Note: Descriptions are shown in the official language in which they were submitted.
~ ~ 559~
BN BONDEI) ~N FIB~R ARTICL~ F~O~ ~N FIH~K
BAGKGROUND OF T~ INVENTION
a) Field of the Inve~tion
This invention relates to boron nitride ribers
and more particularly rolAtes to article~ manu~actured
from integral three d~mensional boron n~tride fiber
mat~. The invention further relatc~ to the ~ethod
ror the manufacture of such articles.
b) ~
Boron nitride (~N) possesses a nu~ber of h~ghly
desirabl~ propertie~ which render it use~ul in a wide
variety of application~. Its high oloctrical re-
sistivity coupled ~ith lts high thermal conductivity
~ake it especially use~ul in olectr~oal and eloctronic
applications requ~r~n~ a material ~hich si~ultaneou~ly
acts as an electrical insulator and a ther~al conductor.
Its excellent thermal ~hock resistance render~ it
e~fective as a re~ractory at temperatures up to 1,600C
or higher in a non-oxidizing at~ospher- and at tom-
peratures as high as ~00 to 900C in alr. It i~ highly
corrosion resistant, baing incrt to ~ost organic ~iQulds
and many corrosive chemical~ and dlsplaying exc-llent
resistance to attack by variou~ molten metals. Purth-r-
more, becau~e of its low dissipation ractor ov r a ~id-
temperature range, this mat0rial i8 w~}l ~ult-d ror u~-
in microwave and radar dielectric compon-nts ~ r dar
windows). Yariou~ method~ for the nanurac~ur- o~ boron
~'
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1~85596
nltrlde ribers are known in the prior art, for exam~le.
it is di~closed in U. ~. Pat~nt 3,429,722 issued to
Js~e3 Economy et al that boron nitride fibers can be
manufactured by heating boron oxido fiber~ ~n an
ammonia atmosphere.
U. S. Patent 3,668,o59 is~ued to Ju~es ~conomy
et al discloses a boron nitride riber having a high
Young' 8 modulus of ela~ticity which i8 prepared by
hcating a partially nitrided fiber in an lnert at-
mosphere at a temperature Or at least 1800C under
longitudinal tension.
While it is well ~no~n in the prior art thatboron nitride fibers can be ~anu~actur-d ha~ing good
characteristics, the use o~ ~uch f~bers has boen
limited due to difficulties in forming three di-
mensional articles from the ribers. Almost any sub-
stance ~hich i8 used to bond the fib~rs to e~ch other
has properties which are inferior to tho properti-
~of the boron nitride fibers thus rosulting in a
bonded article which i8 un~uitable for use in ~any
applications. For example, ~hen a boron nitride
fiber article, w~ich i~ bound by prior art ~at-riols,
i8 used as a scparator material in a corrosive cell
electrolyte such as molt~n lithium chloride ~nd
pota~si~m chloride, the fibers separate ~rom each
other due to the inability Or the binding materia~ to
~ithstand the high temperatur~ corrosive environ~ent.
An att*mpt has been made prior to the pr-s~nt
invention to form articles from boron nitride bonded
~ boron nitride fibers by heating boron nitride fibers
i~pregnRted with boric acid solution to l-vatod
temp~rature~ ~n ammonia as dieclos~d in U. S. Patent
3,~37,997 to Jame~ Economy et al.
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lG855~6
dditic)n to the ~bove-llote~ rert~l~cnc~s r elating
to boron ni tl~ide fib~rs, s~lape~ ~)oron n~tride, usu~J 3y
non-porou6, bodies have also b~en prepare~ in the ~ast.
Such articles are di~closed, for example, by ~ylor, u.s.
~atent 2,888,325, which teaches the use of a ~ultiple
~tage nitriding process comprising intermittant addition
of oxygen-containing boron compound at intarmediate
stages of nitriding, follow~d by furth-r nitriding.
Furthermore, such art~cles ha~e ~en pr~par~d by
sintering boron nitride fiber~ in the pr~sence of boron
oxide.
None of these methods resulted in a non-woven porous
boron nitride fiber article ha~ing suf~icient strength
for use as an electric cell separator ~n molten lithium
chloride environments. The~e bonding proc~sses Yometime~;
resulted in a boron nitride ~iber of r duced strength or
the bond was of insufficient strength or durability to
secure the fibers to ea¢h other in molten llth~um chlorid~
en~ironments.
) ~
In accordance with this inv~ntion, ther~ i~ now
provided a shap~d article comprising ~used boron nltrid-
fibers, wh~ch retains the porous characteristics Or a
f~ber mat and in addition has good dimen~ional str ngth,
iB relatively non-brittle when co~pared with prior art
boron nitride f~ber articles and retsins the high h-at
and chemical resistance of boron nitride fiber.
In aceordance with the in~ention, the boron nltrid-
article i9 manufactured by a method which ¢o~pris~ bl-nd-
ing from about 2 to about 40 weight percent of boron oxidewith ~rom about 60 to about 98 weight percent Or boron
nitride fiber~ or partlally nitridod boron oxide rib-r~.
~h- r~sulting blend i8 then for~ d into a ~h~p-d ~rticl-
and heated in an anhydrous ga~ s-lected rro~ the group
1~85S96
~nsi~ , of in~rt ~a~es, rlitro~ren, ~mmonia ~n~ mixt~re
thereor to -1 temperature above thc melting temperat~lr- o~
the boron oxide for ~ time sufflcient to melt at l~a~t
some of the boron oxide to the fibers. '~he articl~ is
heated for an insufficient time and to an ~nsuffic~ent
temperature to de~troy the fiber~ by molting or de-
compo~ition. ~imultaneously with the or ~ubsequent to
the heating of the a~ticle to melt the boron oxide, the
article i8 heated in an ammonia atmosphere to a ~ufri-
c~ent temperature and for a ~ufricient ti~e to convort
the boron oxide to boron nitride. ~rhe resulting art~cle
comPrises boron nitride ribors fused to each other ~ith
boron nitride which articlc ha~ good strength, good
dimen~ional 6tability, ~ood ¢hemlcal resistance, and
retains the desirable characterlstics, i.e., porosity,
of a fiber artlcle.
3ETAILED D~SCRIPTION OF T~ IN~NTION
The art~cle manufactured in accordanc~ with th~
method of the invention can bo of any de~irable s~ape.
Por example, the article may be spherical, cublc, cy-
lindrical, oval, a bar or in the form of 8 plat- or mat.
The article may be provided with holes or contours 1~
desired for a particular application. The article co~-
prises a body of boron nitride fibers which are secur-d
to each othsr at fiber inter~ections by partlally or
completely nitrided boron oxide. Dasirably, the boron
oxide is completely nitrided to form boron nitride.
One example of a desirable article manufactured in
accordance with the process o~ the invention, iB a fib-r
mat which has ~ufficient poros~ty, str~ngth and ch~nic~l
resistance to be usad as a ~eparator ln lithiu~-~ul~ld~
batteries utilizing molten lithium chlor~d- nd molt-n
~ota88ium chloride a~ the electrolyt-.
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1~85596
In accordance with the method of the invention
from about 2 to about 40 weight percent and preferably
from about 5 to about 20 weight percent of boron oxide
is blended with from about 60 to about 98 weight per-
cent and preferably from about 80 to about 95 weight
percent of boron nitride or partially nitrided boron
oxide fibers. The most desirable concentration of
boron oxide is from about 10 to about 20 weight per-
cent and the most desirable concentration of boron
nitride or partially nitrided boron oxide is from
about 80 to about 90 weight percent.
The boron oxide may be particulate or fibrous
in form. When the boron oxide is fibrous, the boron
oxide (B203) fibers desirably have a maximum diameter
of 20 microns and most desirably, a maximum diameter
of about 10 microns. When the B203 is particulate,
the average particle size may vary from sub micron to
about 100 microns in diameter and the particles may be
of any shape. The boron nitride or partially nitrided
boron oxide fibers similarly have a maximum diameter
of 30 microns, more desirably a maximum diameter of
20 microns and most desirably, a maximum diameter of
about 10 microns.
The boron nitride or partially nitrided boron
oxide is desirably made by heating boron oxide fibers
in an ammonia atmosphere in accordance with known pro-
cedures as for example are disclosed in U.S. Patents
3,429,722 and 3,66~,059 both issued to James Economy.
Boron oxide fibers when used may be made by any known
method including spinning the boron oxide (B203) fibers
from a B203 melt and winding the resulting fibers upon
a reel in an atmosphere protected from moisture.
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1~8SS96
aLtern~tively t~e B2~1 precur~oI fiber~ may be ~lown
in staple form from a 13203 melt.
The boron nitride or partially nitrided boron
oxide fiber~ may be blended with boron oxide by any
suitable means such as by ~lurrying the fibers in
an anhydrous liquid such as keros~ne and subse~uently
removing the liqui~ from the ribors. Oth~r methods
for blending include blowing th~ rib-rs and boron
oxide, whether in fiber or particle ~or~. into a
containsr or mixing ln a n uldized bed.
After the fibers are blendod ~ith boron oxido,
the resulting composition iB formed lnto a shaped
article by any suitable means. For example, the
shaped article may be formed by pressing the com-
posltion into the appropriate shape. ~olds may be
used ir deslred durlng the prossing prooedure. Flb-r
mats and fiber boards can bo manu~acturod by prc881ng
the blond between tlat plates. Pre~cures which can
bc used dur~ng the forming procedur~ pretorr~bly
range between about 0.5 and about 2.5 ~ilogracs p r
square centlmeter absolute. Shaped artlcles can
al~o be formed from the blend by casting a slurry
of the blend in an anhydrous liguid followed by 8ub-
sequent evaporation of the liquld. The slurry ~y
bo cast into a mold or on to a tlat surtace.
After the article i8 rormed, it i8 heat-d in an
anhydrous ga~ selected from the group con~lsting o~
ln~rt gases, nitrogen, ammonia and mixtures thereot
to a temperature above the melting temperature of
th~ boron oxide for a t~me surficicnt to ~u~- at
east ~ome Or th~ boron oxid~ to th- boron nltrld-
ri~ers and for a tlm~ insuff~ci-nt to do~troy th-
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1~855~6
boron nitrld~ or p~rti~lly nitridel b~roll ~xi~ ?r~
by rne:Ltit~g. In t~neral, the heatin~; tem~eI~ature is
from about 4~0 to ~bout 1400C De~irably the l~at-
ing temperature i6 b~low about 750C since hi~her
temperatures tend to re~ult in localized rsther than
uniform fusion of th~ ~ibers to each other by ~oron
oxide throughout the articl~, partlcularly wh~n heat
transfer is not substant~ally enhanc~d by flow of
heated gas through the articlo
The time requir~d to fuse the fibors together,
without destroying the fibers by ~elting or de-
composition is dependent upon the fusion temperaturo
u~d and heat transfer method~ employed At higher
temperatures, short heating times sre required and
rapid heat transfer throughout the article 18 n-eded to
prevent localized evaporation o~ the B203 befor- tho
riber~ throughout the article aro fusod to each other
Such heat tran~fer i8 generally accompl~shod by rapldly
clrculating heated gas through tho fib~rs At highor
temperatures, i e , from about 750C to 1400C, th-
time sufficient to fu~e at least some of the boron
ox~de to the boron nitride fibers ~8 generally bot~e-n
about 3 and about 60 minutes
In general, it ha~ been found that a 810~ ta~-
perature rise to the desired peak temperature over th-
hoating time result~ in a more uniform artlclo
At lower temperatur~s, i e , fro~ about 450 &
to about 750C, longer heating times are required
~or suf~ioient fusion of the boron ox~do to tho boron
nltr~de or partially n~trided boron ox~de f~er~,
~Q~er, e~en at the lowor te~poratur 8, good heat
1085596
transfer between the fibers i~ desirable to obtai~
a uniform article. At lower temperatures, the
~ufficient time to ru~e the flbers i8 generally
between about 1 and 6 hour~. ~gain, it ha~ been
found that a more unifor~ articlo i8 obtained when
the heating temperature i8 810wly levated to the
peak temperature over the heating tlme.
The heating of the article in an am~onia at-
mosphere to a ~ufficient tomperaturo and for a
~ufficient time to convert the boron oxide to boron
nitride may occur si~ultaneously with or subs~quent
to the hoating of the art~cle in an anhydrous gas to
~use tho boron oxide to tho boron nitrld~ or par-
tially nitrided boron oxide fiberfi.
In general, the sur~icient teaperaturc to con-
vert the boron oxide to boron nitride in an am~onia
atmosphere i8 any tomperature abov the reactlon tom-
perature of ammonia with boron oxlde up to th- melt-
ing temperature of boron nitride. In general, the
suf~icient temperature to convert the boron oxide
to boron nitrido i8 from sbout 200C to about 900C.
When partially nitrided boron oxide fibers are used,
they are converted to boron n~tride during thls con-
vorsion step.
The time which is required to convert the boron
oxid~ to boron nitride depends mainly upon the
di~usion rate of ammonia into the fibcr~ ~hich in
turn i8 dep~ndent upon the concentration of am~onia
gas and thQ flow or contaot of th~ ammon~a gas ~lth
the boron oxide and to some ~xtent, the gaB
temperature. In general, the Yufflciont time to
convert the boron ox~de at to~peratures
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1~85596
betwe~n abo~t ?O~C anA about 900~C i~ am~nia g~s
at atmo~pheric pres~qure wlth ~u~f~clent ~`low Or
ammon~a through th~ fibers to proYlde ~xces~ ammo
~8 reactant, is from about 2 to about ~ hours.
Longer times may be u~ed without detriment to the
article but have not b~en found to be n~cessary.
Th~ following xample8 serve to illu~trate
the proce~s and articl~ of the in~ention without
limiting the invention-
~XAMPLJ3 1.
7 grams of BN fiber~ having an averago diameter
of about 4Juand an average length of between about
0.5 to about 0.9 ¢entimeter iB blended with 3 gr ms
Or B203 fibers having an average diameter o~ about
4~and an averago length of bet~een about 1 and
about 2 centi~eters. The blending iB acoo~plished
by coYering a mixture o~ the rlbers with ~orosone
and blending the resulting composition in a rood
bl~nder at about 3,200 rpm for sbout two m~nuto~.
$he composition is then cast into a aheet ln
a mold about 4 centimot~rs s~uare and dried ~nd h-ated
up to 650C over a four hour period in an o~on. Th
re~u~ting sheet i8 then allowed to cool for oight
hours in the oven which 18 nitrogen purged.
The sheet i~ then r~oved, cut in hal~ and
heated in an o~en, at a te~perature rise o~ 100C
p~r hour up to 900C, in ammon~a at atmosph-rlc
pressure. Ammonia ~low through the oY~n i8 lS lit~r~
por mlnute. The resulting sheet i~ Mexibl-. porou~,
strong and 1~ able to ~lthstand a mo~ten llt~iu~
chloride environment ~or an extended tl~e p riod
~ithout deterioration.
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1085596
EXA~PL~ ~.
Example 1 is repeated except 6 grams of BN
fiber and 4 grams of B20~ fiber are used. The re-
sulting sheet i8 flexible, porous, strong and i~
able to with~tand a molten lithium chloride environ-
ment for an extend~d period without dsteriorat~on.
EXA~PLE_~.
Example 2 is r~peated except 5 gram~ of BN
fiber and S gram~ Or B203 fiber are usod. The re-
sults are the same as Example 2 except the sheet
is not flexible and has reduced porosity.
EXAMPLE 4.
The procedure of Exa~ple 1 is followed xcept
8.5 gr~Ds Or BN fib-r and 1.5 grums o~ B203 riber
are uscd and after hcating in ammonia, tho result~n~
sheet i8 heated in air at 600C for 2 hour~. The
resulting 6heet ha~ all of the desirable properties
of the sheet prepared in Example 1 and in addition
i~ more flexible and more uni~orm.
EXAMPLE 5.
The procedure of Example 4 i8 rollowed except
partially nitrided B203 fibers are ~ubstituted for
the BN fiber6. The partially nitridod B203 fib~rs
are prepar~d in accordance with the te~chings of
Example 2 of U. S. Patent 3,668,059 wherein boron
oxide fibers are heated in flowing ammonia gas at
210C for 0.5 hours,rrcm 210C to 550C at ~ rate Or
4C per hour, from 550C to 640C at a rate of 15C
per hour, and then at 640C for ono hour. Tho re-
~0 ~ulting product is a strong BN bonded aN fib-r pap r
which iB resistant to molten lithium chlorid-.
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