U.S. patent application number 11/468424 was filed with the patent office on 2010-01-07 for production of ultrafine boron carbide particles utilizing liquid feed materials.
This patent application is currently assigned to PPG INDUSTRIES OHIO, INC.. Invention is credited to Cheng-Hung Hung, Noel R. Vanier.
Application Number | 20100003180 11/468424 |
Document ID | / |
Family ID | 39721741 |
Filed Date | 2010-01-07 |
United States Patent
Application |
20100003180 |
Kind Code |
A1 |
Hung; Cheng-Hung ; et
al. |
January 7, 2010 |
PRODUCTION OF ULTRAFINE BORON CARBIDE PARTICLES UTILIZING LIQUID
FEED MATERIALS
Abstract
The production of ultrafine boron carbide powders from liquid
boron-containing precursors and/or liquid carbon-containing
precursors is disclosed. The liquid precursors are fed together or
separately to a plasma system where the precursor materials react
to form boron carbide in the form of ultrafine particles.
Inventors: |
Hung; Cheng-Hung; (Wexford,
PA) ; Vanier; Noel R.; (Wexford, PA) |
Correspondence
Address: |
PPG INDUSTRIES INC;INTELLECTUAL PROPERTY DEPT
ONE PPG PLACE
PITTSBURGH
PA
15272
US
|
Assignee: |
PPG INDUSTRIES OHIO, INC.
Cleveland
OH
|
Family ID: |
39721741 |
Appl. No.: |
11/468424 |
Filed: |
August 30, 2006 |
Current U.S.
Class: |
423/291 |
Current CPC
Class: |
C01B 32/991
20170801 |
Class at
Publication: |
423/291 |
International
Class: |
C01B 31/36 20060101
C01B031/36 |
Goverment Interests
GOVERNMENT CONTRACT
[0001] This invention was made with United States government
support under Contract Number W911NF-05-9-0001 awarded by DARPA.
The United States government has certain rights in this invention.
Claims
1. A method for making ultrafine boron carbide particles
comprising: introducing a liquid boron-containing precursor
comprising a borate ester and a carbon-containing precursor into a
plasma; heating the precursors by the plasma to form the ultrafine
boron carbide particles from the precursors; and collecting the
ultrafine boron carbide particles.
2-3. (canceled)
4. The method of claim 1, wherein the borate ester comprises
trimethylboroxine, trimethylborate, triethylborate, or a
combination thereof.
5. The method of claim 1, wherein the liquid boron-containing
precursor comprises trimethylboroxine.
6. The method of claim 1, wherein the liquid boron-containing
precursor comprises trimethylborate.
7. The method of claim 1, wherein the carbon-containing precursor
is liquid.
8. The method of claim 7, wherein the liquid carbon-containing
precursor comprises aliphatic carbon atoms, aromatic carbon atoms,
or a combination thereof.
9. The method of claim 7, wherein the liquid carbon-containing
precursor comprises an organic liquid.
10. The method of claim 9, wherein the organic liquid has a C:H
atomic ratio greater than 1:3.
11. A method of for making ultrafine boron carbide particles
comprising: introducing a liquid boron-containing precursor and a
liquid carbon-containing precursor comprising an organic liquid
into a plasma; heating the precursors by the plasma to form the
ultrafine boron carbide particles from the precursors; and
collecting the ultrafine boron carbide particles, wherein the
organic liquid has a C:O atomic ratio greater than 2:1.
12. The method of claim 7, wherein the liquid carbon-containing
precursor comprises acetone, iso-octane, toluene, or a combination
thereof.
13. The method of claim 7, wherein the liquid carbon-containing
precursor comprises acetone.
14. The method of claim 7, wherein the liquid carbon-containing
precursor comprises iso-octane.
15. The method of claim 1, further comprising mixing the liquid
boron-containing precursor and carbon-containing precursor before
the introduction into the plasma.
16. The method of claim 1, further comprising contacting the liquid
boron-containing precursor and carbon-containing precursor with a
carrier gas before the introduction into the plasma.
17. The method of claim 1, further comprising introducing an
additional reactant into the plasma.
18. The method of claim 17, wherein the additional reactant
comprises nitrogen.
19. A method of for making ultrafine boron carbide particles
comprising: introducing a liquid boron-containing precursor, a
carbon-containing precursor, and an additional reactant into a
plasma; heating the precursors by the plasma to form the ultrafine
boron carbide particles from the precursors; and collecting the
ultrafine boron carbide particles, wherein the additional reactant
comprises silicon.
20. The method of claim 1, wherein the ultrafine boron carbide
particles have an average particle size of less than 100 nm.
21. The method of claim 1, wherein the ultrafine boron carbide
particles comprise B.sub.4C, B.sub.13C.sub.2, B.sub.8C, B.sub.10C,
B.sub.25C, or a combination thereof.
22. The method of claim 1, wherein the ultrafine boron carbide
particles comprise B.sub.4C.
23. The method of claim 1, wherein the carbon-containing precursor
comprises a source of nitrogen.
24. (canceled)
25. A method for making ultrafine boron carbide particles
comprising: at least partially dissolving or suspending a
particulate boron-containing material in an organic liquid to form
a liquid feed; and then introducing the liquid feed into a plasma;
heating the liquid feed by the plasma to form the ultrafine boron
carbide particles from the liquid feed; and collecting the
ultrafine boron carbide particles.
26. (canceled)
27. The method of claim 2, wherein the particulate boron-containing
material comprises B.sub.2O.sub.3, borax, or a combination
thereof.
28. The method of claim 2, wherein the organic liquid comprises
methanol, glycerol, ethylene glycol or dimethyl carbonate.
29-31. (canceled)
32. The method of claim 25, wherein the plasma is produced from an
inert gas.
33. The method of claim 32, wherein the inert gas comprises argon,
helium, or neon.
Description
FIELD OF THE INVENTION
[0002] The present invention relates to boron carbide particles,
and more particularly relates to the production of ultrafine boron
carbide particles utilizing liquid feed materials.
BACKGROUND INFORMATION
[0003] Micron-sized boron carbide particles have been produced by
solid phase synthesis using B.sub.2O.sub.3 and carbon as the
starting reactant materials.
[0004] Boron carbide particles have also been produced by vapor
phase synthesis using BCl.sub.3 and CH.sub.4 gaseous reactants as
the starting materials. Although such vapor phase synthesis is
capable of producing B.sub.4C nanoparticles, the process is
relatively expensive.
SUMMARY OF THE INVENTION
[0005] In certain respects, the present invention is directed to a
method for making ultrafine boron carbide particles comprising:
introducing a liquid boron-containing precursor and a
carbon-containing precursor into a plasma; heating the precursors
by the plasma to form the ultrafine boron carbide particles from
the precursors; and collecting the ultrafine boron carbide
particles.
[0006] In other respects, the present invention is directed to a
method for making ultrafine boron carbide particles comprising:
introducing a boron-containing precursor and a liquid
carbon-containing precursor into a plasma; heating the precursors
by the plasma to form the ultrafine boron carbide particles from
the precursors; and collecting the ultrafine boron carbide
particles.
[0007] In other respects, the present invention is directed to an
apparatus for making ultrafine boron carbide particles comprising:
a source of liquid boron-containing precursor; a source of
carbon-containing precursor; a plasma chamber; and at least one
feed line for delivering the precursors to the plasma chamber.
[0008] In further respects, the present invention is directed to
ultrafine boron carbide particles made from such methods and
apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a flowchart depicting the steps of certain methods
of the present invention.
[0010] FIG. 2 is a partially schematic sectional view of an
apparatus for producing ultrafine boron carbide particles including
a feed line for liquid boron-containing and carbon-containing
precursors of the boron carbide in accordance with certain
embodiments of the present invention.
[0011] FIG. 3 is a photomicrograph of ultrafine boron carbide
particles produced in accordance with an embodiment of the present
invention.
[0012] FIG. 4 is a photomicrograph of ultrafine boron carbide
particles produced in accordance with another embodiment of the
present invention.
[0013] FIG. 5 is a photomicrograph of ultrafine boron carbide
particles produced in accordance with a further embodiment of the
present invention.
[0014] FIG. 6 is an X-ray diffraction pattern from the ultrafine
boron carbide particles shown in FIG. 5.
[0015] FIG. 7 is an X-ray diffraction pattern from ultrafine boron
carbide particles produced in accordance with another embodiment of
the present invention.
[0016] FIG. 8 is an X-ray diffraction pattern from ultrafine boron
carbide particles produced in accordance with a further embodiment
of the present invention.
[0017] FIG. 9 is a photomicrograph of ultrafine boron carbide
particles produced in accordance with another embodiment of the
present invention.
[0018] FIG. 10 is an X-ray diffraction pattern from the ultrafine
boron carbide particles shown in FIG. 9.
DETAILED DESCRIPTION
[0019] For purposes of the following detailed description, it is to
be understood that the invention may assume various alternative
variations and step sequences, except where expressly specified to
the contrary. Moreover, other than in any operating examples, or
where otherwise indicated, all numbers expressing, for example,
quantities of ingredients used in the specification and claims are
to be understood as being modified in all instances by the term
"about". Accordingly, unless indicated to the contrary, the
numerical parameters set forth in the following specification and
attached claims are approximations that may vary depending upon the
desired properties to be obtained by the present invention. At the
very least, and not as an attempt to limit the application of the
doctrine of equivalents to the scope of the claims, each numerical
parameter should at least be construed in light of the number of
reported significant digits and by applying ordinary rounding
techniques.
[0020] Notwithstanding that the numerical ranges and parameters
setting forth the broad scope of the invention are approximations,
the numerical values set forth in the specific examples are
reported as precisely as possible. Any numerical value, however,
inherently contains certain errors necessarily resulting from the
standard variation found in their respective testing
measurements.
[0021] Also, it should be understood that any numerical range
recited herein is intended to include all sub-ranges subsumed
therein. For example, a range of "1 to 10" is intended to include
all sub-ranges between (and including) the recited minimum value of
1 and the recited maximum value of 10, that is, having a minimum
value equal to or greater than 1 and a maximum value of equal to or
less than 10.
[0022] In this application, the use of the singular includes the
plural and plural encompasses singular, unless specifically stated
otherwise. In addition, in this application, the use of "or" means
"and/or" unless specifically stated otherwise, even though "and/or"
may be explicitly used in certain instances.
[0023] Certain embodiments of the present invention are directed to
methods and/or apparatus for making ultrafine boron carbide
particles, as well as the ultrafine boron carbide particles
produced by such methods and apparatus. Examples of ultrafine boron
carbides that may be produced include B.sub.4C, B.sub.13C.sub.2,
B.sub.8C, B.sub.10C, B.sub.25C, and any other boron carbides that
may be produced as ultrafine particles utilizing a liquid
boron-containing precursor as the boron source and/or a liquid
carbon-containing precursor as the carbon source for the boron
carbide. In certain embodiments, the ratio of boron-containing
precursor to carbon-containing precursor is selected in order to
control the boron carbide composition. For example, increasing the
boron to carbon ratio in the precursors may result in the formation
of B.sub.8C or B.sub.10C rather than B.sub.4C.
[0024] As used herein, the term "ultrafine boron carbide particles"
refers to boron carbide particles having a B.E.T. specific surface
area of at least 5 square meters per gram, such as 20 to 200 square
meters per gram, or, in some cases, 30 to 100 square meters per
gram. As used herein, the term "B.E.T. specific surface area"
refers to a specific surface area determined by nitrogen adsorption
according to the ASTMD 3663-78 standard based on the
Brunauer-Emmett-Teller method described in the periodical "The
Journal of the American Chemical Society", 60, 309 (1938).
[0025] In certain embodiments, the ultrafine boron carbide
particles made in accordance with the present invention have a
calculated equivalent spherical diameter of no more than 200
nanometers, such as no more than 100 nanometers, or, in certain
embodiments, 5 to 50 nanometers. As will be understood by those
skilled in the art, a calculated equivalent spherical diameter can
be determined from the B.E.T. specific surface area according to
the following equation:
Diameter (nanometers)=6000/[BET (m.sup.2/g)*.rho.
(grams/cm.sup.3)]
[0026] In certain embodiments, the ultrafine boron carbide
particles have an average particle size of no more than 100
nanometers, in some cases, no more than 50 nanometers or, in yet
other cases, no more than 30 or 40 nanometers. As used herein, the
term "average particle size" refers to a particle size as
determined by visually examining a micrograph of a transmission
electron microscopy ("TEM") image, measuring the diameter of the
particles in the image, and calculating the average particle size
of the measured particles based on magnification of the TEM image.
One of ordinary skill in the art will understand how to prepare
such a TEM image and determine the average particle size based on
the magnification. The size of a particle refers to the smallest
diameter sphere that will completely enclose the individual
particle.
[0027] FIG. 1 is a flow diagram depicting certain embodiments of
the methods of the present invention. A liquid boron-containing
precursor and a liquid carbon-containing precursor are provided as
feed materials. In the embodiment shown in FIG. 1, the liquid
precursors are provided from separate sources. However, a single
liquid comprising both the boron-containing precursor and
carbon-containing precursor may also be used. The term "liquid
precursor" means a precursor material that is liquid at room
temperature.
[0028] In accordance with certain embodiments, suitable liquid
boron-containing precursors include borate esters and other
compounds containing boron-oxygen bonds. For example, the liquid
boron-containing precursor may comprise trimethylboroxine,
trimethylborate and/or triethylborate.
[0029] In certain embodiments, the carbon-containing precursor may
be in liquid form and may comprise aliphatic carbon atoms and/or
aromatic carbon atoms. For example, the liquid carbon-containing
precursor may comprise acetone, iso-octane and/or toluene. In
certain embodiments, the liquid carbon-containing precursor may
also be a source of nitrogen. Examples of such materials include,
but are not limited, to dimethylformamide and methylformamide. In
certain embodiments, the liquid carbon-containing precursor may
comprise an organic liquid with a relatively high C:H atomic ratio,
e.g., greater than 1:3 or greater than 1:2. Furthermore, such
liquid hydrocarbon precursors may also have a relatively high C:O
atomic ratio, e.g., greater than 2:1 or greater than 3:1.
[0030] In certain embodiments, a single liquid may be provided as
the feed material. For example, boron-containing compounds such as
B.sub.2O.sub.3 or borax particles may be suspended or dissolved in
an organic liquid such as methanol, glycerol, ethylene glycol or
dimethyl carbonate. Thus, the liquid boron-containing precursor and
liquid carbon-containing precursor may comprise hydrocarbon
solvents in which particulate boron-containing precursors are at
least partially suspended or dissolved. As another example,
polypropylene powder may be suspended in trimethylborate
liquid.
[0031] In accordance with certain embodiments, the ratio of
boron-containing precursor to carbon-containing precursor is
controlled in order to control the composition of the resultant
boron carbide and/or in order to control the formation of excess
boron or excess carbon in the ultrafine boron carbide particles.
For example, if an excess amount of boron-containing precursor is
used, excess boron may form on or in the ultrafine boron carbide
particles, which may react with oxygen or air to form oxide
compounds. As a further example, an excess amount of
carbon-containing precursor in the starting feed material may cause
the formation of graphite on or in the resultant boron carbide
particles.
[0032] As shown in FIG. 1, in accordance with certain methods of
the present invention, the liquid boron-containing and
carbon-containing precursors are contacted with a carrier. The
carrier may be a gas that acts to suspend or atomize the liquid
precursors in the gas, thereby producing a gas-stream in which the
liquid precursors are entrained. Suitable carrier gases include,
but are not limited to, argon, helium, nitrogen, hydrogen, or a
combination thereof.
[0033] Next, in accordance with certain embodiments of the present
invention, the precursors are heated by a plasma system, e.g., as
the entrained liquid precursors flow into a plasma chamber,
yielding a gaseous stream of the precursors and/or their vaporized
or thermal decomposition products and/or their reaction products.
In certain embodiments, the precursors are heated to a temperature
ranging from 1,500.degree. to 20,000.degree. C., such as
1,700.degree. to 8,000.degree. C.
[0034] In certain embodiments, the gaseous stream may be contacted
with other reactants or dopants, such as tetramethoxy silane,
tetraethoxy silane, alkoxy titanates or dimethyl formamide or
nitrogen, that may be injected into the plasma chamber or which may
be introduced as part of the liquid precursors. For example, the
additional reactants may be used to increase the yield and/or
purity of B.sub.4C, e.g., by reducing carbothermal reactions which
would otherwise tend to form B.sub.2O.sub.3 and graphite.
Furthermore, the additional reactants may result in improved
physical properties of the B.sub.4C such as mechanical properties
and the like. The additional reactants may result in the formation
of additional boron-containing compounds and/or additional
carbon-containing materials or compounds. For example, silanes may
be used to produce silicon carbide on or in the ultrafine boron
carbide particles. Suitable additional reactant materials include,
but are not limited to, hydrogen, nitrogen, ammonia,
nitrogen-containing organic compounds, silicon-containing
compounds, polymers, alkoxy metal compounds and/or metal
carboxylates.
[0035] In certain methods of the present invention, after the
gaseous stream is produced, it is contacted with one or more quench
streams that are injected into the plasma chamber through at least
one quench stream injection port. For example, the quench streams
are injected at flow rates and injection angles that result in
impingement of the quench streams with each other within the
gaseous stream. The material used in the quench streams is not
limited, so long as it adequately cools the gaseous stream to
facilitate the formation or control the particle size of the
ultrafine boron carbide particles. Materials suitable for use in
the quench streams include, but are not limited to, inert gases
such as argon, helium, nitrogen, hydrogen gas, ammonia, mono, di
and polybasic alcohols, hydrocarbons, amines and/or carboxylic
acids.
[0036] In certain embodiments, the particular flow rates and
injection angles of the various quench streams may vary, so long as
they impinge with each other within the gaseous stream to result in
the rapid cooling of the gaseous stream. For example, the quench
streams may primarily cool the gaseous stream through dilution,
rather than adiabatic expansion, thereby causing a rapid quenching
of the gaseous stream, before, during and/or after the formation of
the ultrafine boron carbide particles prior to passing the
particles into and through a converging member, such as a
converging-diverging nozzle, as described below.
[0037] In certain embodiments of the invention, after contacting
the gaseous product stream with the quench streams to cause
production of ultrafine boron carbide particles, the ultrafine
particles may be passed through a converging member, wherein the
plasma system is designed to minimize the fouling thereof. In
certain embodiments, the converging member comprises a
converging-diverging (De Laval) nozzle. In these embodiments, while
the converging-diverging nozzle may act to cool the product stream
to some degree, the quench streams perform much of the cooling so
that a substantial amount of the ultrafine boron carbide particles
are formed upstream of the nozzle. In these embodiments, the
converging-diverging nozzle may primarily act as a choke position
that permits operation of the reactor at higher pressures, thereby
increasing the residence time of the materials therein. The
combination of quench stream dilution cooling with a
converging-diverging nozzle appears to provide a commercially
viable method of producing ultrafine boron carbide particles from
liquid boron-containing and carbon-containing precursors using a
plasma system, since, for example, in certain embodiments the
liquid feed materials can be used effectively without the necessity
of heating the feed materials to a gaseous state before injection
into the plasma. Alternatively, liquid feed materials may be
vaporized prior to introduction to the plasma system.
[0038] As is seen in FIG. 1, in certain embodiments of the methods
of the present invention, after the ultrafine boron carbide
particles exit the plasma system, they are collected. Any suitable
means may be used to separate the ultrafine boron carbide particles
from the gas flow, such as, for example, a bag filter, cyclone
separator or deposition on a substrate.
[0039] FIG. 2 is a partially schematic sectional diagram of an
apparatus for producing ultrafine boron carbide particles in
accordance with certain embodiments of the present invention. A
plasma chamber 20 is provided that includes a liquid feed inlet 50
which, in the embodiment shown in FIG. 2, is used to introduce a
mixture of liquid boron-containing precursor and liquid
carbon-containing precursor into the plasma chamber 20. In another
embodiment, the liquid feed inlet 50 may be replaced with separate
inlets (not shown) for the liquid boron-containing precursor and
the liquid carbon-containing precursor. Also provided is at least
one carrier gas feed inlet 14, through which a carrier gas flows in
the direction of arrow 30 into the plasma chamber 20. The carrier
gas may act to suspend or atomize the liquid precursors in the gas,
thereby producing a gas-stream with the entrained liquid precursors
which flows towards plasma 29. Numerals 23 and 25 designate cooling
inlet and outlet respectively, which may be present for a
double-walled plasma chamber 20. In these embodiments, coolant flow
is indicated by arrows 32 and 34.
[0040] In the embodiment depicted by FIG. 2, a plasma torch 21 is
provided. The torch 21 may thermally decompose or vaporize the
liquid boron-containing and carbon-containing precursors within or
near the plasma 29 as the stream is delivered through the inlet of
the plasma chamber 20, thereby producing a gaseous stream. As is
seen in FIG. 2, the liquid precursors are, in certain embodiments,
injected downstream of the location where the arc attaches to the
annular anode 13 of the plasma generator or torch.
[0041] A plasma is a high temperature luminous gas which is at
least partially (1 to 100%) ionized. A plasma is made up of gas
atoms, gas ions, and electrons. A thermal plasma can be created by
passing a gas through an electric arc. The electric arc will
rapidly heat the gas by resistive and radiative heating to very
high temperatures within microseconds of passing through the arc.
The plasma is often luminous at temperatures above 9,000 K.
[0042] A plasma can be produced with any of a variety of gases.
This can give excellent control over any chemical reactions taking
place in the plasma as the gas may be inert, such as argon, helium,
or neon, reductive, such as hydrogen, methane, ammonia, and carbon
monoxide, or oxidative, such as oxygen, nitrogen, and carbon
dioxide. Inert or reductive gas mixtures may be used to produce
ultrafine boron carbide particles in accordance with the present
invention. In FIG. 2, the plasma gas feed inlet is depicted at
31.
[0043] As the gaseous product stream exits the plasma 29 it
proceeds towards the outlet of the plasma chamber 20. An additional
reactant, as described earlier, can optionally be injected into the
reaction chamber prior to the injection of the quench streams. A
supply inlet for the additional reactant is shown in FIG. 2 at
33.
[0044] As is seen in FIG. 2, in certain embodiments of the present
invention, the gaseous stream is contacted with a plurality of
quench streams which enter the plasma chamber 20 in the direction
of arrows 41 through a plurality of quench stream injection ports
40 located along the circumference of the plasma chamber 20. As
previously indicated, the particular flow rate and injection angle
of the quench streams is not limited so long as they result in
impingement of the quench streams 41 with each other within the
gaseous stream, in some cases at or near the center of the gaseous
stream, to result in the rapid cooling of the gaseous stream to
control the particle size of the ultrafine boron carbide particles.
This may result in a quenching of the gaseous stream through
dilution.
[0045] In certain methods of the present invention, contacting the
gaseous stream with the quench streams may result in the formation
and/or control of the particle size of the ultrafine boron carbide
particles, which are then passed into and through a converging
member. As used herein, the term "converging member" refers to a
device that restricts passage of a flow therethrough, thereby
controlling the residence time of the flow in the plasma chamber
due to pressure differential upstream and downstream of the
converging member.
[0046] In certain embodiments, the converging member comprises a
converging-diverging (De Laval) nozzle, such as that depicted in
FIG. 2, which is positioned within the outlet of the plasma chamber
20. The converging or upstream section of the nozzle, i.e., the
converging member, restricts gas passage and controls the residence
time of the materials within the plasma chamber 20. It is believed
that the contraction that occurs in the cross sectional size of the
stream as it passes through the converging portion of nozzle 22
changes the motion of at least some of the flow from random
directions, including rotational and vibrational motions, to a
straight line motion parallel to the plasma chamber axis. In
certain embodiments, the dimensions of the plasma chamber 20 and
the material flow are selected to achieve sonic velocity within the
restricted nozzle throat.
[0047] As the confined stream of flow enters the diverging or
downstream portion of the nozzle 22, it is subjected to an ultra
fast decrease in pressure as a result of a gradual increase in
volume along the conical walls of the nozzle exit. By proper
selection of nozzle dimensions, the plasma chamber 20 can be
operated at atmospheric pressure, or slightly less than atmospheric
pressure, or, in some cases, at a pressurized condition, to achieve
the desired residence time, while the chamber 26 downstream of the
nozzle 22 is maintained at a vacuum pressure by operation of a
vacuum producing device, such as a vacuum pump 60. Following
passage through nozzle 22, the ultrafine boron carbide particles
may then enter a cool down chamber 26.
[0048] As is apparent from FIG. 2, in certain embodiments of the
present invention, the ultrafine boron carbide particles may flow
from cool down chamber 26 to a collection station 27 via a cooling
section 45, which may comprise, for example, a jacketed cooling
tube. In certain embodiments, the collection station 27 comprises a
bag filter or other collection means. A downstream scrubber 28 may
be used if desired to condense and collect material within the flow
prior to the flow entering vacuum pump 60.
[0049] In certain embodiments, the residence times for materials
within the plasma chamber 20 are on the order of milliseconds. The
liquid boron-containing and carbon-containing precursors may be
injected under pressure (such as from 1 to 300 psi) through a small
orifice to achieve sufficient velocity to penetrate and mix with
the plasma. In addition, in many cases the injected liquid stream
is injected normal (90.degree. angle) to the flow of the plasma
gases. In some cases, positive or negative deviations from the
90.degree. angle by as much as 30.degree. may be desired.
[0050] The high temperature of the plasma may rapidly decompose
and/or vaporize the liquid precursors. There can be a substantial
difference in temperature gradients and gaseous flow patterns along
the length of the plasma chamber 20. It is believed that, at the
plasma arc inlet, flow is turbulent and there is a high temperature
gradient from temperatures of about 20,000 K at the axis of the
chamber to about 375 K at the chamber walls. At the nozzle throat,
it is believed, the flow is laminar and there is a very low
temperature gradient across its restricted open area.
[0051] The plasma chamber is often constructed of water cooled
stainless steel, nickel, titanium, copper, aluminum, or other
suitable materials. The plasma chamber can also be constructed of
ceramic materials to withstand a vigorous chemical and thermal
environment.
[0052] The plasma chamber walls may be internally heated by a
combination of radiation, convection and conduction. In certain
embodiments, cooling of the plasma chamber walls prevents unwanted
melting and/or corrosion at their surfaces. The system used to
control such cooling should maintain the walls at as high a
temperature as can be permitted by the selected wall material,
which often is inert to the materials within the plasma chamber at
the expected wall temperatures. This is true also with regard to
the nozzle walls, which may be subjected to heat by convection and
conduction.
[0053] The length of the plasma chamber is often determined
experimentally by first using an elongated tube within which the
user can locate the target threshold temperature. The plasma
chamber can then be designed long enough so that the materials have
sufficient residence time at the high temperature to reach an
equilibrium state and complete the formation of the desired end
products.
[0054] The inside diameter of the plasma chamber 20 may be
determined by the fluid properties of the plasma and moving gaseous
stream. It should be sufficiently great to permit necessary gaseous
flow, but not so large that recirculating eddies or stagnant zones
are formed along the walls of the chamber. Such detrimental flow
patterns can cool the gases prematurely and precipitate unwanted
products. In many cases, the inside diameter of the plasma chamber
20 is more than 100% of the plasma diameter at the inlet end of the
plasma chamber.
[0055] In certain embodiments, the converging section of the nozzle
has a high aspect ratio change in diameter that maintains smooth
transitions to a first steep angle (such as >45.degree.) and
then to lesser angles (such as <45.degree. degree.) leading into
the nozzle throat. The purpose of the nozzle throat is often to
compress the gases and achieve sonic velocities in the flow. The
velocities achieved in the nozzle throat and in the downstream
diverging section of the nozzle are controlled by the pressure
differential between the plasma chamber and the section downstream
of the diverging section of the nozzle. Negative pressure can be
applied downstream or positive pressure applied upstream for this
purpose. A converging-diverging nozzle of the type suitable for use
in the present invention is described in U.S. Pat. No. RE37,853 at
col. 9, line 65 to col. 11, line 32, the cited portion of which
being incorporated by reference herein.
[0056] The following examples are intended to illustrate certain
embodiments of the present invention, and are not intended to limit
the scope of the invention.
Example 1
[0057] Boron carbide particles were produced using a DC thermal
plasma reactor system similar to that shown in FIG. 2. The main
reactor system included a DC plasma torch (Model SG-100 Plasma
Spray Gun commercially available from Praxair Technology, Inc.,
Danbury, Conn.) operated with 80 standard liters per minute of
argon carrier gas and 12 kilowatts of power delivered to the torch.
A liquid precursor feed composition comprising the materials and
amounts listed in Table 1 was prepared and fed to the reactor at a
rate of 7 grams per minute through a gas assisted liquid nebulizer
located about 0.5 inch down stream of the plasma torch outlet. At
the nebulizer, 15 standard liters per minute of argon were
delivered to assist in atomization of the liquid precursors.
Following a 4 inch long reactor section, a plurality of quench
stream injection ports were provided that included 61/8 inch
diameter nozzles located 60.degree. apart radially. A 10 millimeter
diameter converging-diverging nozzle was provided 4 inches
downstream of the quench stream injection port. Quench argon gas
was injected through the quench stream injection ports at a rate of
145 standard liters per minute.
TABLE-US-00001 TABLE 1 Material Amount Trimethoxy Boroxine.sup.1
640 grams Acetone 400 grams .sup.1Commercially available from Alfa
Aesar, Ward Hill, Massachusetts.
[0058] The measured B.E.T. specific surface area of the produced
material was 28 square meters per gram using a Gemini model 2360
analyzer (available from Micromeritics Instrument Corp., Norcross,
Ga.), and the calculated equivalent spherical diameter was 85
nanometers. FIG. 3 is a micrograph of a TEM image of a
representative portion of the particles (25,000.times.
magnification). The micrograph was prepared by weighing out 0.2 to
0.4 grams of the particles and adding those particles to methanol
present in an amount sufficient to yield an adequate particle
density on a TEM grid. The mixture was placed in a sonicator for 20
minutes and then dispersed onto a 3 millimeter TEM grid coated with
a uniform carbon film using a disposable pipette. After allowing
the methanol to evaporate, the grid was loaded into a specimen
holder which was then inserted into a TEM instrument.
Example 2
[0059] Boron carbide particles from liquid precursors were prepared
using the apparatus and conditions identified in Example 1, except
that plasma was operated at 24 kilowatts of power delivered to the
torch, a 7 millimeter diameter converging-diverging nozzle was
used, and the feed materials and amounts used are listed in Table
2.
TABLE-US-00002 TABLE 2 Material Amount Trimethyl Borate.sup.2 1000
grams Iso-Octane.sup.3 34.4 grams .sup.2Commercially available from
Alfa Aesar, Ward Hill, Massachusetts. .sup.3Commercially available
from Alfa Aesar, Ward Hill, Massachusetts.
[0060] The measured B.E.T. specific surface area of the produced
material was 38 square meters per gram using a Gemini model 2360
analyzer (available from Micromeritics Instrument Corp., Norcross,
Ga.), and the calculated equivalent spherical diameter was 63
nanometers. FIG. 4 is a TEM image of a representative portion of
the particles (45,000.times. magnification) prepared in the same
manner described in Example 1.
Example 3
[0061] Boron carbide particles from liquid precursors were prepared
using the apparatus and conditions identified in Example 2, except
that the feed materials and amounts used are listed in Table 3.
TABLE-US-00003 TABLE 3 Material Amount Trimethyl Borate 1000 grams
N,N-Dimethylformamide.sup.4 87.9 grams .sup.4Commercially available
from Alfa Aesar, Ward Hill, Massachusetts.
[0062] The measured B.E.T. specific surface area of the produced
material was 37 square meters per gram using a Gemini model 2360
analyzer (available from Micromeritics Instrument Corp., Norcross,
Ga.), and the calculated equivalent spherical diameter was 64
nanometers. FIG. 5 is a TEM image of a representative portion of
the particles (100,000.times. magnification) prepared in the same
manner described in Example 1. FIG. 6 is an X-ray diffraction
spectrum of the particles measured using an X-ray diffractometer
(Philips X' Pert MPD model). The analysis indicated that the powder
comprised crystalline boron carbide having a hexagonal crystal
structure and small amounts of boron oxide and graphite.
Example 4
[0063] Boron carbide particles from liquid precursors were prepared
using the apparatus and conditions identified in Example 2, except
that quench argon gas was injected at a rate of 290 standard liters
per minute, and the feed materials and amounts are listed in Table
4.
TABLE-US-00004 TABLE 4 Material Amount Trimethyl Borate 1000 grams
N,N-Dimethylformamide 93.4 grams Tetramethoxysilane.sup.5 22.9
grams .sup.5Commercially available from Alfa Aesar, Ward Hill,
Massachusetts.
[0064] The measured B.E.T. specific surface area of the produced
material was 33 square meters per gram using a Gemini model 2360
analyzer (available from Micromeritics Instrument Corp., Norcross,
Ga.), and the calculated equivalent spherical diameter was 72
nanometers. FIG. 7 is an X-ray diffraction spectrum of the
particles measured using an X-ray diffractometer (Philips X' Pert
MPD model). The analysis indicated that the powder comprised
crystalline boron carbide having a hexagonal crystal structure.
Example 5
[0065] Boron carbide particles from liquid precursors were prepared
using the apparatus and conditions identified in Example 4, except
that the feed materials and amounts are listed in Table 5.
TABLE-US-00005 TABLE 5 Material Amount Trimethyl Borate 1000 grams
Iso-Octane 35.8 grams Tetramethoxysilane 22.9 grams
[0066] The measured B.E.T. specific surface area of the produced
material was 39 square meters per gram using a Gemini model 2360
analyzer (available from Micromeritics Instrument Corp., Norcross,
Ga.), and the calculated equivalent spherical diameter was 61
nanometers. FIG. 8 is an X-ray diffraction spectrum of the
particles measured using an X-ray diffractometer (Philips X' Pert
MPD model). The analysis indicated that the powder comprised
crystalline boron carbide.
Example 6
[0067] Boron carbide particles from liquid precursors were prepared
using the apparatus and conditions identified in Example 5, except
that the feed materials and amounts are listed in Table 6.
TABLE-US-00006 Material Amount Trimethyl Borate 1000 grams
Iso-Octane 17.2 grams Tetraethoxysilane.sup.6 83.5 grams
.sup.6Commercially available from Alfa Aesar, Ward Hill,
Massachusetts.
[0068] The measured B.E.T. specific surface area of the produced
material was 47 square meters per gram using a Gemini model 2360
analyzer (available from Micromeritics Instrument Corp., Norcross,
Ga.), and the calculated equivalent spherical diameter was 51
nanometers. FIG. 9 is a TEM image of a representative portion of
the particles (100,000.times. magnification) prepared in the manner
described in Example 1. FIG. 10 is an X-ray diffraction spectrum of
the particles measured using an X-ray diffractometer (Philips X'
Pert MPD model). The analysis indicated that the powder comprised
crystalline boron carbide and small amounts of boron oxide, silicon
carbide, and graphite.
[0069] It will be readily appreciated by those skilled in the art
that modifications may be made to the invention without departing
from the concepts disclosed in the foregoing description. Such
modifications are to be considered as included within the following
claims unless the claims, by their language, expressly state
otherwise. Accordingly, the particular embodiments described in
detail herein are illustrative only and are not limiting to the
scope of the invention which is to be given the full breadth of the
appended claims and any and all equivalents thereof.
* * * * *