John & Peter MILEWSKI
Single
Crystal Light Filament
Dr. John V. Milewski
Professional Engineer
Dr.
John V. Milewski is an Internationally recognized leader
and consultant in his field of Advanced Materials. He is a
professional engineer, scientist, inventor, entrepreneur,
writer, publisher, editor and lecturer. He is a retired staff
member of Los Alamos National Labs and has worked previously
as a scientific staff member at Exxon Research Center and at
Thiokol Chemical Rocket Engine Div. He recently founded his
own research company called Superkinetic, Inc. where he is
currently working on a revolutionary new electric light bulb
based on using a single crystal fiber as a filament.
He
is a graduate of the University of Notre Dame in Chemical
Engineering (1951, Stevens Institute of Technology with an MS
in Metallurgy (1959), and has his Ph.D. in Ceramic Engineering
from Rutgers University (1873). Dr. Milewski holds 30 patents
and has over 42 publications and has edited 4 books in his
field.
In
this write up Dr. Milewski will present information about his
crystal growing business and his revolutionary unified field
theory on SuperLight energy and he hopes to tie these two
together to bring about cheap, clean energy and healing and
regeneration.
In
March 2000 Dr. John V. Milewski along with his son Dr. Peter
D. Milewski were honored to have their crystal filament light
bulb invention Patent # 4,864,186 put on permanent exhibit at
the Smithsonian American History Museum. This is part of a new
exhibit titled "Lighting a Revolution" honoring the six most
significant ideas in lighting field from 1950 to 2000.
Peter Milewski also
advocates the existence of Super-Light (C^2), concerning which
visit his website:
http://www.hbci.com/~wenonah/new/milewski.htm
USP # 4,864,186
Single
Crystal Whisker Electric Light Filament
John &
Peter Milewski
( September 5,
1989 )
Abstract ---
An electric light filament comprising a single crystal whisker
is disclosed. In the preferred embodiment the whisker consists
essentially of silicon carbide (SiC), preferably beta silicon
carbide, doped with a sufficient amount of nitrogen to render
the whisker sufficiently electrically conductive to be useful
as a light bulb filament at household voltages. Filaments made
of such materials are characterized by high strength,
durability, and resilience, and have higher electrical
emissivities than conventioanl tungsten filaments.
U.S. Class:
313/341 ; 313/578; 313/633
International
Class: H01K 1/00 (20060101); H01K 1/04 (20060101);
H01K 001/10 ()
References
Cited:
U.S.
Patent Documents: 821017 May 1906 Clark // 3875477
April 1975 Fredriksson et al. // 4513030 April 1985 Milewski
// 4565600 January 1986 Ricard
Other References
Encylopedia of
Materials Science and Engineering, "Whiskers", Milewski,
Bever, ed., pp. 5344-5346, 1/86, Pergamon Press. .
Journal of Materials Science, "Growth of Beta-Silicon Carbide
Whiskers by the VLS process", Milewski et al., pp. 1160-1166,
1/85, Chapman and Hall..
Description
BACKGROUND OF THE
INVENTION
1. Field of the
Invention
The invention
described and claimed herein is generally related to electric
light filaments, and more particularly to materials used as
such filaments.
2. Description of
the related art.
Previously known
electric light filaments are typically made of materials which
are either polycrystalline in nature or which are amorphous,
or noncrystalline, in nature. Such materials suffer from the
disadvantage that they become brittle with time at elevated
temperatures.
Polycrystalline
materials, which include the majority of commercially
available metallic filaments, are characterized by the
presence of crystal grain boundaries, dislocations, voids and
various other microstructural imperfections. These
microstructural imperfections lead to grain growth and
recrystallization, particularly at elevated temperatures,
which in turn lead to increased brittleness and diminished
strength.
Metallic filaments
also suffer from a disadvantage that is a consequence of the
relatively low electrical resistance that characterizes
metallic filaments. The low electrical resistance requires
that the filaments be made quite long, which in turn requires
the filament to be tightly coiled in order to fit it into a
light bulb of suitable size. Coiling of the filament
effectively reduces the radiating surface area because the
coiled filament partially occludes itself, thereby diminishing
the radiative efficiency of the filament.
Another disadvantage
of metallic filaments is that metals in general, and
particularly tungsten, have a relatively high
resistivity/temperature coefficient. From room temperatures to
approximately 1200.degree. C. the resistance of metal
filaments increases as much a six-fold, resulting in high
electrical power consumption at operating temperatures.
Amorphous metals
used as filaments undergo various degrees of crystallization
at elevated temperatures, resulting in the development of
grain boundaries that decrease the strength and toughness of
these materials also. Additionally amorphous materials are
often of lower strengths initially relative to crystalline
materials.
Accordingly, it an
object and purpose of the present invention to provide an
electric light filament which is of improved strength,
durability and resilience, particularly at elevated
temperatures.
It is also an object
and purpose of the present invention to provide an electric
light filament which does not undergo progressive
crystallization or recrystallization at incandescent
temperatures.
SUMMARY OF THE
INVENTION
The present
invention provides an electric light filament comprising a
single crystal fiber known as a "whisker." The whisker is
preferably a high emissivity ceramic whisker. In the preferred
embodiment the whisker consists essentially of a
monocrystalline fiber of silicon carbide, most preferably beta
silicon carbide. Such filaments are characterized by high
mechanical strength and durability at the elevated
temperatures required to achieve incandescence. Also, such
crystals are characterized by their high surface area to
volume ratios, as a result of their typically small
cross-sectional diameters, which are on the order of about 5
microns. They are also characterized by their high electrical
resistances and high emissivities relative to metals.
Additionally, the resistance of silicon carbide does not
increase with temperature as much as tungsten, such that power
consumption of a silicon carbide whisker is lower at
incandescent operating temperatures. In accordance with
another aspect of the invention, the silicon carbide whisker
is doped with nitrogen to increase the conductivity of the
whisker to a level appropriate for use of the whisker as an
electric light filament.
These and other
aspects of the present invention will be more apparent upon
consideration of the following detailed description of the
preferred embodiments of the invention.
The invention will
be more readily understood by referring to the figure of the
drawing.
FIG. 1 is a
plan view of the electric light filament.
In greater detail
FIG. 1 shows Single Crystal Filament (1).
DESCRIPTION OF
THE PREFERRED EMBODIMENTS
Whiskers are minute,
high purity, single crystal fibers. More than a hundred
materials, including metals, oxides, carbides, halides,
nitrides, an carbonaceous materials have been prepared as
whiskers. As a consequence of their high chemical purity and
monocrystalline structures, whiskers are characterized by very
high mechanical tensile strengths, which in the case of some
materials approach the theoretical maximum strength of the
material based on actual interatomic bonding forces. Because
of their high tensile strengths, whiskers have been primarily
of interest as agents used to reinforce ceramic, metallic and
even polymeric matrices.
In addition to the
high mechanical strength that results from the highly ordered
crystalline structure of whiskers, other significant and, to
some extent, unexpected changes are obtained in the optical,
magnetic, dielectric and electrical conductivity of materials
that are formed as whiskers.
Ceramic whiskers are
unique in that they can be strained elastically as much as
three percent without permanent deformation, compared with
about 0.1 percent for bulk ceramic materials. In addition,
whiskers exhibit considerably less strength deterioration with
increasing temperatures than the best conventional
high-strength metal alloys. Further, no appreciable fatigue
effects have been observed in whiskers. They can be handled
roughly, milled or chopped, elevated to high temperatures, and
otherwise worked without any appreciable loss of strength.
Whiskers can be
produced in a range of fiber sizes and fiber forms. A number
of processes are known for producing whiskers in various
forms, including forms known by terms such as grown wool,
felted paper and loose fibers.
Some ceramic
materials are semiconductors, and are known to be very
resistant to current flow. However, it is known that by doping
silicon carbide with nitrogen, which becomes located
interstitially within the silicon carbide crystal structure,
the electrical conductivity of the silicon carbide can be
increased to a level that permits its use as an incandescent
electric light filament. In this regard, the emissivity of
silicon carbide is also particularly conducive to this use,
lying in the range of 0.9, which is considerably higher than
the emissivity of approximately 0.4 that characterizes most
metallic filaments.
These
characteristics are all conducive to the new use of whiskers,
provided by the present invention, as electric light
filaments.
A demonstration of
the present invention was conducted using a number of single
crystal whiskers of beta silicon carbide (SiC). The whiskers
were doped green with nitrogen. The whiskers ranged from three
(3) millimeters to thirty (30) millimeters in length and were
approximately five (5) microns in diameter. The whiskers were
mounted between two wire binding posts which were spaced
approximately three millimeters apart. A direct voltage was
applied to the whiskers across the binding posts, and the
temperatures achieved in the whiskers were measured with an
optical pyrometer. When 30 volts (d.c.) was applied to the
whiskers, the whiskers glowed in the high red heat
(800.degree.-1,000.degree. C.) region. Higher voltages in air
caused the whiskers to burn out due to oxidation. In partial
vacuum temperatures were of 1100.degree. C. to 1440.degree. C.
were achieved in the whiskers before burnout.
The silicon carbide
filaments were compared with conventional tungsten filaments.
For both types of filaments, the properties of electrical
resistance, filament length, filament diameter and filament
weight were measured. From the measured voltages and current
readings, power requirements at various filament temperatures
were calculated and are set forth below. Qualitative analyses
of light output in lumens were done by comparing a silicon
carbide filament to that of a candle, and a candle to a 4 watt
clear glass tungsten filament light bulb. This was done using
the dual screen method.
The temperatures,
voltages and currents were measured simultaneously on each
filament as the voltage was increased. Temperatures were
measured using an optical pyrometer. Voltage and current
readings were performed on a digital multimeter. The voltages
were regulated using a variable transformer. The masses,
lengths, and cross-sectional areas were also measured and/or
calculated. For comparison, similar measurements were made on
clear glass conventional 25-watt and 4-watt tungsten filament
light bulbs. Data obtained from these tests, which compare the
silicon carbide filament to the tungsten filament, are given
in Tables I through V below.
TABLE I
______________________________________ Comparative Physical
Property Data Beta SiC Coiled Ratio Whisker Tungsten SiC/W
______________________________________ Mass (mg) .002 4.5
1/2500 Length (mm) 30 32 1/1 300 (uncoiled) 10/1 Diameter
(microns) 5 250 1/50 Resistance (Ohms) 1800-3100 74 (25 watts)
25/1 560 (4 watts) 5/1 Effective Radiating 2.86 .077 36/1
Surface Area to Volume Ratio Emissivity at 1200.degree. C. .90
.40 2.3/1 Resistance Change, Room Temp. to 1200.degree. C. 2x
6x 1/3 ______________________________________
TABLE II
______________________________________ Tungsten 4 Watt Bulb
Voltage Current Power Resistance Temperature (volts) (ma)
(watt) (ohms) (.degree.C.)
______________________________________ .77 1.36 .001 560 --
6.0 4.65 .028 1300 -- 10.2 7.36 .075 1390 -- 15.0 9.48 .142
1580 first light 19.6 11.11 .218 1760 800 23.0 12.79 .294 1800
860 28.3 14.32 .405 1980 920 32.7 15.76 .515 2070 980 37.0
17.12 .633 2160 1030 41.6 18.52 .770 2250 1080 46.1 19.72 .909
2340 1120 50.8 20.92 1.06 2430 1180 55.4 22.17 1.23 2500 1235
59.8 23.07 1.38 2590 1290 64.1 24.33 1.56 2630 1340 68.4 25.35
1.73 2700 1390 72.7 26.44 1.92 2750 1390 77.3 27.48 2.12 2810
1420 81.8 28.41 2.32 2880 1420 86.0 29.32 2.52 2930 1430 90.2
30.00 2.71 3000 1480 94.6 31.05 2.94 3050 1480 98.9 31.99 3.16
3090 1510 103.0 32.76 3.374 3140 1530 107.0 33.67 3.603 3180
1530 111.4 34.30 3.821 3190 1550 115.7 35.94 4.158 3220 1560
119.7 35.89 4.296 3340 1575 123.6 36.72 4.539 3370 1585
______________________________________
TABLE III
______________________________________ Tungsten 25 Watt Bulb
Voltage Current Power Resistance Temperature (volts) (ma)
(watt) (ohms) (.degree.C.)
______________________________________ .77 10.4 .008 74 --
10.2 74.9 .76 136 -- 19.6 90.8 1.8 216 -- 28.3 102.2 2.9 277
-- 37.0 110.5 4.1 335 930 46.1 118.6 5.5 389 1070 55.4 126.9
7.0 437 1212 64.1 134.3 8.6 478 1330 72.7 141.0 10.3 516 1410
81.8 148.0 12.1 553 1450 90.2 156.6 14.1 576 1550 98.9 162.8
16.1 607 1630 107.0 169.4 18.1 632 1720 115.7 175.9 20.4 658
1830 123.6 181.1 22.4 682 1900
______________________________________
TABLE IV
______________________________________ Silicon Carbide Whisker
Filament Voltage Current Power Resistance Temperature (volts)
(ma) (watt) (ohms) (.degree.C.)
______________________________________ .775 .43 -- 1802 --
10.6 5.89 -- 1800 -- 19.6 10.00 -- 1960 -- 28.29 13.3 -- 2127
-- 36.99 15.43 -- 2400 -- 46.09 17.07 .78 2700 800 50.77 17.50
.89 2900 850 55.44 18.00 1.00 3080 950 59.79 18.45 1.10 3240
1060 64.13 18.80 1.21 3410 1170 68.42 19.10 1.31 3582 1260
______________________________________
TABLE V
______________________________________ Silicon Carbide Whisker
Filament Voltage Current Power Resistance Temperature (volts)
(ma) (watt) (ohms) (.degree.C.)
______________________________________ .77 .25 .0002 3080 --
6.0 1.71 .010 3500 -- 10.2 2.50 .025 4080 -- 15.0 3.28 .049
4570 760 19.6 3.96 .078 4950 850 23.0 4.49 .103 5120 900 28.3
4.92 .139 5750 1050 32.7 5.16 .169 6330 1140 37.0 5.45 .191
6790 1250 41.6 5.40 .227 7700 1340 46.1 5.60 .258 8230 1400
______________________________________
As summarized in
Table I, the resistance of the tungsten filaments increases
six-fold over the temperature range from room temperature to
1200.degree. C., wherease the silicon carbide filaments
increase in resistance only two-fold. Also, the emissivity of
the silicon carbide whisker filaments at 1200.degree. C. is on
the order of 0.9, whereas the emissivity of the tungsten
filament is on the order of 0.4.
One surprising
discovery is that silicon carbide whiskers are considerably
more efficient as electric light filaments than conventional
tungsten filaments. Comparisons with conventional tungsten
filaments have indicated that, to achieve a particular
incandescent temperature, silicon carbide filaments require
significantly less electrical power than a comparble tungsten
filament. This is thought to be a consequence of a higher
surface area to volume ratio in the silicon carbide whiskers
than in tungsten filaments, and possible also due to a higher
emissivity in silicon carbide whiskers than in tungsten
filaments.
These advantages are
considered to be a consequence of a higher resistance and a
higher surface area to volume ratio in the silicon carbide
whiskers than in tungsten filaments, as well as a higher
emissivity in silicon carbide than in tungsten filaments.
From a review of the
foregoing data, it is evident that because of the physical
structure of the silicon carbide whisker and its significantly
different physical, mechanical and electrical properties,
single crystal whisker filaments have many superior
performance properties and as a result produce a more
efficient light bulb filament when compared to conventional
polycrystalline metallic tungsten wire filaments.
Although the present
invention is described herein by reference to a preferred
embodiment of the invention, it will be understood that
various modifications, alterations and substitutions which may
be apparent to one of ordinary skill in the art may be made
without departing from the essential invention. Accordingly,
the present invention is defined by the following claims.
USP # 5,404,836
Method
and Apparatus for Continuous Controlled Production of
Single Crystal Whiskers
John V.
Milewski ( April 11, 1995 )
Abstract --- Described
herein is a method and apparatus for continuously growing
single crystal whiskers of silicon carbide, silicon nitride,
boron carbide and boron nitride by the VLS process under
controlled reaction conditions. A growth substrate such as a
plate of solid graphite is coated with a suitable VLS catalyst
and is conveyed through a tubular furnace, into which is
separately introduced two feed gases. The first feed gas
contains a cationic suboxide precursor such as silicon
monoxide or boron monoxide. The second feed gas contains an
anionic precursor compound such as methane or ammonia. The
precursor compounds react upon exposure to the catalyst by the
VLS process to produce crystalline whiskers. The associated
apparatus includes a conveyor assembly that continuously
circulates multiple substrate growth plates through the
furnace and past a harvesting device which brushes the
whiskers from the plates and removes them by vacuum
collection. Whiskers of uniform size, shape, and purity are
produced.
Current U.S.
Class: 117/87 ; 117/205; 117/91; 117/921; 117/952;
117/98; 423/290; 423/291; 423/344; 423/346; 501/87; 501/95.1
Current
International Class: C30B 25/00 (20060101); C30B
029/36 (); C30B 029/38 (); C30B 029/62 (); C30B 035/00 ()
References Cited:
U.S.
Patent Documents: 2122982 July 1938 Leo //
3580731 May 1971 Milewski // 3622272 November 1971 Shyne
et al. // 3632405 January 1972 Kippenberg // 3692478 September
1972 Kippenberg et al. // 3721732 March 1973 Knippenberg et
al. // 3808087 April 1974 Milewski et al. // 3855395 October
1974 Cutler // 4013503 March 1977 Kippenberg et al. // 4155781
May 1979 Diepers // 4500504 February 1985 Yamamoto // 4504453
March 1985 Tanaka et al. // 4702901 October 1987 Shalek
4789536 December 1988 Schramm et al. // 4789537 December 1988
Shalek et al. // 4911781 March 1970 Fox et al. // 4915924
April 1990 Nadkarni et al. // 5116679 May 1992 Nadarni et al.
Foreign Patent
Documents : 62-91499 Apr., 1987 JP
// 63-291900 Nov., 1988 JP // 1-197400
Aug., 1989 JP
Description
BACKGROUND OF THE
INVENTION
1. Field of the
Invention
The invention
described and claimed herein is generally related to the
production of single crystal fibers known as whiskers. More
particularly, the present invention is related to apparatus
and methods for continuously making whiskers by the
vapor-liquid-solid (VLS) method.
2. Description Of
Related Art Including Information Disclosed Under 37
C.F.R. 1.97-1.99
Crystalline fibers
of materials such as silicon carbide, silicon nitride, boron
carbide and boron nitride are commonly referred to as
whiskers. In bulk form these materials are known for their
hardness. In whisker form, they are further noted for their
high tensile strength and high modulus relative to other
fibrous materials, as well as their high chemical stability
and high resistance to corrosion and oxidation. All of these
characteristics are retained even at relatively high
temperatures, making whiskers of these materials desirable as
reinforcing elements in ceramics, metals, polymeric composite
materials, and in other applications. Further, yarns and other
woven materials made from these whiskers would have widespread
utility, but there has not been heretofore available a process
for making significant quantities of high quality whiskers
that are also sufficiently long to be spun into yarn.
It has been
previously known to prepare silicon carbide and silicon
nitride whiskers by processes such as those described in U.S.
Pat. Nos. 2,122,982 and 3,855,395. These processes suffer from
the disadvantage that they produce a significant amount of
non-fiber particulate material as well as particular
impurities, which must be separated from the whisker product
before it can be used.
Also, the
above-referenced previously known processes produce whiskers
of varying thicknesses as well as varying lengths. In this
regard, it is known that whiskers having thicknesses of less
than about a micron constitute a health hazard similar to that
of asbestos, and a three micron thickness has been considered
a minimum diameter for safe handling in the absence of special
precautions. The whisker product of these prior art processes
contains a significant amount of sub-micron sized whiskers,
and for this reason must be considered hazardous.
Whiskers are also
known to be produced by the Tokai process, disclosed in U.S.
Pat. No. 4,500,504 to Yamamoto, which involves exchanging a
water soluble catalyst onto a silica gel, mixing the dried
silica gel with a fine particle carbon black, and reacting the
mixture in a nonoxidative atmosphere at elevated temperature.
This process produces a sub-micron diameter whisker.
U.S. Pat. No.
4,504,453 to Tanaka, et al., discloses the use of thin, porous
silicon-and carbon-containing material, which is passed
through a series of increasing temperature zones to produce
sub-micron silicon carbide whiskers in the material.
The
vapor-liquid-solid (VLS) process for growing whiskers is
described, for example, in U.S. Pat. No. 3,622,272 to Shyne,
et al., U.S. Pat. No. 3,692,478 to Knippenberg, et al., U.S.
Pat. No. 3,721,732 to Knippenberg, et al., and U.S. Pat. No.
4,013,503 to Knippenberg, et al. In the VLS process gaseous
compounds of whisker constituents such as silicon and carbon
are introduced into a reaction chamber at an elevated
temperature. There is also introduced into the reaction
chamber a solid substrate that is coated with a particulate
catalyst, which melts at the temperature of the chamber to
form small molten beads. The gaseous reactants dissolve in the
molten catalyst beads. Suitable catalysts for carbide type
whiskers for this purpose include iron, manganese, nickel,
cobalt, ferro silicon, stainless steel and various glasses for
nitride type whiskers.
Solid silicon
carbide crystallizes out of the molten catalyst at the
interface between the molten catalyst and the underlying solid
substrate. The silicon carbide typically crystallizes in the
form of a slender whisker of solid crystalline beta silicon
carbide, with the molten catalyst bead resting on and being
raised upwardly on the end of the whisker as the whisker
grows. The whisker is typically of a diameter similar to that
of the molten catalyst bead.
Other carbide and
nitride whiskers, for example B.sub.4 C, TiC, S.sub.3 N.sub.4
and BN can also be grown by the VLS process, using various
transition metals, iron alloys and glasses as catalysts.
U.S. Pat. No.
4,789,537 to Shalek, et al., discloses an improvement to the
VLS process, whereby a catalyst is prealloyed with silicon and
carbon to accelerate the initiation of the VLS process for
growing silicon carbide whiskers. A disadvantage however of
using prealloyed catalysts is that prealloying lowers the melt
viscosity of the catalyst, thereby causing excessive wetting
and spreading of the molten catalyst, which breaks into small
parts. This in turn produces a large number of submicron
diameter whiskers, which as noted above are recognized to be a
health hazard.
It has been
previously known to grow submicron diameter silicon carbide
whiskers on carbon yarn that is pulled through a tubular
furnace, as disclosed in U.S. Pat. No. 3,580,731. Although the
process disclosed therein allows for continuous whisker
production in small quantities, it does not utilize the
advantages of the VLS process, and there is also no means for
controlling the stoichiometry of the reactant gases in the
furnace, so that there is no means for controlling the crystal
sizes or morphologies obtained by the process.
SUMMARY OF THE
INVENTION
Accordingly, it is
the object and purpose of the present invention to provide an
improved method and a related apparatus for growing single
crystal whiskers of silicon carbide, silicon nitride, boron
carbide and boron nitride.
It is also an object
and purpose of the present invention to provide a method and
apparatus for producing such whiskers on a continuous basis.
It is another object
and purpose of the present invention to provide a method and
apparatus for continuously producing single crystal whiskers
characterized by high purity, low particulate levels, and high
aspect ratios.
Still another object
of the invention is to provide a method and apparatus for
producing whiskers which are uniformly more than about 3
microns in diameter and which therefore pose a less health
hazard than whiskers products having varying whisker
diameters, including submicron diameters which are recognized
to be hazardous.
It is another object
of the present invention to provide a method and apparatus for
continuously producing whiskers by the VLS process, wherein
the stoichiometry of the reactant gas mixture and the
concentration of the whisker constituents in the molten VLS
catalyst can be continuously controlled to obtain optimum
whisker sizes and morphologies.
With regard to the
present invention, it if noted that the compounds SiC,
Si.sub.3 N.sub.4, B.sub.4 C and BN may all be characterized as
consisting of a cationic element (silicon or boron) and an
anionic element (carbon or nitrogen). Consequently, in this
specification the term cationic precursor feed gas is used to
refer to gases that are useful in the VLS process and which
contain silicon or boron and the term anionic precursor feed
gas is used to refer to gases which are useful in the VLS
process and which contain carbon or nitrogen.
The present
invention provides a method for continuously growing whiskers
of silicon carbide, silicon nitride, boron carbide or boron
nitride. In accordance with the method, a suitable growth
substrate is first coated with a solid particulate catalyst,
and is subsequently conveyed through a furnace maintained at
an elevated temperature sufficiently high to melt the
catalyst. There are separately introduced into the heated
furnace cationic and anionic precursor feed gases which
contain gaseous precursor compounds containing the elemental
components of the whiskers to be crystallized by the VLS
process. The relative rates of introduction of the precursor
compounds may be varied so to enable the stoichiometry of the
gaseous reactants in the furnace, as well as the degree of
supersaturation of the gases in the VLS catalyst, to be varied
as may be necessary to obtain optimum crystalline
morphologies, sizes or other whisker characteristics.
In the preparation
of either silicon carbide or silicon nitride whiskers by the
method of the invention, silicon is preferably introduced into
the furnace in the form of gaseous silicon monoxide. The
silicon monoxide is preferably produced by a silicon monoxide
generator located within the furnace. The generator includes a
porous refractory brick material that is impregnated with
powdered silica and carbon. The silica and carbon react to
form silicon monoxide. The rate of generation of silicon
monoxide and the rate of its introduction into the furnace are
preferably controlled by passing a moderating gas through the
porous generator brick. The moderating gas consists
essentially of a reducing carrier such as hydrogen, and a
variable concentration of carbon monoxide. Carbon monoxide
suppresses the generation of silicon monoxide, and is thus
used to moderate the introduction of silicon monoxide into the
furnace.
The anionic
precursors carbon and nitrogen are introduced into the furnace
in form of a light hydrocarbon and ammonia, respectively.
These compounds are introduced in a reducing carrier gas that
preferably includes an inert diluent, for example nitrogen. In
the preferred embodiment, carbon is introduced as methane in a
carrier of hydrogen and nitrogen. Nitrogen is preferably
introduced as ammonia in a carrier of hydrogen and nitrogen.
Ammonia thermally decomposes to hydrogen and nacent-nitrogen,
which an active specie of nitrogen.
Boron is introduced
as a cationic precursor in the form of gaseous boron monoxide.
The boron monoxide is generated within the furnace by a
solid-state disproportionation reaction of solid boron and
solid boric oxide, which can be combined for example in a
pelletized bed. The rate of generation of boron monoxide and
its introduction into the furnace may be preferably controlled
by passing a moderating carrier gas through the boron monoxide
generator, the moderating gas preferably consisting of an
inert diluent such as nitrogen, in a carrier of hydrogen.
In the furnace, the
gaseous cationic and anionic precursors react in the molten
catalyst beads to form crystalline whiskers by the VLS
process. For example, silicon monoxide and methane react in a
molten metal catalyst to form crystalline beta silicon carbide
in the VLS process, and to release water vapor and hydrogen
waste byproducts. Likewise, silicon monoxide and ammonia are
introduced to form silicon nitride whiskers; boron monoxide
and methane may be introduced to form boron carbide whiskers;
and boron monoxide and ammonia may be introduced to form boron
nitride whiskers.
The present
invention also is directed to an apparatus particularly
adapted for conducting the process of the invention, in that
it continuously produces crystalline whiskers of uniform size
and quality. The apparatus comprises a furnace and a conveyor
means supporting associated growth substrate means. The
conveyor means is operable to convey the growth substrate
through the furnace, preferably in a continuous loop. The
apparatus further comprises means for applying a VLS catalyst
on the growth substrate means as the growth substrate means is
conveyed into the furnace by the conveyor means, and
collection means for collecting whiskers grown on the growth
substrate means upon exit of the growth substrate means from
the furnace. The apparatus further includes means for
separately introducing cationic and antonic precursor feed
gases into the furnace in contact with the substrate growth
means.
In the preferred
embodiment the conveyor means includes a conveyor system that
carries a plurality of growth substrate plates in a continuous
loop through the furnace, which is preferably tubular in
structure. The growth substrate plates are preferably composed
of graphite, alumina, or other refractory materials, with the
choice of a particular substrate material depending on the
type of whiskers to be grown.
The means for
applying the VLS catalyst preferably includes a particulate
dispersal means for uniformly dispersing a powdered
particulate catalyst material onto the growth substrate plates
as they enter the furnace.
The collection means
preferably includes a brushing apparatus for brushing the
whiskers from the growth substrate plates as they exit from
the furnace, and a vacuum collection system for transporting
the whiskers to a collection container.
These and other
aspects of the present invention will be more apparent upon
consideration of the following detailed description of the
invention, when taken with the accompanying drawings.
BRIEF DESCRIPTION
OF THE DRAWINGS
The Figures set
forth in the accompanying drawings from a part of this
specification and are hereby incorporated by reference. In the
Figures:
FIG. 1 is a
plan view of the whisker growth apparatus of the present
invention;
FIG. 2 is a
cross sectional view of the furnace tube of the apparatus of
FIG. 1, taken as section A--A;
FIG. 3 is a
cross sectional view of an alternative embodiment having
multiple vertically oriented substrate growth plates; and
FIG. 4 is an
illustration of the VLS whisker growth process.
DESCRIPTION OF
THE PREFERRED EMBODIMENT
Referring to FIGS. 1
and 2, the apparatus of the present invention includes a
conveyor belt system 1 which carries a number of growth
substrate plates 2 in a continuous loop. The conveyor system
may be a conveyor belt, chain or other suitable arrangement.
The catalyst spray coater 3 coats the growth substrate plates
2 with catalyst particles 4.
The conveyor belt
system 1 carries the growth substrate plates 2, coated with
catalyst particles 4, into a furnace tube 5. The furnace 5
includes a gas inlet tube 6 for introduction of a moderating
gas, for example carbon monoxide and carrier gas. The furnace
tube 5 extends through a high temperature furnace 7, which
includes a central hot zone 8.
In the hot zone 8 of
the furnace 7 is located a cationic precursor suboxide gas
generator 9, which generates a suboxide feed gas as more
particularly described below. Spent gases are discharged
through a spent gas discharge tube 10. Monocrystalline
whiskers 11 grow on the upper surface of the growth substrate
plates 2 as they are conveyed through the furnace. Upon
existing from the furnace 7 the whiskers 11 are brushed from
the plates 2 by a rotating brush 12, harvested by a vacuum
collection system 13, and collected in container 14.
An anionic precursor
feed gas is introduced through a feed tube 15 and dispersed
throughout the furnace by a distribution manifold 16 having
gas mixing jets 17.
Referring
particularly to FIG. 2, the feed gases are mixed in a mixing
chamber 18 which is below the suboxide gas generator 9 and
above the plates 2.
FIG. 3 illustrates
an alternative embodiment in which there are multiple plates 2
which are vertically oriented. The plates 2 are pushed through
the furnace, sliding in upper and lower slots which are
provided for guiding the plates 2. In this regard, the plates
2 in some cases can be made of graphite, as can the supporting
slots, giving a low-friction sliding structure even at high
temperatures. Above the plates 2 is provided a gas mixing
chamber 18, in which the precursor feed gases are mixed. The
anionic precursor gases, preferably containing carbon or
nitrogen in the form of methane or ammonia in suitable
carriers, are introduced through manifolds 16 and into the
reaction chamber through associated mixing jets 17. The porous
cationic precursor suboxide generator 9 is suspended above the
mixing changer 18. Above the suboxide generator 9 is a gas
inlet tube 6 through which a moderating gas is introduced and
caused to flow downwardly through the porous suboxide
generator 9. Between the vertical plates 2 are whisker growth
areas in which the mixed precursor gases are exposed to the
molten catalyst beads on the plates 2. The whiskers 11 grow
outwardly from the plates 2 and are harvested upon exit from
the furnace by brushing and vacuum collection, as described
above with respect to the embodiment shown in FIGS. 1 and 2,
or by other suitable means.
Still referring to
FIG. 3, between the furnace walls 20 and at the bottom of the
furnace is a spent gas collection manifold 21, in which there
is a spent gas outlet tube 22.
The anionic
precursor feed gas introduced through tube 15 contains carbon
in the form of a hydrocarbon gas, preferably methane, which is
preferably contained in a reducing carrier consisting
essentially of hydrogen and nitrogen. The carrier preferably
consists primarily of hydrogen, containing nitrogen in a
concentration of between about 4% and 12%, and most preferably
about 7%, by volume; and also containing methane in a
concentration of between about 1% and 3% by volume, and most
preferably about 2%.
In the production of
silicon carbide whiskers, silicon is supplied to the reaction
in the form of silicon monoxide, which is generated by a solid
stated reaction between silica (SiO.sub.2) and carbon, which
may be in the form of carbon black. Generation of silicon
monoxide is moderated and controlled by passing a gaseous
mixture of carbon monoxide and hydrogen through a porous
refractory brick impregnated with a mixture of powdered carbon
and silica. Carbon monoxide inhibits the generation of silicon
monoxide in this system, and hence by controlling the amount
of carbon monoxide in the hydrogen stream the rate of
generation of silicon monoxide can be controlled. In practice,
the carbon monoxide concentration in the hydrogen stream is
preferably between about 5% and 20% by volume, with a range of
7% to 8% by volume being most preferred.
The silicon monoxide
generator preferably includes a porous, refractory,
aluminosilicate brick impregnated with a mixture of finely
powdered silica and carbon. A suitable brick material is sold
by Babcock and Wilcox Co. and is identified as K-30 brick.
Within the impregnated brick, silicon monoxide gas is formed
by the reaction:
It will be
understood from the above reaction that excess carbon monoxide
will suppress the generation of silicon monoxide. This is the
basis for the use of externally supplied carbon monoxide to
moderate the production of silicon monoxide. This is
desireable for two reasons. First, the solid state silicon
monoxide generator typically does not generate silicon
monoxide at a constant rate. The generation rate for a freshly
composed SiO generator typically rises quickly to a maximum
rate and then progressively decreases as the solid reactants
are gradually depleted. Consequently it is desireable to
moderate the generation of silicon monoxide bypassing
externally supplied carbon monoxide through the generator. The
carbon monoxide content of the moderating gas is gradually
decreased, to counter the decreasing generation of SiO
mentioned above, to thereby obtain a relatively constant
supply of silicon monoxide being introduced into the furnace
reaction chamber. In practice the composition of the gaseous
reaction mixture in the furnace chamber may be monitored, for
example by spectroscopic techniques, to determine the optimum
use of the moderating gas.
The catalyst
particles melt to molten beads upon entering the furnace. The
feed gases, for example methane and silicon monoxide, dissolve
in the molten metal catalyst. Under the effect of the molten
catalyst the feed gases react by the reaction:
It is believed that
the methane and silicon monoxide actually decompose to
elemental silicon and carbon at the interface between the
furnace atmosphere and the molten catalyst, and that it is
primarily elemental silicon and carbon which dissolve in the
catalyst and subsequently react with one another to
crystallize as silicon carbide.
The solid silicon
carbide crystallizes out of the molten catalyst in the form of
a slender whisker of crystalline silicon carbide. The molten
catalyst bead is elevated on the end of the whisker as the
whisker continuously crystallizes and grows longer. Growth
continues as the feed gases dissolve in the molten catalyst
bead and crystallize therefrom at the solid-liquid interface
between the bead and the underlying whisker. The length of the
whisker is determined by the length of time the catalyst is
exposed to the feed gases. The diameter of the whisker is
determined largely by the size of the molten bead. By
selecting the size of the solid particles which melt to become
the catalyst beads, the diameters of the resulting whiskers
can be selected. Thus, with this process the aspect ratio,
which is the ratio of the length to the diameter of the
whiskers, can also be controlled. This is significant in
micropacking for fabrication of efficiently reinforced
short-fiber composites.
A sequence for the
growth of beta silicon carbide (SiC) whiskers by the VLS
process is shown in FIG. 4. Referring to FIG. 4, solid
particles 4 of a suitable metal catalyst, for example #304
stainless steel, are melted at about 1,400.degree. C. to form
spherical liquid catalyst beads 23. Gaseous feed vapors are
introduced into the furnace and supersaturate the molten beads
23. Silicon carbide crystallized out of the molten bead 23 at
the liquid-solid interface 26 between the bead 23 and the
growth substrate plate 2. As whiskers 11 form and grow, the
beads 23 are raised upwardly on the ends of the whiskers 11,
and continue to absorb feed gases from the surrounding furnace
atmosphere to continue the growth process.
In the production of
silicon nitride (Si.sub.3 N.sub.4) whiskers, silicon monoxide
is generated, moderated with carbon monoxide, and introduced
into the furnace as described above. Nitrogen is introduced
into the furnace in a second feed gas, or an anionic precursor
feed gas, in the form of ammonia (NH.sub.3). The ammonia is
carried in a carrier of hydrogen and an inert diluent such as
nitrogen. This feed gas preferably includes between 30% and
65% by volume hydrogen, 30% to 55% by volume ammonia, and 2%
to 15% by volume nitrogen. Optimum composition of the anionic
precursor feed gas is approximately 55% hydrogen,
approximately 40% ammonia, and approximately 5% nitrogen.
In the production of
boron carbide (B.sub.4 C) whiskers, carbon is introduced into
the furnace as described above with respect to the production
of silicon carbide. That is, it is introduced in the form of a
light hydrocarbon, preferably methane, in a reducing carrier
preferably consisting of hydrogen and an inert diluent such as
nitrogen. The carrier preferably consists primarily of
hydrogen, containing nitrogen in a concentration of between
about 4% and 12% and most preferably about 7% by volume; and
also containing methane in a concentration of between about 1%
and 3% by volume, and most preferably about 2%
Boron is introduced
into the furnace in the form of gaseous boron monoxide (BO),
which is produced by a solid-state disproportionation reaction
between boron and boric oxide (B.sub.2 O.sub.3), according to
the following reaction:
This
disproportionation reaction may be carried out in a suitable
porous material, for example alumina, when impregnated with
powdered boron and boric oxide. Alternatively, this reaction
may be conducted in a pelletized bed of solid boron and boric
oxide. In either case, the rate of generation of BO and the
rate of its introduction into the furnace are regulated by
passing a stream of an inert diluent gas, such argon or
preferably nitrogen in a carrier of hydrogen, through the
impregnated alumina or the pelletized bed. The boron monoxide
and methane react under the influence of the metallic catalyst
to form crystalline boron carbide according to the following
reaction:
As noted above, It
is believed that this overall reaction occurs partly at the
surface of the molten catalyst, with boron and carbon
dissolving in the catalyst in elemental form and reacting
therein to form boron carbide whiskers. Metal catalysts are
operable for the formation of boron carbide.
In the production of
boron nitride (BN) whiskers, boron is introduced as boron
monoxide and is produced by the disproportionation reaction
discussed above, and nitrogen is introduced in the form of
ammonia, also as described above. Production of boron monoxide
is controlled with an inert diluent such as nitrogen in a
carrier of hydrogen, which is passed through the boron
monoxide generator. The anionic precursor feed gas consists of
ammonia, carried in a carrier such as hydrogen and an inert
diluent such as nitrogen. The anionic precursor feed gas
preferably contains between 30% and 65% by weight hydrogen;
between 30% and 55% by volume ammonia, and between 2% and 15%
by volume nitrogen. An optimum composition of the feed gas is
approximately 55% by volume hydrogen, 40% ammonia, and 5%
nitrogen. A glass catalyst is preferred, with a borate glass
being most preferred.
It will be noted
from the discussion above that glass catalysts are preferred
for generation of the nitride whiskers, and metal catalysts
are preferred for generation of the carbide whiskers.
In all of the above
reactions, the use of an inert diluent such as nitrogen is
useful for preventing the development of excess
supersaturation of the reactive gases in the catalyst, thereby
keeping the crystallization reaction from proceeding too fast,
which can otherwise result in poor whisker quality. The amount
of the nitrogen or other diluent, such as argon, can be varied
to achieve optimum crystal growth.
It will recognized
that in the production of all of the whiskers described above,
the stoichiometry of the gaseous mixture of precursor
compounds in the reaction chamber can be varied in several
ways. First, the rate of generation of the suboxide precursor,
i.e. either silicon monoxide or boron monoxide, can be varied
by varying the absolute rate of flow of the moderating gas
through the suboxide generator. Secondly, in the case of the
silicon monoxide generator the rate of generation of silicon
monoxide may also be moderated by varying the amount of carbon
monoxide in the moderating gas. Further, the rates of
introduction of the anionic precursor compounds (methane or
ammonia) can be varied by either varying the absolute flow
rate of the feed gas, or by varying the concentration of the
anionic precursor in the feed gas. Finally, the amount of
inert diluent in the feed gas can be varied to either dilute
or concentrate the concentration of the anionic precursor
compound in the reaction chamber. This combination of
variables enables very precise yet extensive control over the
composition and reaction conditions of the gaseous mixture
exposed to the VLS catalyst, and thereby allows for optimum
whisker growth conditions to be achieved.
The following
examples explain how the present invention may be practiced to
produce the whiskers described above.
EXAMPLE 1
Silicon Carbide
Whiskers
Beta silicon carbide
whiskers may be grown on a growth substrate consisting of a
number of graphite plates which are passed sequentially
through a tubular muffle furnace. A suitable catalyst for the
VLS reaction is nominal 15.mu. diameter spherical 304
stainless steel powder. The catalyst is dispersed evenly onto
each graphite plate at a concentration of about 0.5 grams per
square foot. A suitable tubular furnace is of a cross section
of about 50 square inches and is maintained at a temperature
of 1,400.degree. C. A silicon monoxide generator, consisting
of a porous brick impregnated with fine-grained silica and
carbon powder, is suspended in the furnace above the path of
the graphite plates. A moderating gas is caused to flow
through the silicon monoxide generator at a rate which may be
varied to effect optimum concentration of silicon monoxide in
the furnace. The moderating gas may consist of hydrogen with
carbon monoxide at a concentration of about 8% by volume. The
gas supplied as the cationic precursor feed gas may consist of
hydrogen with about 7% to 8% nitrogen and about 2% by volume
methane. The nominal flow rate of each gas may be
approximately 5 liters per minute per foot of hot zone within
the furnace. Beta silicon carbide whiskers 4.mu. to 6.mu. in
diameter can be produced by this method, having aspect ratios
of 300 to 10,000. The whiskers may be harvested from the
graphite plates upon exit from the furnace by brushing and
vacuum collection as described above.
EXAMPLE 2
Silicon Nitride
Whiskers
Alpha silicon
nitride whiskers may be grown on a growth substrate consisting
of a number of alumina plates. A catalyst consisting of
nominal 5.mu. diameter glass beads may be used. The glass is
preferably a soda-lime silicate glass. The catalyst is
dispersed on the alumina plates at a concentration of about
0.5 grams per square foot. A tubular muffle furnace having a
cross section of 50 square inches and maintained at a
temperature of 1,440.degree. C. is suitable.
Silicon is
introduced into the furnace as silicon monoxide, which is
generated with a silicon monoxide generator as described
above, consisting of a porous silicate brick impregnated with
silica and carbon. The rate of introduction of silicon
monoxide into the furnace is moderated by a stream of hydrogen
and carbon monoxide passed through the generator.
Nitrogen is
introduced into the furnace in the form of ammonia in a
carrier consisting of hydrogen and nitrogen.
EXAMPLE 3
Boron Carbide
Whiskers
In this example,
boron carbide whiskers may be produced using the apparatus and
method of the invention. A growth substrate may consist of a
number of graphite plates. A catalyst consisting of nominal
15.mu. spherical nickel powder is dispersed on the graphite
growth plates at a concentration of about 0.5 grams per square
foot. A tubular muffle furnace having a cross section of 50
square inches, maintained at a temperature of 1,500.degree.
C., is suitable.
A boron monoxide
generator consists of a pelletized bed of boron and boric
oxide. A moderating gas consists of an inert diluent such as
nitrogen in a carrier of hydrogen. A total flow of the
moderating gas is approximately 5 liters per minute per foot
of hot zone, although flow rates of between 2 liters and 15
liters per minutes may be used, depending on the crystal
morphology desired. Carbon is supplied in an anionic precursor
feed gas, supplied to the distribution manifold 16. This feed
gas may consist of hydrogen with nitrogen at a concentration
of about 7% and methane at a concentration of about 2%. The
total flow of these gases is about 5 liters per minute per
foot of hot zone.
Whisker growth is
initiated in about twenty minutes. The growth rate varies from
about 0.5.mu. to 2.0.mu. per second, with 1.0.mu. per second
being average. Using the 15.mu. nickel catalyst, boron carbide
whiskers 4.mu. to 6.mu. in diameter are produced. Whiskers
having an aspect ratio (length to diameter) of from 300 to
10,000 are produced depending on the time in the furnace.
EXAMPLE 4
Boron Nitride
Whiskers
In this example,
boron is generated in a boron/boric oxide generator as
described above, and is introduced into the furnace in a
stream of moderating gas consisting of an inert diluent such
as nitrogen in a carrier of hydrogen. Nitrogen is introduced
into the furnace as ammonia in a carrier of hydrogen and
nitrogen. A suitable catalyst is a borate glass. A 10.mu.
diameter catalyst will produce about a 5.mu. diameter whisker
of cubic boron nitride.
The anionic
precursor feed gas consists of ammonia in a carrier consisting
of a mixture of hydrogen and an inert diluent such as
nitrogen. The anionic precursor feed gas preferably contains
between 30% and 65% by weight hydrogen, between 30% and 55% by
volume ammonia, and between 2% and 15% by volume nitrogen. An
optimum composition of the feed gas is approximately 55% by
volume hydrogen, 40% ammonia, and 5% nitrogen.
Utility
The present
invention is useful for continuously producing relatively
large quantities of whiskers of high quality and uniform shape
and size. Precise stoichiometric adjustments in the feed
gases, resulting in desired whisker characteristics, can be
made by selectively adjusting the compositions of the anionic
and cationic precursor feed gases.
Whiskers of the type
produced by this invention are characterized by high tensile
strength, which strength is maintained at high temperatures.
Consequently whiskers are of particular utility in the
formation of fiber reinforced composite materials, including
fiber reinforced ceramics, polymers and metals.
Whiskers such as
silicon carbide are recognized as a health hazard similar to
asbestos when the diameter of the whiskers is below 3.mu..
Consequently the present invention is of substantial value in
producing whiskers which are of uniform diameter larger than
about 3.mu., and which can, as a result, be more safely
handled and processed.
Although the
invention has been described with reference to these preferred
embodiments, other embodiments can achieve the same results.
Variations and modifications of the present invention will be
obvious to those skilled in the art and it is intended to
cover in the appended claims all such modifications and
equivalent
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