; filtering the third gaseous mixture to
remove particulates; and recovering B.sub.2F.sub.4 from the third
gaseous mixture.
[0016] In a further aspect, the disclosure relates to an apparatus
for production of B.sub.2F.sub.4, comprising a reactor containing a
boron reactant that is reactive with boron trifluoride, BF.sub.3,
to yield boron fluoride, BF, wherein said reactor is configured to
provide process conditions effective for reaction of BF.sub.3 and
said boron reactant to yield BF as a reaction product, and to
discharge BF and unreacted BF.sub.3 as a reactor effluent; a source
of BF.sub.3 arranged to supply BF.sub.3 to the reactor; a
condensation zone configured to receive the reactor effluent from
the reactor and to provide process conditions effective for
condensation of BF and BF.sub.3 to yield B.sub.2F.sub.4; and a
recirculation loop for flowing to the reactor unreacted BF.sub.3
recoverable from the condensation, wherein the recirculation loop
comprises a purification unit configured to purify recirculated
BF.sub.3 flowed to the reactor.
[0017] Other aspects, features and embodiments of the disclosure
will be more fully apparent from the ensuing disclosure and
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a perspective view of a reactor system according
to one embodiment of the disclosure.
[0019] FIG. 2 is a perspective view of the reactor system according
to the embodiment of FIG. 1.
[0020] FIG. 3 is a perspective view of an installed reactor
system.
[0021] FIG. 4 is a cross-sectional view of a reactor system
according to the embodiment of FIG. 2.
[0022] FIG. 5 is a magnified view of the split flange area of the
reactor system illustrated in FIG. 4.
[0023] FIG. 6 is a perspective view of an illustrative trap
configuration according to an embodiment of the disclosure.
[0024] FIGS. 7A and 7B are process diagrams of integrated reactor
systems for reacting solid reactants with fluids, according to an
embodiment of the disclosure.
DETAILED DESCRIPTION
[0025] The present disclosure relates to a reactor system and
methods for producing intermediate and final products from reaction
of fluids with solid reactant materials. In a specific aspect, the
present disclosure relates to a reactor system and methods for the
manufacture of boron precursors such as B.sub.2F.sub.4.
[0026] In a more specific aspect, the present disclosure provides a
reaction system and manufacturing process for B.sub.2F.sub.4.
Diboron tetrafluoride is a liquefiable gas, having a melting point
of -56.degree. C. and a normal boiling point of -34.degree. C., and
a vapor pressure at 21.degree. C. of 9.54 bar.
[0027] In a specific aspect of the present disclosure, diboron
tetrafluoride (I3.sub.2F.sub.4) is a precursor compound that
provides advantages over the traditional boron trifluoride
precursor, in part because of the nature of the boron-boron
chemical bond, which makes B.sub.2F.sub.4 significantly easier lo
dissociate and ionize than boron trifluoride. In consequence, the
use of diboron tetrafluoride enables significantly higher beam
currents to be achieved. Atomic mass unit (AMU) magnet selection
can be utilized to select the same ions for implantation (1.sup.1B
or .sup.11BF.sub.2) as are selected when boron trifluoride is used
as the precursor in a same ion implant tool. In addition, diboron
tetrafluoride can be supplied in enhanced safety vessels, e.g.,
pressure-regulated supply vessels such as those commercially
available under the trademark VAC from ATMI, Inc. (Danbury, Conn.,
USA), or in a sorbent-containing supply vessels in which the
sorbent serves as a storage medium for diboron tetrafluoride.
[0028] To produce diboron tetrafluoride in accordance with the
present disclosure, boron or a boron-containing compound is
contacted with BF.sub.3 at elevated temperature to generate BF as
an intermediate. In specific embodiments of this arrangement, the
boron trifluoride is passed through a bed of boron-containing
solids with the reaction zone at elevated temperature up to
2200.degree. C., e.g., a temperature in a range of from
I000.degree. C. to 2200.degree. C. Sub-ranges of temperature within
such broad range of I000.degree. C. to 2200.degree. C. that may be
usefully employed in specific applications include sub-ranges in
which the lower temperature limit of the sub-range may have any
suitable value, e.g., 1000.degree. C., 1050.degree. C.,
1100.degree. C., 1150.degree. C., 1200.degree. C., 1250.degree. C.,
1300.degree. C., 1350.degree. C., 1400.degree. C., 1450.degree. C.,
1500.degree. C., 1550.degree. C., 1600.degree. C., 1650.degree. C.,
1700.degree. C., 1750.degree. C., 1800.degree. C., 1850.degree. C.,
1900.degree. C., 1950.degree. C., 2000.degree. C., 2050.degree. C.,
2100.degree. C. or 2150.degree. C., and wherein the upper limit of
such sub-range has a value that is greater than the lower limit of
the sub-range, e.g., an upper temperature limit of 1050.degree. C.,
1100.degree. C., 1150.degree. C., 1200.degree. C., 1250.degree. C.,
1300.degree. 7, 1350.degree. C., 1400.degree. C., 1450.degree. C.,
15007, 1550.degree. C., 1600.degree. C., 1650.degree. C.,
1700.degree. C., 1750.degree. C., 1800.degree. C., 1850.degree. C.,
1900.degree. C., 1950.degree. C., 2000.degree. C., 2050.degree. C.,
2100.degree. C., 2150.degree. C. or 2200.degree. C. in specific
embodiments. Still other temperature ranges can be employed in the
broad practice of the disclosure to produce BF, or other
temperature ranges of suitable character can be utilized when the
reactor system and methods herein disclosed are employed in the
production of other intermediates and final products.
[0029] The present disclosure contemplates thermal control of the
reaction of the boron-containing compound and BF.sub.3 gas in an
integrated assembly in a reaction zone. Such control of temperature
regulates the change of temperature in the reaction zone and is
measured or otherwise detected such that the passage of heat energy
into or out of the reaction zone is adjusted to achieve a desired
average temperature. In particular, the temperature conditions in
the reaction zone are thermally controlled to a predetermined
temperature range.
[0030] Such thermal control may be accomplished through the use of
thermocouples, thermostatic sensors, pyrometric sensors, or other
devices that are adapted to sense or monitor temperature, in
combination with heating and cooling equipment, processors, and
CPUs and/or other controllers operatively linked with the heating
and cooling equipment. Such system may be operated so that a
temperature sensor generates a temperature sensing signal, which is
conveyed to the processor, which in turn actuates a heater or a
cooling device as necessary to maintain a desired set point
temperature.
[0031] Boron monofluoride is a B.sub.2F.sub.4 precursor that is
generated in a reaction zone or high temperature part of a reactor.
A simple temporal temperature profile may be used which is intended
(i) to elevate the temperature of the reaction zone to a certain
level for the BF.sub.3+2B=3BF reaction to start and proceed in a
controlled way, and (ii) to prevent thermal shock to reactor
components due to high rate of temperature change.
[0032] In other embodiments, other temporal temperature profiles
may offer certain advantages. For instance, an initial temperature
ramp may be linear, step-wise, exponential or any other profile
intended to increase the temperature of the reaction zone in a
sufficiently rapid fashion. The temperature profile may be
maintained as a linear profile or it may be of other shape, for
example, to increase utilization of boron solids as they become
depleted in the course of reaction. The temperature profile may
also be synchronized with the BF.sub.3 flow profile. The
temperature ramp-down may be linear, step-wise, exponential or of
any other shape that is optimized for fast cooling of the high
temperature area without causing thermal shock to reactor
components.
[0033] It also may be possible to optimize the spatial temperature
profile to maximize BF production. For example, by using more than
one radio frequency (RF) heating coil, such profile may be tuned to
constitute the reaction zone in a certain volume of the reaction
cavity. By using constructive interference of a few RF coils, the
reaction zone can be migrated, changed in size or shape or
modulated to improve production of BF. This also may be enhanced by
mechanical movement of the reaction zone.
[0034] Pressure in the reaction of boron-containing solids and
boron trifluoride, in specific embodiments, can be at any suitable
value, e.g., pressure in a range of from 10 -6 to 1,000 torr.
[0035] The boron-containing solids utilized for the formation of
intermediate BF can be of any suitable size and shape
characteristics, e.g., size and shape characteristics that permit
the solids to be loaded into the reaction zone to which BF.sub.3 is
delivered, so that the reactive solids in the reaction zone are
contacted sufficiently with boron trifluoride to produce boron
fluoride (BF) intermediate in a desired quantity.
[0036] Consistent with the preceding discussion, the reactive solid
used in the broad practice of the present disclosure may be
provided in any size particles or discontinuous forms, including
powder, granules, pellets, platelets, solid films on substrate
carrier particles, etc. In particular, optimized particle size
distribution including uniform, normal, bi-, tri- and multimodal
distributions may be used for better compacting of the
boron-containing solids in the reaction zone. Such compacting
results in prolonged reaction and, thus, higher B.sub.2F.sub.4
throughput. In addition, multimodal particle size distributions can
be employed, to reduce void volume in the bed of solid reactive
particles, and to maximize the amount of reactive solid per unit
volume of the casing in which the bed of solid reactive particles
is reposed. Solid particle size and shape may be optimized for
better thermal uniformity of the particle bed and/or better gas
flow behavior through the bed.
[0037] In one aspect of the disclosure, the boron-containing solids
are engineered to expose lattice planes which are more reactive
toward BF.sub.3 to increase yield and throughput. For instance, if
a (lmn) lattice plane is found to have higher reaction rate in BF
generation, use of monocrystals of boron-containing material with
high fraction of surface area of the (lmn) plane can be beneficial.
Alternatively, crystallites with low fraction of the beneficial
(lmn) lattice plane can be arranged into a macro structure to hide
less reactive lattice planes and preferentially expose the more
reactive one(s).
[0038] The reactive solid used in the higher temperature reaction
zone can be of any appropriate type. In the production of boron
compounds such as diboron tetrafluoride, solid boron has been
illustratively described as a suitable solid reactant. In other
embodiments, it may be desirable to use reactive solids other than
boron metal for the contacting of BF.sub.3. For example, the boron
can be present in any suitable boron-containing compound. In a
preferred aspect of the present disclosure, the boron-containing
compound will be one wherein the boron atoms are not in the highest
oxidation state. In one aspect of the invention, the following
boron-containing solids or their mixtures may be used for more
effective generation of BF in reaction with BF.sub.3: (i)
boron-containing compounds with stoichiometric formulae, for
example, such as M.sub.4B, M.sub.3R, M.sub.5B.sub.2,
M.sub.7B.sub.3, M.sub.2B, M.sub.5B.sub.3, M.sub.3B.sub.2,
M.sub.11B.sub.5, MB, M.sub.10B.sub.11, M.sub.3B.sub.4,
M.sub.2B.sub.3, M.sub.3B.sub.5, MB.sub.2, M.sub.2B.sub.5, MB.sub.3,
MB.sub.4, MB.sub.6, M.sub.2B.sub.13, MB.sub.10, MB.sub.12,
MB.sub.15, MB.sub.18, MB.sub.66, where M represents an element of
the periodic table with which a boride of certain stoichiometry
exists, e.g., B.sub.4C and C (carbon); or (ii) boron-containing
compounds with non-stoichiometric formulae. Such boron-containing
solids may also be used in mixtures of any two or more
boron-containing compounds, wherein each compound in the mixture is
independently selected. Mixtures of the compounds within the
particle bed may be homogeneous or stratified for optimal reaction
design and control.
[0039] In one aspect of the disclosure, it may be desirable to
match compositions of the reaction zone with the composition of the
boron-containing solids. In particular, boron carbides such as
B.sub.4C may offer an additional benefit of not introducing an
additional element into the reaction zone if the constituent parts
and components of the reaction zone are made of carbon or
graphite.
[0040] The boron-containing solids can for example have a diameter
or characteristic major dimension that is in a range of from 1 mm
up to 5 cm or more depending on the scale of the reactor apparatus
and the retention structure that is used to retain the solids in
the casing within the reaction cavity. Retention structures can be
of any suitable type or types, including for example screens,
grids, rods, offset plates, cups, metal or non-metal wool plugs,
etc. The boron-containing solids can be arranged in any suitable
manner that permits the BF intermediate to be discharged from the
reaction zone so that it is captured in the condensation zone.
[0041] After the boron-containing solids are contacted with
BF.sub.3 at elevated temperature to generate BF as an intermediate,
the BF intermediate and unreacted BF.sub.3 are discharged from the
reaction zone into a condensation zone where the BF intermediate
and the unreacted BF.sub.3 condense. The condensation zone, for
example, may comprise a cold trap, where the BF intermediate and
the unreacted BF.sub.3 condense on a cooled or chilled surface of
the cold trap, with BF reacting with BF.sub.3 to form
B.sub.2F.sub.4.
[0042] In one embodiment of the present disclosure, the reactor
system is utilized by placing the boron-containing solid reactant
inside a casing with openings on its surface, with the bottom of
the casing being provided with a retention structure that supports
the boron-containing solids and retains them in the interior volume
of the casing, while allowing flow of reactive gas through the
retention structure. The casing is inserted in a reaction cavity,
which may for example be in the shape of a hollow cylinder formed
of suitable material such as, for example, quartz or graphite. The
inner diameter of such cylinder is larger than the diameter of the
casing so that the inner cylinder surface is in spaced relationship
to the casing. In a specific arrangement, the casing and cylinder
can be arranged coaxially with respect to one another, so that an
annular space is formed between them. The cylinder and casing are
in direct flow communication with a condensation zone that is
cooled by a suitable coolant, such as liquid nitrogen or other heat
transfer coolant medium, or other refrigeration source.
[0043] In this arrangement, boron-containing solids can be loaded
in the casing so as to form a bed of boron-containing solids, with
the casing disposed in a reaction zone of cylindrical geometry.
This assembly can be suspended by a tube through which boron
trifluoride is delivered to the reaction zone, so that the boron
trifluoride reacts with the boron-containing solids in such
reaction zone, producing a boron fluoride (BF) intermediate.
[0044] The BF intermediate and the unreacted BF.sub.3 preferably is
discharged from the reaction zone into a condensation zone
directly. The BF intermediate and the unreacted BF.sub.3 in such
arrangement condense, for example, on a surface of a cold trap,
with BF reacting with BF.sub.3 to form B.sub.2F.sub.4. Once a
sufficient amount of BF, BF.sub.3 and B.sub.2F.sub.4, has
condensed, the reaction is stopped and the condensation zone is
brought to higher temperature to allow evaporation of reaction
products such as B.sub.2F.sub.4 and B.sub.xF.sub.y
(higher/polymeric boron fluoride species, wherein x and y have
stoichiometrically appropriate values) and unreacted BF.sub.3. The
B.sub.2F.sub.4-containing gas mixture then can be pumped out of the
condensation zone and subjected to recovery treatment such as
distillation to recover the B.sub.2F.sub.4, with BF.sub.3
concurrently being recovered and recycled back to the reactor or
subjected to other disposition or use.
[0045] Recycle of reaction product components from the condensation
zone may be useful in specific implementations of the disclosure.
For example, B.sub.xF.sub.y components of the reaction product
mixture from the lower temperature reaction zone can be subjected
to heating, to decompose same to form solid boron as a recovered
byproduct of the condensation zone reaction process.
[0046] In a specific embodiment, the boron-containing solids are
loaded in a cylindrical casing that may also be formed of graphite
or ceramics, or alternatively of another suitable material of
construction, and the casing then is deployed in a cylindrical
reaction cavity, preferably so that the casing is centered in the
cylindrical cavity, although other non-centered arrangements can
also be effected. The casing advantageously is provided with holes
in a lower section thereof, so that BF.sub.3 enters from the top of
the casing, passing through the boron-containing solids and
reaching the perforated section where the reaction cavity is
located. The resulting BF generated in the reaction zone from the
reaction of the BF.sub.3 and boron-containing reactants exits the
holes of the casing and then flows downwardly into the condensation
zone. Clogging at the bottom of the casing and reaction cavity is
reduced by the provision of a gap between the casing and reaction
cavity. An example is shown in FIG. 4, wherein the casing assembly
31 comprises solid reactant 32, a reaction cavity 33 and a casing
34.
[0047] The positioning of the reactor assembly including the casing
and the reaction cavity may employ a horizontal or vertical
orientation or an angular orientation at any angle between the
horizontal and vertical positions of the reactor assembly. The
shape of the openings in the perforated casing disposed in the
reaction cavity can be uniform or non-uniform in character, and may
be circular or may have other geometric shapes, consistent with the
objective of maintaining open pathways for flow of boron fluoride
and boron trifluoride out of the casing, while concurrently
effecting appropriate contact between boron trifluoride and the
boron-containing solids. The shape and/or size of the
boron-containing solids can be uniform or non-uniform in
character.
[0048] More generally, the characteristics of the openings in the
casing within the reaction cavity, and the morphology, shape,
crystallinity and size of the boron-containing solids deployed in
the reaction are parameters that can be individually or
combinatorially tailored to optimize the reaction between the
boron-containing solids and boron trifluoride.
[0049] The casing and/or reaction cavity can be advantageously
formed of metal, quartz, graphite or other carbonaccous material.
Graphite is preferred for the casing for such purpose because heat
can be readily resistively generated in such material, such as by
oscillating currents induced by a radio frequency (RF) field of an
electrical coil that is circumscribingly positioned in relation to
the reaction zone, for example, a zone comprising a cylindrical
cavity and the casing containing the reactive solids. The RF coil
provides a simple and effective arrangement for achieving the
elevated temperatures required for reaction of boron trifluoride
with the boron-containing solids in the casing holding such solids.
The casing as previously described may be foraminous in character,
being constructed of a porous sorbent permeable material, or
alternatively having one or more openings therein for egress of the
intermediate that is formed in the high temperature reaction
therein.
[0050] Concerning the use of graphite materials for the reactive
solid contacting zone through which reactant gas is flowed, and the
use of RF coils to resistively heat such graphite materials to
reaction temperatures, it will be appreciated that the coil
spacing, diameter, shape, use of multiple coils, etc. will
influence the geometry and strength of the exerted RF field. The RF
coils can be appropriately constructed and arranged to provide
effective axial and radial temperature profiles inside the elevated
temperature reaction zone, in order to achieve efficient production
of the BF intermediate as a reaction product of the reactive solid
and reactant gas in the reaction zone, within the skill of the art,
based on the disclosure herein.
[0051] Although graphite is preferred for construction of the boron
solids-containing casing, metals, ceramics or other high
temperature materials can be employed, provided that they withstand
temperatures of up to 2200.degree. C., and are inert to boron
trifluoride across the full temperature range of interest.
[0052] For example, the heat source for the high temperature of the
reaction zone can be adapted to maintain predetermined temperature
in the reaction zone by a heating modality comprising at least one
of conduction heating, induction heating, convective heating,
resistive heating, and radiation heating. The heat source in such
respect can be adapted to maintain temperature in the reaction zone
that is effective to prevent deposition and/or condensation of
material in the reaction region, e.g., temperature in the reaction
zone in a range of from 1000.degree. C. to 2200.degree. C.
[0053] The reactor system in one embodiment comprises three major
sections. As shown in FIG. 1, the reactor system 200 comprises a
top section 1 where the process and purge gases are supplied, a
middle section 2 comprising the reaction zone where the high
temperature reaction occurs, and a bottom section 3 comprising the
condensation zone where products are cooled and condensed at
cryogenic temperatures (for example, by liquid nitrogen). The
reactor system in this embodiment includes an induction heating
system that provides the energy for the high temperature reaction,
a cooling water system for thermal management, and a liquid
nitrogen dewar for cooling the reactants in a cryogenic temperature
range. The liquid nitrogen dewar has an associated hydraulic lift
to aid in the control of the temperature of the bottom section of
the reactor system. FIG. 2 and FIG. 3 illustrate this reactor
system.
[0054] As shown in FIG. 2, the top section of the reactor system
200 of this embodiment comprises a view port 11, a gas inlet
chamber 12, an adapter flange 13, and a transition chamber 14. The
middle section of the reactor system comprises a quartz jacket 19.
The middle section also comprises the casing assembly 31 shown in
FIG. 4. The bottom section of the reactor system comprises a split
flange 16, a bottom flange 17 and a cold trap 18.
[0055] As shown in FIG. 3, the installed reactor system including
reactor system 200 according to an embodiment of the disclosure
comprises a reactor 21, a reactor support 22, a liquid nitrogen
container 80, a hydraulic lift 90 and an induction heal station
23.
[0056] In an embodiment of the disclosure, a reactor system is
provided comprising a reaction zone for contacting a gaseous
reagent with a solid material under temperature and pressure
conditions effective to form an intermediate species, an opening
for allowing an unreacted portion of the gaseous reagent and the
intermediate species to exit the reaction zone into a condensation
zone, and a seal surrounding the opening sealingly connecting the
reaction zone to the condensation zone. In specific embodiments,
the opening may variously be constituted by a single opening, or
alternatively by a multiplicity of openings, to accommodate
discharge of the intermediate species and unreacted gaseous reagent
from the reaction region. The opening may be constituted by a lumen
or passage, or other structural arrangement enabling the specified
discharge of the intermediate species and unreacted gaseous
reagent, e.g., to a further reactor in which the intermediate
species and unreacted gaseous reagent are further reacted to form
final product.
[0057] The seal connecting the reaction zone with the condensation
zone is selected according to requirements such as chemical
compatibility, application temperature, sealing pressure, size and
cost. The seal should be capable of sealingly connecting the
reaction zone and the condensation zone under conditions of vacuum
or super-atmospheric pressures. The seal further will completely
surround the opening which allows an unreacted portion of the
gaseous reagent and the intermediate species to exit the reaction
zone.
[0058] The seal may be of any suitable size, shape and character
which provides a sufficient seal between the reaction zone and the
condensation zone under the pressure and temperature conditions
extant during operation of the reaction system. The seal, in one
embodiment, will be an O-ring. In a further embodiment, the seal
may comprise a perfluoroelastomer material.
[0059] In one embodiment, the reaction zone in the middle section
of the reactor system may comprise a reactor which is a double
walled quartz jacket that is water cooled. In the center of this
jacket is the casing or a crucible that is heated for the high
temperature reaction. The bottom section or condensation zone of
the reactor system is a vessel constructed of stainless steel and
is submersed in liquid nitrogen so that the reaction products will
condense and solidify on the walls.
[0060] By way of example, hydraulic lift 90 as shown in FIG. 3 may
be used to move the dewar or vessel containing the liquid nitrogen
or other material which is capable of cooling at cryogenic
temperatures up and down as required for cooling and condensing the
reactants in the condensation zone. This movement may be automated
according to predetermined requirements for effective cooling and
warming as needed. For example, the cryogenic temperature range is
controlled by use of a hydraulic lift configured to translate a
vessel containing material that can cool at cryogenic temperatures,
in a selected one of upward and downward directions, between an
uppermost position and a lowermost position.
[0061] In an embodiment wherein a double walled quartz jacket is
used for the reaction zone and a stainless steel cylinder is used
for the condensation zone, to maintain the seal between the quartz
and metal components under both vacuum and super-atmospheric
pressures, a perfluoroelastomer (FFKM) O-ring is used. To protect
this O-ring from both the high and low temperature extremes
associated with the reaction one or heated crucible and the
condensation zone, or cooled lower part of the reactor, a water
channel can be used to minimize the temperature extremes to which
the O-ring would otherwise be exposed. These temperature extremes
would otherwise reduce the life of the O-ring or prevent it from
maintaining the seal under all process conditions.
[0062] To physically maintain the compression on the O-ring, a
split flange constructed of a high performance engineering
thermoplastic may be used. The thermoplastic may for example
comprise 40% glass-filled polyphenylene sulfide, a bismaleimide,
polysulfone, or other high performance engineering thermoplastic.
FIG. 4 is a model cross-section view of a reactor system 200
according to an embodiment of the disclosure. FIG. 5 is an
enlargement of one section of the reactor system 200 of FIG. 4. As
shown, the o-ring 41 is disposed below split flange 16 on bottom
flange 17. In the magnification of FIG. 5, the O-ring 41, split
flange 16, bottom flange 17 and a cushion layer 42 are arranged
above cold trap 18 (FIG. 4) such that the compression on the O-ring
is maintained according to the requirements for operation of the
reactor system. Quartz jacket 19 is also shown in FIG. 4 and FIG.
5.
[0063] The composite material for the split flange was selected for
its ability to operate in the elevated temperature reaction zone
and not couple with the RF field generated by an induction heating
system. The bolts used for securing the split flange are brass, so
as to minimize the coupling effect as well. Between the split
flange and quartz jacket as well as between the brass bolts and the
split flange, a fluorocarbon, such as polytetrafluoroethylene, may
be used both as a cushion and to provide an additional layer of
thermal isolation. In one embodiment, nitrogen gas is blown through
channels in the split flange to provide cooling to the bolts as
well as to displace oxygen from the region directly around the
O-ring seal.
[0064] In an embodiment of the disclosure, the water channel allows
water to be circulated through the bottom flange. The temperature
of the water is controlled to .about.21.degree. C. using a heat
exchanger. During the high temperature step of the reaction the
water is used to draw heat out of the bottom flange and away from
the o-ring to protect it from being over heated. Overheating the
o-ring may cause damage. After the high temperature step when the
bottom of the reactor is still in liquid nitrogen, the water
channel is used to prevent the flange and more specifically the
o-ring from being over cooled. Overcooling the o-ring can cause it
to lose its integrity and form a leak.
[0065] In a separate embodiment, the reaction zone may comprise a
metal reactor, i.e., a reactor wherein all the materials of
construction are metal, such as the casing and reaction cavity. In
this embodiment, metal seals may be used and the heat source may be
located within the reaction zone. For example, the heat source can
be adapted to maintain predetermined temperature in the reaction
zone by a heating modality comprising at least one of conduction
heating, induction heating, convective heating, resistive heating,
and radiation heating. The heat source in such respect can be
adapted to maintain temperature in the reaction zone that is
effective to prevent deposition and/or condensation of material in
the reaction region, e.g., temperature in the reaction zone in a
range of from 1000.degree. C. to 2200.degree. C. The all-metal
reactor will be able to withstand high pressures and shock that may
occur due to the high temperature gradient that exists between the
reaction zone and condensation zone or from external factors, such
as system or process failures.
[0066] Downstream from the high temperature reactor in which boron
trifluoride and elemental boron or boron-containing solids are
reacted to form boron monofluoride, the condensation zone comprises
a lower temperature reactor. e.g., a cold trap, whose size, shape
and character are additional parameters that can be selectively
varied. These parameters may be adjusted to optimally achieve more
uniform deposits of material from the intermediate-containing
stream flowed from the higher temperature reaction zone to the
condensation zone, while maintaining suitable vacuum conditions in
the condensation zone. Where a cold trap is used, for example,
extended surface structures, e.g., cold fingers, fins, or the like,
can be provided in the cold trap to increase surface area for
condensation of material that is present in the
intermediate-containing stream flowed from the higher temperature
reaction zone to the cold trap.
[0067] In the condensation zone or lower temperature zone employed
for condensation of solid intermediate (BF), liquid nitrogen has
been described as an illustrative coolant. Other refrigerant media
can be employed in specific implementations of the disclosure,
including, without limitation, refrigerants such as liquid argon,
liquid oxygen, liquid helium, etc.
[0068] The cold trap can be periodically cleaned with water to
remove solid boron fluoride residues, e.g., higher/polylmeric boron
fluoride species of the formula B.sub.xF.sub.y, wherein x and y
have stoichiometrically appropriate values. In operation, the cold
trap can be cooled with liquid nitrogen or other suitable cryogen.
Temperature at the bottom of the cold trap can be on the order of
-196.degree. C. when liquid nitrogen is used as the coolant.
[0069] In one embodiment, the cold trap may also be operated to
pre-condense boron trifluoride therein, and thereby increase
production of diboron tetrafluoride when the
intermediate-containing reaction mixture is subsequently flowed to
the cold trap.
[0070] The reaction product mixture that is extracted or off-loaded
from the cold trap may contain 5%, 10%, 20%, 30%, 40%, 50% or
higher concentrations of diboron tetrafluoride, with the remainder
being boron trifluoride and trace amounts of volatile heavy boron
fluorides.
[0071] The reaction system of the present disclosure can be
deployed with various specific features and in various specific
arrangements for high-efficiency continuous operation. For example,
instead of a reaction zone containing a single reaction cavity and
a single casing loaded with boron-containing solids, high
temperature reactor assemblies can be provided with multiple,
independent reaction cavities and multiple casings that can be
operated, maintained, and refilled independently of one
another.
[0072] Considering the condensation zone used to produce diboron
tetrafluoride by reaction of boron monofluoride and boron
trifluoride, such zone can be provided as a cold trap comprising a
sufficiently cooled surface element, such as a plate member or a
plate member with extended surface, which is conveyed into a cavity
of the cold trap to receive and condense BF, BF.sub.3 and
B.sub.2F.sub.4. Once such receiving plate member is sufficiently
coated with condensed material, it is conveyed out of the
condensation zone and replaced with a fresh receiving plate member,
e.g., by a suitably configured conveyor or other transport
system.
[0073] In another arrangement, the high temperature assembly can be
positioned over a low-temperature compartmented cold trap within a
reactor system. Such assembly can be translated in any suitable
manner to different compartments of the reactor system, with the
specific cold trap compartment being arranged for deposition of BF,
BF.sub.3 and B.sub.2F.sub.4 in an isolated state relative to the
rest of the reactor (e.g., with the rest of the reactor undergoing
cleaning or "off-loading" of condensed products that have already
been deposited in other cold trap compartments).
[0074] After the B.sub.2F.sub.4 product is formed in the
condensation zone, the next step in B.sub.2F.sub.4 production is
extraction of a B.sub.2F.sub.4-containing gas mixture from the
reactor for further distillation purification and recovery of
B.sub.2F.sub.4.
[0075] More generally, as contemplated by the present disclosure,
boron-containing compounds of widely varied types can be
synthesized utilizing boron-containing solids with which reactant
gases are contacted, and can include any number of boron atoms. In
one embodiment, the boron-containing compound contains at least two
boron atoms. In another embodiment, the boron-containing compound
contains from 2 to 80 boron atoms, including diboron compounds such
as B.sub.2F.sub.4, triboron compounds, tetraboron compounds such as
(F.sub.2B).sub.3BCO, pentaboron compounds, hexaboron compounds,
septaboron compounds, octaboron compounds, nonaboron compounds,
decaboron compounds, undecaboron compounds, dodecaboron compounds,
etc., up to B80 compounds such as B80 analogs of fullerenes.
[0076] In other embodiments, the boron-containing compound can
contain 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11 boron atoms. Additional
embodiments may comprise cluster boron compounds. In still other
embodiments, the boron-containing compound can be a diboron
compound. In other embodiments, the boron-containing compound can
be defined in terms excluding certain compounds, e.g., as
comprising diboron compounds other than diborane. It will therefore
be appreciated that the present disclosure contemplates a wide
variety of classes of boron-containing compounds, within the broad
scope of the disclosure, that may be variously specified, in
inclusive or alternatively in exclusionary specification
thereof.
[0077] According to an embodiment of the present disclosure, a
recovery zone may be deployed downstream from the condensation zone
for recovering the reaction product and unreacted BF.sub.4 gas. In
such embodiment, after B.sub.2F.sub.4 production in the
condensation zone, the condensed material is warmed to form a gas
mixture. The gas mixture from the reactor is extracted for further
distillation of B.sub.2F.sub.4. Such gas mixture obtained from the
reaction system typically contains B.sub.2F.sub.4, BF.sub.3 and
volatile heavy boron fluorides (B.sub.xF.sub.y). Normally,
B.sub.xF.sub.y content is 1-2% maximum; however, some
B.sub.xF.sub.y species may become unstable and trigger
B.sub.2F.sub.4 decomposition. It is highly desirable to either (i)
prevent B.sub.xF.sub.y extraction from the condensation zone of the
reaction system, or (ii) remove these B.sub.xF.sub.y species during
product extraction or recovery so that only B.sub.2F.sub.4 and
BF.sub.3 are captured. Additionally, it is important to prevent
migration of any particles from the reactor down the transfer line
into a collection vessel since such particles can damage valves and
other components of the system.
[0078] In one embodiment, the recovery zone is operated to achieve
extraction of the gas mixture from the condensation zone for
further collection/distillation of B.sub.2F.sub.4, by gradual
warming of the condensation zone to volatilize condensed BF.sub.3
and B.sub.2F.sub.4. In this arrangement, the gases are transferred
into a collection vessel at cryogenic temperature, employing a
pressure drop between the condensation zone and the vessel. Since
BP, species have lower than BF.sub.3 and B.sub.2F.sub.4 vapor
pressures at any given temperature, an inline low temperature trap
may be employed for condensing the B.sub.xF.sub.y species from the
gas mixture stream. The temperature and the trap inlet pressure
chosen for this purpose should be selected to prevent excessive
condensation of B.sub.2F.sub.4 or BF.sub.3. For example, at -57
C..degree. B.sub.2F.sub.4 exhibits .about.160 Torr pressure, which
allows the reactor to operate up to this pressure without
condensing B.sub.2F.sub.4 in the trap.
[0079] In one embodiment, the process for extracting crude product,
BF.sub.3/B.sub.2F.sub.4, from the reactor relies on a pressure
differential between condensation zone of the reactor and the
collection cylinder or vessel. The pressure differential is
established by cooling the collection vessel, preferably a one
gallon stainless steel cylinder, by partially submerging it in
liquid nitrogen. By doing this the BF.sub.3 and B.sub.2F.sub.4 will
condense in the cylinder resulting in such pressure differential
between the reactor and collection vessel allowing material to
flow. In the reaction system, the height of the dewar is at a set
height to allow the condensed gases to evaporate. However, the
temperature of the condensation zone may be controlled in any
manner that sufficiently provides a desired temperature, pressure
and flow rate. For example, by automating the height control of the
dewar, the rate of evaporation can be controlled.
[0080] The recovery zone may further comprise a filtration zone,
whereby particles in the system may be removed by appropriate
filters. Such filters include any type which may be conveniently
placed into the gas-carrying lines of the system. For example, the
filters may be inline filters including a 15 .mu.m filter
element.
[0081] To expedite recovery or extraction of the gas mixture for
further collection/distillation of B.sub.2F.sub.4, a carrier gas
may be used. By way of example, in one embodiment helium is blown
into the reactor (i) to facilitate warm up of the condensed gases,
and (ii) to increase material flow through the transfer line into
the collection vessel.
[0082] In various embodiments, faster extraction rates may be
achieved by selective thermal control of the reactor system. In one
embodiment, the condensation zone is allowed to warm up naturally
and its pressure is controlled by lowering or increasing the level
of a dewar with liquid nitrogen. This, however, may create
temperature gradients resulting in poor control of vaporization
rate, so that the content of the gas mixture may not be
sufficiently controlled during extraction. In one preferred
embodiment, an improved thermal control arrangement is used to
effect preferential removal of BF.sub.3 and B.sub.2F.sub.4 without
extracting other B.sub.xF.sub.y species. This may for example be
achieved by maintaining a uniform temperature of the entire low
temperature assembly of the reactor system through the warm up
step, and by limiting the temperature to a predetermined
temperature at which the other (undesired) B.sub.xF.sub.y species
do not volatilize. The utilization of this approach can be further
augmented with the use of a carrier gas, low temperature traps and
filters to achieve particularly beneficial results.
[0083] Substantial amounts of heavy boron fluorides,
B.sub.xF.sub.y, may be generated as by-products in the production
of B.sub.2F.sub.4. Major amounts are usually retained in the
reactor system components and traps; however, smaller quantities
are scattered throughout the system. Accumulation and spread of the
heavy B.sub.xF.sub.y species can result in a number of unwanted
effects (line clogging, valve failure, etc.) but more importantly
excessive build-up of the heavy B.sub.xF.sub.y species in the
reactor can elevate reactor pressure during the high temperature
reaction step, and prevent adequate cooling of incoming BF and
BF.sub.3, thereby lowering B.sub.2F.sub.4 throughput and yield. To
ameliorate this circumstance, in one embodiment, the heavy
B.sub.xF.sub.y species are hydrolyzed and removed from the reactor
system. This can be done manually, but is a laborious and hazardous
task. Preferably, an automated method is used to achieve high
productivity. Various methods may be employed to facilitate residue
removal. In one approach, initial hydrolysis of the heavy
B.sub.xF.sub.y species with water vapor is followed by water wash
and drying with warm gas. In another approach, thermal
decomposition of B.sub.xF.sub.y is achieved by baking out the
reaction zone or any component therein, with recycling of the
resultingly produced BF.sub.3 and boron. In another approach,
decontamination of B.sub.xF.sub.y is effected by reacting the heavy
B.sub.xF.sub.y species with an active compound such as XeF.sub.2,
F.sub.2, NF.sub.3, O.sub.2, O.sub.3, CO, etc., to form a residue,
followed by mechanically removing the remaining residue. In another
approach, the heavy B.sub.xF.sub.y species may be reacted with
halogen-containing agents to volatilize the residue. Any of these
approaches to minimize the effects of B.sub.xF.sub.y in the system
may be used, as a single method or in combination(s).
[0084] The reactor system, including a high temperature assembly
for solid/fluid reaction to form the intermediate, and a lower
temperature assembly such as a cold trap for further fluid reaction
with the intermediate, may have any suitable geometry, size and
scale, in which the reaction zone and the cold trap are in fluid
flow communication to permit flow of a gas mixture including the
intermediate product into the cold trap zone for further reaction
to form the final product.
[0085] An apparatus and reactor system and method for preparation
of a compound or intermediates thereof from a solid material is
disclosed in U.S. patent application Ser. No. 13/219,706, filed
Aug. 28, 2011, the disclosure of which is hereby incorporated
herein by reference in its entirety, for all purposes.
[0086] Various reactor configurations are contemplated within the
scope of the present disclosure, wherein boron trifluoride is
reacted with elemental boron or boron-containing solids at high
temperature to form boron monofluoride, and the resulting boron
monofluoride is reacted with boron trifluoride at low temperature,
e.g., cryogenic temperature, to form diboron tetrafluoride
(B.sub.2F.sub.4) and heavier boron fluoride compounds. Since the
reaction of boron trifluoride and elemental boron is reversible,
the reactor configurations are desirably adapted to maximize the
production of the intermediate boron monofluoride in any suitable
manner that controls the corresponding reaction to favor BF
production.
[0087] In addition to single-pass (once through) flow arrangements
for contacting of boron trifluoride gas with boron-containing
solids, the present disclosure contemplates arrangements in which
boron trifluoride is recirculated, and augmented by make-up boron
trifluoride introduced in a recirculation loop, to achieve
high-rate continuous production of boron fluoride (BF).
[0088] In the production of B.sub.2F.sub.4 based on reaction
between BF.sub.3 and boron forming BF intermediate, it may be
advantageous to utilize BF.sub.3 recycled from previous
B.sub.2F.sub.4 production cycles, for example, in a recycling zone.
Using such approach, however, the amount of B.sub.2F.sub.4 produced
may be reduced due to impurity in the recycled BF.sub.3. Although
the nature of such impurity is not fully understood, it is believed
that the impurity may be generated from a precursor that itself is
relatively volatile and is extracted from the reaction system along
with BF.sub.3. The exact mechanism of B.sub.2F.sub.4 throughput
suppression by the impurity, although not fully understood, is
believed to relate to the inhibiting action of the impurity on the
reaction between BF.sub.3 and boron.
[0089] A simple freeze-pump-thaw purification of recycled BF.sub.3
has been demonstrated to improve B.sub.2F.sub.a throughput by
30-40%. Freeze-pump-thaw methods, for example, may be used for
fractionation of a mixture composed of compounds with substantially
dissimilar vapor pressure and low miscibility in each other. For
instance, nitrogen and oxygen could be effectively removed from a
number of solvents by freezing them with liquid nitrogen and them
pumping on the solvent while allowing it to melt slightly for some
time. Such procedure allows purification of a solvent from air
component to high degree.
[0090] The same method may be employed for purification of recycled
BF.sub.3. A working hypothesis is that an impurity is generated
during B.sub.2F.sub.4 process that migrates along with BF.sub.3.
The impurity is a precursor for another volatile chemical that
inhibits the reaction rate between BF.sub.3 and boron. Use of a
single freeze-pump-thaw purification of recycled BF.sub.3 may
improve B.sub.2F.sub.4 throughput by 30-40%. In such approach, the
vessel with recycled BF.sub.3 is cooled to appropriate temperature
by an appropriate refrigerant, e.g., cooling to cryogenic
temperatures, such as temperature of -196.degree. C. by liquid
nitrogen, followed by pumping of the vessel head space for a
predetermined time, after which the vessel is warmed up. Vessel
temperatures, rates of cooling and warming, as well as the number
of cooling/warming cycles, may be chosen to optimize BF.sub.3
purity and/or reduce purification time.
[0091] The reactor system of the present disclosure may employ any
suitable monitoring and control components, assemblies and
arrangements, to achieve desired operational conditions during
processing of feed gas for contacting with the reactive solid to
form the intermediate product, and subsequent reaction of the
intermediate to obtain final product.
[0092] For example, the pressure of the cold trap can be monitored
to ensure that consistent low vacuum conditions are maintained in
the trap, and flow controllers such as mass flow controllers, flow
control valves, restricted flow orifice elements, gas flow
regulators, pressure switches, etc., may be employed to regulate
flow of reactant gas to the higher temperature reaction zone for
contacting with the reactive solid. In general, any suitable
monitoring and control arrangements may be implemented, which serve
to beneficially operate the reactor system, so that intermediate
and final products of appropriate character are obtained.
[0093] Accordingly, monitoring and control components, assemblies
and arrangements can be employed to control temperature and
pressure in the reactive solid contacting reaction zone, as well as
in the intermediate product processing zone that is utilized to
produce the final product. Alternatively, other monitoring and
control modalities may be employed to modulate other system
variables and parameters, to achieve beneficial operation of the
process system.
[0094] The reaction system of the disclosure can include a process
control system that is constructed and arranged to establish and
maintain selected temperature and pressure conditions in the
reaction region, e.g., pressure in the reaction region in a
selected range, and/or temperature in the reaction region in a
selected range. In various embodiments, temperature in the reaction
region may be in a range of from 1000.degree. C. to 2200.degree. C.
The pressure in the reaction region can be maintained at any
suitable level. In various embodiments, the pressure in the
reaction region can be in a range of from 10 -6 to 1,000 torr, and
more preferably in a range of from 0.10 torr to 10 torr. A pressure
gradient exists in the reaction region, from an upstream portion to
a downstream portion thereof. In a specific example, wherein the
reaction region is in an interior volume of a reactor housing of
cylindrical form, the pressure at the exit of the cylinder in
various embodiments can be in a range of from 10 -2 to 10 -3
torr.
[0095] The flow rate of the gaseous reagent can be selected to
provide appropriate levels of production of the intermediate
product. In one embodiment, wherein the gaseous reagent is on
trifluoride, the flow rate can be in a range of from 500 sccm to
1200 sccm, or even higher with more efficient cooling, and the
orientation of flow can be varied to optimize contact with the
reactive solid. With scale-up to large size reactors,
correspondingly larger flows of BF.sub.3 can be utilized.
[0096] In order to efficiently employ the reactor system of the
present disclosure, down time associated with the cleaning of the
reactor system should be reduced or minimized. Notwithstanding the
low incidence of deposits on surfaces of the high temperature
reactor that is used to form boron monofluoride by reaction of
solid boron with boron trifluoride, some residues will form in the
reactor and accumulate over long periods of operation, requiring
periodic cleaning of the reactor. Such cleaning may be effected in
any suitable manner, and with any suitable cleaning reagents. In
various embodiments, cleaning of the reactor surfaces can be
carried out with vapor-phase cleaning reagents such as fluorine,
xenon difluoride, nitrogen trifluoride, and the like, with or
without plasma activation of such cleaning agents.
[0097] The condensation zone or lower temperature reaction zone, in
accordance with the present disclosure, can be augmented in various
ways, to remove specific components of the reaction product
mixture, such as species that may be deleterious to downstream
pumps, compressors or other flow circuitry components. For example,
multiple traps may be provided for product recovery.
[0098] According to one embodiment, cleaning of the reactor system
may be accomplished by the use of double valves at each connection
point. By way of example, double valves may be used at the
connection point for the process gas, at the connection point for
the purge gas and for the connection point for the product
extraction. On FIG. 7, described below, the valves would be located
next to AV10, AV11 and MV05. When the cleaning takes place, the
connections between the valves are separated and the reactor
hardware can be relocated to a separate area where it can be
cleaned offline, if desired. While the used system is being cleaned
up, a spare, clean system can be reinstalled and used to continue
to produce the product material.
[0099] Alternatively, the system may be configured in a manner
enabling water or other cleaning solutions to be injected into
different parts of the system to clean the parts in place. This
arrangement allows all the byproducts of the cleaning process to be
contained and directed to a scrubber system and/or drained from the
system, to capture the waste.
[0100] The advantages and features of the disclosure are further
illustrated with reference to the following description, which is
not to be construed as in any way limiting the scope of the
disclosure, but rather as illustrative of embodiments of the
disclosure in specific applications thereof.
[0101] FIG. 7 is a process diagram of an integrated reactor system
according to one embodiment of the present disclosure. The process
diagram includes a reactor system and the other components of an
integrated system for the recovery and production of
B.sub.2F.sub.4. The process control devices are represented as
shown. The process flow for producing a crude
B.sub.2F.sub.4-containing material (a BF.sub.3/B.sub.2F.sub.4 mix)
involves flow of BF.sub.3 gas from a supply cylinder and delivery
of the gas to a reactor at a controlled rate of flow using a mass
flow controller. In the reactor, the BF.sub.3 gas reacts with the
metallic boron pieces that have been loaded into a crucible that is
heated in the middle section of the reactor. The products of the
reaction of BF.sub.3 gas and the boron are captured in the bottom
of the reactor, which is cooled with liquid nitrogen. Once this
high temperature reaction step is completed, the BF.sub.3 gas flow
is stopped, the liquid nitrogen bath is lowered, and the reaction
products are allowed to warm up. Gas is extracted from the bottom
of the reactor as it warms up and is directed through a cold trap,
then a filtration zone, and finally collected in a capture
cylinder. Additional equipment and piping as shown are included for
evacuating, purging, and leak checking the system.
[0102] As shown in FIG. 7, BF.sub.3 gas from a supply cylinder or
cylinders 10 or 20 flows via lines 5 or 15 into BF.sub.3 gas supply
zone 36 and then to the reaction zone by flow through line 25 to
the reactor 50. The BF.sub.3 gas supply manifold contains
appropriate flow controller, FC1, pressure transducers, PT1 and
PT2, pressure regulator RG1, pressure switches, PS1 and PS2,
pneumatic valves AV02-AV09, manual valve MV04, particle filter FT1,
and solenoid valve manifold SVM1. The solenoid valve manifold is a
set of pneumatic valves that receives an electrical signal from the
control box that is used to actuate the process valves.
[0103] A mass flow profile may be used to control the amount of
BF.sub.3 that is required for the reaction to take place. The
system includes a mass flow controller F 1, two transducers, PT1
and PT2, and a dual stage regulator RG1. The system uses HP (high
pressure) BF.sub.3 and/or VAC BF.sub.3 as a source from cylinders
10 or 20. The outlet pressure from the manual pressure control is
maintained at .about.20-22 torr as monitored using the pressure
transducers. The BF.sub.3 flow from the HP cylinder is controlled
by a dual stage regulator RG1 such that the mass flow controller
inlet pressure is on the order of 650 torr. The IP BF.sub.3
cylinder may contain recycled BF.sub.3 from the reaction, formed as
a useful by-product. Waste gas may be vented in vent line 37.
[0104] A remote heat station (not shown) can be used to heat the
reaction zone. Other heater arrangements can be employed, such as
inductive heating coils (not shown) surrounding the reactor 50 to
provide heat to the reaction zone. Reactor 50 contains reactant
boron metal as a solid starting material that is manually loaded
into the reactor prior to sealing the system for each run. The
integrated system can be run as a batch operation.
[0105] In operation, BF.sub.3 gas flows into reactor 50 where it
reacts with the boron metal at high temperature to form an
intermediate, BF. BF and other products of the reaction in the
reactor are captured in condensation zone 70 and cooled with liquid
nitrogen. Hydraulic lift 90 raises and lowers the liquid nitrogen
bath 80 as needed to cool the reactants in the condensation zone.
The reactor is maintained at -195.degree. C. during the high
temperature run, e.g., by an auto liquid nitrogen transfill method.
In this method, a liquid nitrogen dewar having .about.180 L of
liquefied nitrogen is connected with a dispensing line (e.g., a
Ratermann Cryogenic dispensing line) fitted with a frit at its end.
The line is operated in accordance with a PLC program that controls
dispensing of liquid nitrogen to maintain a desired temperature
profile of the low temperature assembly.
[0106] After the products of the reaction are cooled, the BF.sub.3
gas flow is stopped and the hydraulic lift 90 lowers the nitrogen
bath 80 and the reaction products are allowed to warm up. After the
reaction products warm up to a predetermined level, the products
are directed through line 45 to a cold trap 46.
[0107] Cold trap 46 is illustrated in FIG. 6. The cold trap is used
to prevent heavy boron fluoride materials from migrating from the
main reactor vessel to the collection cylinder. FIG. 6 shows the
configuration of one cold trap according to an illustrative
embodiment of the present disclosure. The cold trap assembly 61
comprises an inlet 62 to the cold traps from the reactor zone, a
purge gas inlet 63, a trap outlet 64 to the filtration zone, a trap
outlet 67 to a vent and dewar 66. The dewar 66 holds a slush that
has been cooled with dry ice or liquid nitrogen to obtain a
temperature that is favorable for trapping out the heavy boron
fluorides while allowing the BF.sub.3 and B.sub.2F.sub.4 material
to pass through to the collection cylinder 100.
[0108] As shown, there are 4 traps in a series/parallel
configuration that offer alternate paths for the gas to flow in the
event that a line becomes restricted. The configuration of the
system will be optimized based on trapping efficiency. Parameters
that may vary include the temperature of the cooling solution and
the geometry of the traps. To further control the process, a
chiller (not shown) may be used to more accurately control the
temperature of the traps or to adjust the temperature at different
stages of the process to maximize yield and throughput.
[0109] After passing through the cold trap 46, the product flows
through line 55 into filter zone 58 containing filters 56 and
57.
[0110] The filtration system includes two pressure transducers (PT6
and PT7) that measure inlet and outlet pressures as high as 5000
torr (i.e., pressure drop across the filter element) and two inline
filters 56 and 57 are arranged in parallel, with a 15 um filter
element. During the crude material (reaction product mixture of
B.sub.xF.sub.y, BF.sub.3, B.sub.xF.sub.y) transfer, the mixture
flows through one of the selected filters to remove any solid
particulates entering the crude collection cylinder 100 that could
otherwise constitute or contribute to a clogging problem. The
pressure drop is measured across the filter. In the case of a new
filter element, the pressure differential (delta P) is .about.30-40
torr, whereas an old/used filler may have a delta P of .about.40-70
torr. If there is a clog in the filter, the control elements and
control program operate to allow the filters to be switched during
active processing without stopping the crude flow. The filtration
system includes appropriate equipment elements for conducting flow
through the filters, including valves AV23-AV31, particle filters
FT2 and FT3, pressure switches PS7 and PS9, valve MV07, pressure
regulator RG and SVM2.
[0111] After filtration, the product gas flows via line 65 and is
finally collected in collection cylinder 100 and removed via outlet
line 75. Inlet 85 may be used to flush the system with nitrogen.
Byproducts of the reactions within the reaction system are removed
via outlet 95 to a scrubber unit or cabinet (not shown). Scrubber
units of appropriate character for such treatment are known in the
art. For example, one scrubber unit that is potentially useful in
the reaction system of the present disclosure is a dual scrubber
cabinet system equipped with a CS Clean System 32 gal Novapure.RTM.
Canister down-flow S447D (100%) scrubber, and 32 gal Novapure.RTM.
Canister down-flow S520 (100%) scrubber, connected in series with
one another. These dual scrubbers include a fluoride scrubber and a
hydride scrubber. The S447D unit employs LiOH on molecular sieves
and the S520 unit utilizes hopcalite (MnO.sub.2/CuO 3:1 mix). Such
scrubber unit has a 90% point that is monitored with two Honeywell
MDA sensors units equipped with HF and B.sub.2H.sub.6 sensors.
BF.sub.3 and B.sub.2F.sub.4 are the main constituents of the
hazardous gaseous effluents from the reactor system pumps, but some
amount of heavy boron fluorides (B.sub.3F.sub.5, B.sub.8F.sub.12,
and other B.sub.xF.sub.y species) and HF are expected as well.
Gases are sent to the scrubber either during manifold purge
routines or while discarding unwanted gases from system lines. The
following table demonstrates the scrubbing capacity with respect to
each gas:
TABLE-US-00001 CS Clean System 32 gal Novapure .RTM. Canister
Scrubbing Capacity Resin S447D S520 Gas wt if Volume fraction Max
any 100% B2F4 100% 100% Scrubber time amount decomposes moles/L g/L
of moles/L g/L of total of gas on into Gas of resin resin of resin
resin capacity, g system, g respective B2F4 no data no data no data
400 BF3 1.4 97.24 0.00 0.00 12104 3334 370 BxFy no data no data no
data <50 HF 4.00 76.00 0.00 0.00 9460 <50 310 B2H6 0.00 0.00
0.57 15.96 1987 <50 114
[0112] A vacuum assembly is used during the operation of the
reactor system which includes two dry pumps, a MDP (Molecular Drag
pump) 48 and an ACP 122P vacuum pump 86, which can pump the entire
process system to a vacuum level of .about.10.sup.-6 torr, as
necessary for the reaction. The MDP has a low rotational speed of
27000 rpm and is sufficiently robust to operate under conditions
involving accidental air in-rush, shock venting or gyroscopic
effects, with a pumping performance between 10 mbar and 10.sup.-5
mbar, and a maximum flow rate of 400 sccm. The ADP pump runs at
100.degree. C. temperature and includes a frictionless and oil-free
pump mechanism that is capable of maintaining a pressure level of
10.sup.-2 torr.
[0113] Helium from tank 30 via line 35 may be used to clean or
purge the process system.
[0114] While the system and method have been described with respect
to various aspects, implementations and embodiments, it will be
appreciated that any of such aspects, implementations and
embodiments can be present in any combination with any other
aspects, implementations and embodiments of the disclosure. The
disclosure therefore is to be regarded as comprehending all
permutations and combinations of compatible features individually
or specifically described, in corresponding aggregations of such
features. It further is to be recognized that any one or more of
the individual features specifically disclosed herein may be
selectively excluded from any other feature or combination of
features disclosed herein, in specific implementations of the
reactor system and method of the present disclosure, as further
embodiments thereof.
[0115] While the disclosure has been has been set forth herein in
reference to specific aspects, features and illustrative
embodiments, it will be appreciated that the utility of the
disclosure is not thus limited, but rather extends to and
encompasses numerous other variations, modifications and
alternative embodiments, as will suggest themselves to those of
ordinary skill in the field of the present disclosure, based on the
description herein. Correspondingly, the invention as hereinafter
claimed is intended to be broadly construed and interpreted, as
including all such variations, modifications and alternative
embodiments, within its spirit and scope.
* * * * *