CA1085596A

CA1085596A - Bn bonded bn fiber article from bn fiber

Info

Publication number: CA1085596A
Application number: CA287,634A
Authority: CA
Inventors: Robert S. Hamilton
Original assignee: Carborundum Co
Current assignee: Unifrax 1 LLC
Priority date: 1977-03-02
Filing date: 1977-09-27
Publication date: 1980-09-16
Also published as: NL7712016A; FR2382411B1; FR2382411A1; DE2748853C2; JPS53106714A; DE2748853A1; GB1574077A; BE860178A; JPS6120508B2

Abstract

ABSTRACT OF THE INVENTION

A boron nitride fiber article and its method of manufacture which comprises blending boron oxide with boron nitride or partially nitrided boron oxide fibers, heating the blend in an anhydrous gas to a temperature above the melting temperature of boron oxide and nitriding the resulting article in ammonia.

Description

~ ~ 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 ~' ~' 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.

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-.

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.

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--6_ 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 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 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.
_9_ 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-.

Claims

WHAT IS CLAIMED IS:

1. A method for manufacturing a boron nitride article comprising:
(a) blending from about 2 to about 40 weight percent of boron oxide in particulate or fiber form with about 60 to about 98 weight percent of boron nitride fiber or partially nitrided boron oxide fiber;
(b) forming a shaped article with the resulting blend;
(c) heating the article in an anhydrous gas selected from the group consisting of inert gases, nitrogen, ammonia and mixtures thereof to a temperature between about 460 and about 1400°C, above the melting temperature of the boron oxide and below the melting or de-composition temperature of the fiber, for a time sufficient to melt at least some of the boron oxide to the fibers; and (d) heating the article in an ammonia atmosphere to a sufficient temperature and for a sufficient time to convert essentially all of the boron oxide to boron nitride.

2. The method claimed in Claim 1 wherein from about 5 to about 20 weight percent of boron oxide is blended with from about 80 to about 95 weight percent of fiber.

3. The method of Claim 2 wherein the boron oxide is in particulate form.

4. The method claimed in Claim 1 wherein the heating to melt the boron oxide to boron nitride are done simul-taneously in an ammonia atmosphere.

5. The method of Claim 2 wherein the boron oxide is in fiber form.

6. The method of Claim 2 wherein the fiber is boron nitride.

7. The method of Claim 1 wherein, in Step (c), the article is heated to between about 460°C and about 1400°C for from about three mintues to about six hours to melt the boron oxide; and the gas is an inert gas or nitrogen.

8. The method of Claim 1 wherein, in Step (d), the sufficient temperature to convert boron oxide is from about 200 to about 900°C and the sufficient conversion time is from about 2 to about 18 hours.

9. The method of Claim 8 wherein said boron oxide is in fiber form.

10. The method claimed in Calim 2 wherein from about 80 to about 90 weight percent boron nitride fiber is blended with from about 10 to about 20 weight percent of boron oxide and said heating temperature is from about 460°C to about 1400°C.

11. The method claimed in Claim 2 wherein the fibers are blended with boron oxide by slurrying the fibers and boron oxide in an anhydrous liquid and removing the liquid from the fibers.

12. The method claimed in Claim 11 wherein the liquid is kerosene and the liquid is removed by evaporation.

13. The method claimed in Claim 2 wherein the fibers and boron oxide are blended by blowing them randomly into a container.

14. The method claimed in Claim 2 wherein the fibers and boron oxide are blended by mixing them in a fluidized bed.

15. The method of Claim 1 wherein the shaped article is a fiber mat.

16. The method of Claim 6 wherein the shaped article is a fiber mat.

17. The method of Claim 1 wherein from about 0.5 kilograms per square centimeter to about 2.5 kilograms per square centimeter of absolute pressure is applied to said shaped article during said heating.

18. The method of Claim 6 wherein from about 0.5 kilograms per square centimeter to about 2.5 kilograms per square centimeter of absolute pressure is applied to said shaped article during said heating.