Note: Descriptions are shown in the official language in which they were submitted.
~ ~ 2 ~
1--
RD~19,549
T~S~U~N~ OX~ D~
GL~$8 AN~ ARTIC~
CROSS REFERE~CE TO RELATED APPL:rCATIONS: The
subject application relates to copending applications for RD-
17,455, Serial No. 359,619, RD-19,110, Serial No. 386,327 and
RD-19,818, Serial No. ~.
10The presen~ invention relates to glass compositions
and in particular to translucent glass compositions
comprising silicon, oxygen, and carbon.
Amorphous silica is a refractory glass, however, it
devitrifies readily at temperatures gre~ter than 1100-C. De- ;
vitrification refers to the orderlng or crystallization of
the random structures tha~ glasses are mad~ of. Crys~al-
lization drastically reduces one of the predominant
attributes of vitreous silica, i.~., its low thermal
expansion, as well as many of its other desirable properties.
As a result, much research ha~ been directed to seeking ways
to increase the resistance to devitrification in silica glass
compositions.
Reactions between silicon, carbon, and oxygen have
been studied exténsively. Some of the known reactions in a
silicon, carbon, and oxygen system include oxygen combining
with silicon to form silica, SiO2. ~t temperatures in excess
of 1100 C silica begins to crystallize to form cristobalite,
one of the common mineral fo~ms o silica. Carbon c~n react
with available silica to form crystalline silicon carbide or
escape as carbon monoxide gas. ~y carbon remaining as
,.: , . . . .
,
,
2 ~ $~
--2--
RD-19,549
elemental carbon readily oxidizes above 600 C when exposed to
air.
The ~hermodynamics o~ silicon, carbon and oxygen
reactions is discussed in ~The High-Temperi~ture Oxidation,
S Reduction, and ~olatilization Reactions~ S~l~con and Sili-
con Carbide", Gulbransen, E.A., and Jansson, S~.~ Oxidation
o~ Metals, Volume 4, Number 3, 1972. The t:hermodynamic
analysis of Gulbransen et al. ~hows that at 1700~C silica and
carbon should form gaseous silicon monoxide and carbon mono-
xide or ~olid silicon carbide, SiC. ~owever, no materialcontaining silicon, oxygen and carbon would be expected to
form. Gulbransen et al. conclu~e that silica was not recom-
mended ~or use in reducing atmospheres above 112S-C due to
the formation of volatile silicon monoxide gas. Also silicon
carbide was not recommended for use in oxygen containing en-
vironments where active oxidation may occur due to oxidation
of the silicon carbide.
Ther~ is an opaque, black, glass that is
functionally described as carbon modified vitreous silica and
herein referred to as l'black glass" where 1-3 percent carbon
has been added to ~ilica. The metho~ for making black glass
is disclosed by Smith et al. in U.5. patent 3,378,431.
Carbonaceous organics ~uch as carbowax are added to silica
and the mixture is hot pressed at about 1200'C to form black
glass. Smith, C.F., Jr. has further characterized black
glass by in~rared pectroscopy ln "The Vibrational Spectra of
High Purity and C~emically Substituted Vitreous Silicas~', PhD
Thesis, Alfred Univer3ity, Alfr~d, N.. ~ May 1973. Smith
discloses that in addi ion to elemental carbon dispersed in
the glass, carbon in black glass is associated with oxygen in
carbonato type groups. A carbonato group is the description
:,
:, - - : -
.. . . .
--3--
RD-19,549
of a particular way th~t a carbon a~om bonds with three
oxygen atoms and has the structure,
~\
/ C=O
The mechanical strength of black glass is si~ilar
to the strength of carbon free silica glass, however black
glass has an increased resistance to devitrifica~ion over
conventional silica glass which begins to devitrify at about
1100C while black glass begins to devitrify at about
1250-C. The increased thermal ~tability o~ black glass
allows it to ba used at temperAtures higher than vitreous
silica can with~tand.
In a commercially produced continuous silicon car-
bi~e ceramic fibre sold under the trademark "Nicalon", about
10 percent oxygen is introduced into the fibre to crosslink
it. After crosslinking, ~he fibres are pyrolized and it is
believed that the oxygen becomes part of the fibre as an
amorphous contamina~t, probably in the form of silica. The
degradat~on behavior of such fibres after heat treatment in
various environments was reported in the article "Thermal
Stability of SiC Fibres (Nicalon~ ah, T., et al., Journal
of Material Science, Vol. 19, pp. 1191 1201 ~1984). Mah et
al. ~ound that regardless of the en~ironmental conditions
duriny heat treatm~nt, the "Nicalon" flbre strength degraded
when the fibres were subjected to temperatures greater than
1000 C. Tha fibre degradation was associated with loss of
carbon monoxide from the fibres and beta-silicon carbide
grain growth in the fibres.
2 ~ 2 ~
--4--
RD-19,549
In the copending applica~ion, serial number
359,619, a glass composition is disclosed wherein silicon
atoms are bonded to oxygen and ca~bon atoms to form a glass
that resists devitrification and.decomposi.tion at
temperat-lres up to at lea~t 1650 C. The glass in the '619
applicatlon additionally contains about 3 to 9 weight percent
elemental carbon dispersed atomically or in small clusters in
the glass matrix, and this free carbon makes the glass opaque
and black in appearance.
Ceram~c materials generally exhibit brittle be-
havior as characterized by their high strength and low frac-
ture toughne~s. Fracture toughness is the resistance to
crack propagation in materials. The development of ceramic
composites has been investiqated as a way to allevia~e the
brittle behavior of ceramics. "Nicalon" is an excellent
ceramic fibre but it degrades at temperatures above 1200 C.
Integrating "Nicalon" fibre3 in a protective c~ramic matrix
having desirable mechanical properties and oapable of with-
~tanding temperatures substantially higher than 1200-C, would
be one way of forming an improved ceramic composite.
~owever, ~rom the discussion above, it is app~rent that the
properties of knswn ceramic or glass compositions, and
specifically those containing silicon, oxygen and carbon, are
degraded ~y decomposltion or devitrification of the glass or
25 ceramic at temperatures above llOû C to 1250-C.
Therefore, ~t is an object of this invention to
form glass composition~, comprised of chemically bonded
silioon, oxygen and oarbon wherelr:l a substantial portion of
the carbQn atoms are bonded to silicon atoms, and the glass
30 is formed from select methyl ~ilicone resins.
., .: . ,
,;
, . .. .
2~2.l6~
--5--
RD-19,549
It is another object o~ this invention to form a
translucent glass, comprising chemically bonded silicon,
oxygen and carbon wherein a substantial portion of the carbon
atoms are bonded to 9ili~0n atoms with up to trace amounts of
elemental carbon dispersed in the glass matrix. Such glass
compositions remain structurally stable and do not decompose
in oxidizing or reducing atmospheres at temperatures up to at
least 1600-C.
Ano~her object of this invention is a process for
forming quch glasses comprised of silicon, oxygen and carbon
by pyrolizing select methyl silicone resins.
Still another object of this invention is the
formation of such a glass comprised of silicon, oxygen and
carbon into articles.
We have found that some silicone resins can be py-
rolized in a non-oxidizing atmosphere to form unique glass
20 compositions. Surprisingly, we have found that these sili-
cone resins when pyrolized in a non-oxidizing atmosphere do
not form ~ilica, cristobalite, silicon carbide~ carbon
monoxide or mixtures of silica and carbon. Additionally, we
have found select ~ilicone resins that pyrolize to form
translucen~ glass compositions containing up to trace amounts
of free carbon that permit at least the partial kransmission
of light through the glass 30 that the glass is not opaque or
black in appearance.
Glasses of this invention are made by pyrolizing a
methyl silicone resin to form a glass composition, comprising
silicon, oxygen, and carbon wherein a signlficant poxtion of
: . . ...:
,
.. . ..
2~21~
--6--
RD-19,549
the carbon atoms are chemically bonded to silicon atoms.
According to one method of this invention, a methyl silicone
resin is heated lrl a non-oxidizing a~mosphere ~o pyrolize the
resin. As used herein, a non oxidizing atmosphere is an
atmosphere that will remove reaction products f~om the
pyxolizing resin withou~ in~luencing the reac~ions occurring
during pyrolysis. Exampl~s o~ such non-oxidizing atmospheres
are inert atmospheres like hellum, argon, or nitrogen, and
reducing atmospheres, such as hydrogen. Pyrolysis may also
occur in a vacuum having a pressure below about 10-4
atmospheres.
~ ethyl silicone resins suitable for use in the
method of this invention can be prepared by the method
described in U.S. Patent 2,676,182, which is incorporated by
lS reference hereln. In particular~ ~xamples 2 and 9 In the
'182 patent modified by replacing ethanol with aleohols, and
replacing dlmethylphenylchlorosilane and
trime~hylethoxysilane with tr~methylchlorosilane, and the use
of toluene to aid in the hydrocarbon separation is
particl11arly relevant for preparing the methyl silicone
resin~ used in the method of this invention.
Methyl ~il$cones are made up of siloxane chains
with methyl groupQ attached ~o the silicon atoms. Siloxane
chain3 contain an al~ernating linkage of silicon and oxygen
atoms. Several combinations of methyl groups can be present
on the 5110xane chains to foxm polymethylpolysiloxanes.
The basic unit ~tructures in polymethylpolysilox-
anes are trimethylsiloxy, dimethylsiloxy, and monomethyl
siloxane. The trimethylsiloxy monofunctional unit at the end
of a siloxane chain has ~he structure,
- . .
- 7 -
RD~l9, 5q9
CH3
I
CH3 - Si - O -
c~3
Dime~hylsiloxy is a difunctional unit that builds
chains vr rings and has the structure;
C~3
I
-- O -- si -- o -
CH3
Monomethyl siloxane is a txi~unctional unit and not
onl!r extends siloxane chains, but al~o crosslinks between
chains and has the structure;
CH3
2~ 1
-- O -- si -- ~ --
V
I
Methyl silicone resins may also contain unsub-
stituted tetra~u~ctional units~ hereln referred to as Q
urlits, and haviny the ~tructure;
.. ~ .. . .
. : ,. : ::
.. ..
.
--8--
RD-l9,S99
-- O -- Si -- O
Polymeric s~ructures can be built from these unit
structures to form polymethylpolysiloxanes having a desired
number of methyl groups per silicon atom. By varying the
ratio o~ methyl group~ to ~ilicon atoms, di~ferent methyl
silicone resins ~re ~or~ed having more or les5 organic sub-
stituent, the organic 3ubstitu~ion being the methyl ~roups.
Methyl silicone resin~ general}y contain a ratio of methyl
groups to silicon atoms of ~bout 2:1 or less . The methyl
silicone resins used in this ~nvention consist of, trimethyl-
siloxy and the unsubstituted te~rafunctional Q unit ln ratios
up to the maximum amou~t of trimethylsiloxy that can be
polymerlzed with Q units or up to about 3:1, preferably in
the ratio of about 0.7:1 up to about 3:~ and mos~ preferably
in the ratio o~ about 1:1 up to about 3:1. Such methyl
~ilicone resins are hereafter re~erred to and claim~d as a
methyl slllcone precursor resln or ~ometimes as the precursor
resin or resin. It ~hould be under~tood that the ratio of
trimeth~lsiloxy to Q units in the precursor resins is
specified according to the lnitial stoichiometry of the resin
a-~ prep~red by the processes de~cribed above, however the
polymexized ratio of trimethylsiloxy to Q u~it can be lower
in the resin.
During pyrolysis, the precursor resin densifies as
gases are evolved causing a weight loss from the xesin.
~ ~ : : - . : - .
2~2~
g
RD-19, 54 9
Although the pyrolizing resin experiences a weight loss the
density of ~he pyrolizing resin is increalsing due to a
reduction in volume of the p~rolizing res:in. The pyrolysis
reactions are essen~ially complete~ when a substan~ially
constant weight w~s achieved in the pyrolizing resin.
Further densification of the pyroli~ing resin may occur after
weight loss has ended, i~ heating i9 continued. Therefore,
it sometimes may be desirable to stop heating and pyrolysis
of the resin after it has completely densified, or in other
words, stops reducing in volume. Weight loss during
pyrolysis was determined to be from about 17 to 54 percent.
Methyl silicone precursGr resins can be pyrolized at
temperatures ranging from about 900 C to 1600-C.
Glasses ~ormed by the method of this invention
possess unique properties and characteristics. These glasses
both resist crystallization and do not decompose ~n oxidizing
or reducing atmospheres at temperatures up to at least
16QO~C. In addition, a significant portion of the carbon
present in the glasses of this ~nvention is bonded to silicon
wlth the remainder present as elemental carbon dispersed
within the glass matrix sc that there are no detectable car-
bonato grvups. The carbon-~ilicon bonds discovered in the
glasse of this invention have heretofore been unknown in
silica glasses. In silica glasses, and specifically in black
glass, carbon has only been known tv be present as an un-
bonded element in the silica m~atrix or in carbonato qroups
where carbon is bonded with oxygen. Glasses formed by the
method of this invention and characterized by such unique
properties are herein referred to as silicon-oxy-carbide
glass.
Pyrolysis of the methyl silicone precursor resin
~orms a silicon-oxy-carbide glass that is characterized by a
.
. ,. :,
:
~: : . ::. , .
2 ~
10-
RD-19,549
continued sharing o~ electrons be~ween atoms of silicon, oxy-
gen and carbon. In silicon-oxy~carbide glass, ilicon atoms
are present in ~our polyatomic units. In one unit, herein
referred to as te~raoxysilicon, a sllicon atom is bonded to
5 ~oux oxygen atoms. In a ~econd unit~ herlein referred to as
monocarbosiloxane, a silic~n atom iS bonded to three oxygen
atoms and one carbon atom. In a third uni~, herein referred
to as d~carbosiloxane, a silicon atom is bonded to two oxygen
atoms and two carbon atoms. In a ~ourth unit, herein re-
ferred to as tetracarbosilicon, a ~ilicon atom is bonded to~our carbon atoms.
Silicon-oxy-carbide gla95 ig formed by pyrolysis of
precursor resins containing trimethylsiloxy and Q units
polymerized in any ratio, but, surprisingly, we have found
that the ratio of trimethylsiloxy polymerized to Q ~mits in
the precursor re3in has an effect on the composition and
properties of the silicon-oxy-carbide glass tha~ i~ formed.
When precursor resins con~aining trimethylsiloxy and Q units
in a ratio below the pre~erred precursor resins described
above, or~ in other words, precursor resins having an M to Q
ratio below about 0.7:1 are used, an opaque silicQn
carbide glass is formed that ls bl ck in appearance.
When the preferred precursor resin containing
trimethylsiloxy and Q units in the ratio of 0.7:1 or greater
are pyrolized, a translucent silicon-oxy-carbid~ gIass is
formed having at least a distribut~on of polyatomic units
comprislng in weight percent, about 18 to 2~ percent
t~traoxysilicon, about 21 to 31 percent monoca~o~iloxane,
about 12 to 22 percent dicarbosiloxane, about 28 to 38
percent tetracarbosilicon, with up to tr~ce amQUnts of ele-
mental carbon dispersed ~tomically or in small clusters
within the gl as s matrix . A trace amount of elemental carbon
~ "' .' ' .
- ,.. .
- ., , ,: ;
2~2~
RD-19,549
i~ an amount that is insufficient to make the glass opaque,
or in other wor~s, permits at least the partial transmission
of light through the glass. ~enerally, a trace amount of
elemental carbon is less ~han about 0.1 w~eight percent.. The
polyatomic units are linked primarily by ~ilicon-oxygen bonds
with a small an~ insigni~icant number of bonds between carbon
and oxyge~ atoms.
The translucent ylass can alternatively be
described as a composition of silicon, oxygen, a~d carbon in
a mass of translucen~ silicon-oxy-carbide glass wherein about
73 to 83 percent o~ the ~ilicon atoms are each bonded to at
least an individual carbon atom, with up to trace amounts of
elemental carbon dlspersed atomically or in small clusters
within the glass matrix.
Articles of silicon-oxy-carbide glass can be ~ormed
by pulverizing the pyrolized resin into a powder. The sili-
con-oxy-carbide powder is then consolidated by hst pressing
to form an article. ~ne method for hot pressing is to apply
a uniaxial pressure of at least about 5 ksi at about 1550-C
~0 to 160Q-C to the powder. The unit k~i is kips per square
inch; the equivalent of 1000 pounds per square inch. Such
pressures and temperatures are sufficient to form a densified
article.
Shaped articles oan also be formed directly from
the methyl silicone precursor resin. First, the resin is
dissolved in a solvent such as toluene a~d then cast into a
desired ~hape. The cast resin ls dried at room temperature
and slowly pyrolized in a non-oxidizing atmosphere as de
scribed herein. Pyrolysis is performed at a low rate of
h~ating that avoids formation of voids and bubbles as gases
evolve and cause a weight loss ln the resin. When the weight
. : .
: .... :.. .
~2~
RD-19,549
of the pyrolizing resin stabilizes, pyrolysis is complete.
When the preferred precursor resins described above are
pyrolized, the cask resin densifles to fvxm a translucent
~ilicon-oxy-carbid~ glass having at least a distribution of
S polyatomic units as described above, however when the
precursor resins having a ratio of tr~me~hyl~iloxy to Q units
of less than 0.7:1 are pyrolized the cast resin densifies to
form an opaque ~ilicon-oxy-carbide glass that is black in
appearance.
The precursor resin in a toluene solution may also
be drawn into ~ibres. The precursor resin solution is
treated with a base or acid to increase the viscosity to a
point where a solid object can be dipped into the solution
and withdrawn, pulling a strand of the resin from the
lS soluti~n~ Fibres can then be drawn or pulled from the resin
solution by ~uch dlpping proCe~BeS. Alternatively, the resin
~olution can be drawn into a teflon tube with a Qlight
vacuum. As the resin increases i~ visoosity and toluene
~vaporates, the fibre shrinks and can b~ pushed out of the
tube. Fibres can be strengthened ~or easier handling by
heating them to about 50 C. The ~ibr~s are then pyrolized in
a non-oxidizing atmosphere or a vacuum as described above.
Ceramic composite~ may be formed having ceramic
fibres in a matrix of silicon-oxy-carbide glass and ceramic
filler. The precursor resin is dissolved in a sol~ent ~nd
ceramic particles are disper~ed i~ the ~olution to ~orm an
inflltrant ~lurry. The particulate ceramic filler ~ontrols
~hrinkage of the composite matrix during pyrolysis and can be
chosen so the matrix is compatible with the ~ibre
reinforcement to be used. Some examples of ceramic fillers
are powdered silicon carbide, diatomac20u9 e~rth and the
2SiO2-3A1203 aluminosi~icate referred to as mullite,
2~2~6~ 1'
-13-
RD-19,549
A ceramic Pibre or fibres, or a cloth of the fibres
is drawn through an agitated ~ath of the .infiltrant slurry.
Some examples of ceramic ~ibres are carbon fibre, silicon
carbide fibre and alumino-boro-silicat~ f:ibres. The im-
pregnated fibre is than shaped and dried l:o allow evaporationof the 301vent. One shaping method includes winding an
impregnat~d fibre spira~ly on a drum to form a p~nel~ I,ayers
of the fibre can b~ con~olidated through the application of
heat and pressure to form a continuous resin matrix
surrounding the ceramic fibres. ~he composite is then
pyrolized in a non-oxidizing atmosphere or a vacuum as
described above. The re~in densifie~ into a substantially
amorphous silicon-oxy-carbide glass that binds the ceramic
filler, thus forming a continuous matrix around the fibres.
lS Vepending on the pyrolysis temperature used, the ceramic
filler may be dispersed, partially sintered or fully ~intered
within the glass.
Optionally, ~he ceramic composite can be re-in-
flltrated with a solution of precursor resin dissolved in a
~olvent to reduce porosity in the composite. The composite
is placed in the re-infiltrant.solution while in a vacuum.
Pressure i~ applied to the solution to force the solution
into the pores of the composite. After re-in~iltrating, the
solvent i5 allowed to evaporate and the re-infiltrated com-
po~ite i~ pyrollzed in a non-oxidizi~g atmosphere or vacuum
a-~ described above. Re infiltration and pyrolysis can be re-
peated a~ often as needed to achieve the desir~d degree of
density in the matrix.
The matrix of amorphous ~ilicon-oxy-carbide glas~
binding a ceramic filler urrounds and protects the ceramic
~ibres from decomposition in oxidi ing and reducing atmo-
spheres at ternperatures up to at least 1600 C . It was found
,: '` ;~ '' -:
.
2 ~ 2 11 ~
~19--
RD-19,549
that the inert na~ure of silicon-oxy-carbide glass readily
accepts ceramic ~lbres without reacting w.ith them and
degrading their properties. As a result, 3ilicon-oxy-carbide
glass containing appropriate ceramic ~ erg can be used as a
matxix material for many known ceramic fibres.
The following description of the invention will be
more easily under~tood by making reference to the figures
briefly described below.
Figure 1 i5 a graph showing the weight lost during
pyrolysis of methyl silicone precursor resins.
Figure 2 is a graphical presentation of the
29Silicon nuclear magnetic resonance spectrum of translucent
silicon-oxy-carbide glass.
Figure 3 is a graphical presentation of the
29Silicon nuclear magnetic resonan~e spectrum of "Nicalon"
5ilicon carbide.
Glasses can be defined by two of their basic fea-
tures; one feature belng that ylasse~ are formed from an ex-
tremely viscous ~upercooled liquid, and a second faature
~eing that the liquid3 which ~orm glasses possess a
polymerlzed network structure wi.th shor~-range order. The
glasses of thi.~ in~e~t~on are not made from supercooled
llquids, but they do possess a network structure with short-
range order. Instead of supercooling a li~uid, the glasses
of this invention are formed by pyrollzing a rnethyl silicone
precursor resin in a non-oxidizing atmosphere. Rowever, the
2 ~ 2 ~
--15--
RD-l9, 549
glasses o~ this invention have the short-range ordering char-
acteristic of conventional glasses.
5ilicone resins have a three dinnensional structure
with short-range order and silicone resins can be de:scribed
5 in terms of their stoichiometric compo~itions. The
stoichiometric units in .~ilicone resins contain a silicon
atom honded to oxy~en atoms and radical groups. Radical
groups in silicone resins ~hat may be pyrolized to form
glasses are formed from the monovalent hydrocarbon radicals
10 and halogenateà monovalent hydroc:arbon radicals including
alkyls; such as me~hyl, ethyl, propyl, isopropyl, butyl,
octyl, dodecyl, and the like; cycloalkyls, such as
cyclopentyl~, cyclohexyl, cycloheptyl, and the like; aryls
such as phenyl, naphthyl, tolyl~ xylyl, and the like;
aralkyls, such as benzyl, phenylethyl, phenylpropyl, and the
like; halogenated derivative~ of the aforesaid radicals
including chloromethyl, trifluoromethyl~ chloropropyl,
chlorophenyl, dibromophenyl, tetrachlorophenyl,
difluorophenyl~ and the l~ke; and alkenylsy such as vinyl,
allyl, methallyl, butenyl, penty~, and the like.
The ~our basic units in ~ilicone resins are herein
referred to as ~ gxoups in which a .qilicon atom is bonded to
one oxygen atom and three organic radicals, D groups in which
a ~illcon atom is bonded to two oxygen atom~ and two organic
radical~, T groups in which a silicon atom is bonded to three
oxygen atoms and one organic radical, and Q groups in which
the ~ilicon atom is bonded to four oxygen atoms. Silicone
resins that may be pyrolized to ~orm glasses contain a
combinatlon of M, T, D, and Q groups so that the r~tio of
organlc radicals to silicon atoms is between about Q.5:1 and
less than about 3 1.
- .
2 ~
-16-
RD-19,549
The glasses of this invention xesist devitri~
fication, and remain struc~urally stable at ~emperatur~s up
to at least 1600~C. The term nstructural:Ly stable" refers to
a bulk material that essentially retai~s t:he same micro-
structure from room temperature up to the elevated tem-
peratures indicated. This means that minor changes may occur
in the microstructure. ~inor changes, such as the formation
o~ small crys~allized areas up to about 100 angstroms in an
otherwise amorphous matrix have substa~tially no adverse or
deleterious effect on ~he proper~ies of the bulk material.
Th~refore, structurally 3table glasses of the present
invention are essentially amorphous but may contain small
crystallized ar~a~ of, for example, graphite, cristobalite or
3ilicon carbide within the glass, or display minor amounts of
15 cristobalite on the surfaces o~ the glass.
Silicon-oxy-carbide glass articles, can be made ac-
cording.to several methods in this invention. In one method,
the pyrolized re~in is pulverized into a powder having a par-
ticle ~ize ranging from 0.1 up to 2 microns. Grinding mills,
~uch a~ an attritor or planetary mill, have been used to pro-
duce ~ilicon-oxy-carbide particle sizes of 0.1 to 2 microns.
Attritor milling is performed by impellor stirring of a solu-
tion comprised of about 52 percent llquid, ~uch as w~ter,
about 3~ percent milllng media, such as 1.2 mm diameter balls
that ar~ harder than the material to be ground, and the re-
mainder i9 crushed particles of silicon-oxy-carbide ~lass.
Impellor stirring of the ~olution at 1000 rpm pulverizes the
gla~s part$cles into a powder. Planetary milling is per-
formed with a ~imilar solution exeept the milling m~dia are 5
to 8 mm diameter balls and th~ solution is agitated by ro-
tating the milling vessel in a planetary fashion at slower
speeds.
2 ~
-17-
RD-19,549
The mi~led powder is then dried and consolidated by
application of heat and pressur~ to orm a shaped article.
Consolidation can be achie~ed through app.lica~ion of a uni-
axial pressure of at lea.qt about 5 ksi at about 1550-C -
1600-C, or application of isostatic pressure of at least
about 8 ksi at abou~ 1200-C to 1600 C. Heat and pressure are
applied until the article has been densified the desired
amount or until fully densified.
In another me~hod for forming silicon oxy-carbide
glass articles from cas~ or shaped precursor resins~ the
methyl ~ilicone precursor resin i5 dissolved in a solvent and
cast into the desired form. Illustrativç of the solvents
that have been ~ound 3uitable for dissolving the precursor
resin are toluene and mlxtures of ~olu~ne with isopropyl
alcohol. Precuryor resins can be dissolved in the solvent at
ratios up to about eight par~s resin to five parts solvent.
The cast precursor resin is dried at room temperature.
Preferably, the east precursor resin is dried at a rate that
allows solvent to evapoxate from the resin without forming
voids in the resin. For example, a rate of evaporation that
inhibits void ~ormation in the drying resin was established
by plac~ng the resin olution in a cylindrical dish open at
one end and placing a piece of paper over the ope~ end.
Alternatively, the precursor resin, which is normally in the
~5 form of a powder, can b~ shaped by hot pressing.
The cas~ precursor resin i9 then pyroli7ed in a
nvn~oxidlzing atmosphere as described herein. The heating
rate during pyrolysis must be controlled to allow evolution
o gases without forming voids or bubbles in the resin. Pre-
f~rably, heating rates of less than l.O'C per minute are usedto allow sufficient gas evol~tion without forming bubbles,
voids or d~fects in the gl~ss. Pyrolysis is essentially com-
: . .
2~2~
-18-
RD-l9, 549
plete when weight loss from the evolution of watex~ methyl
groups and other decomposition products fxom the precursor
resir~ substantially ends. Pyrolysis may be continued l~ntil
the glass has completely densified or stops reducing in
5 volume. The precursor resin ~ensifies during pyrolysis and
orms silicon-oxy-carbide glass.
~,~
The following examples are offered to fu:rther il-
lustrate the silicon-oxy-carbide glass o~ this invention and
methods for producing the glass and glass articles. In the
following examples, precursor resins formed by the method
described above in the '182 patent and having methyl radical
groups were used, with a first resin consisting of M and Q
unlts in a ratio of about 0.5:1, a second MQ re~in had a
ratio of 1:1, a third MQ resin had a ratio of 2:1, and a
fourth MQ resin had a ratio of 3:1.
Methyl silicone precursor resins were pyrolized by
heating them to temperatures ranging from llOO'C to 1250-C.
i~ a ~on-oxidizing atmosphere. During pyrolysi~, the
precursor resins experienced weight loss as water, mPthyl
groups, and othex decomposition products cvolved. When the
weight o~ the pyrolizing resin stabilizes, pyrolysis is
~ubstan~ially complete. ~owever, after the weight loss has
ended, same densification of the ~ilicon-oxy-carbide glass
can occur; therefore, heating a~d pyrolysis may continue
until the silicon-oxy-carbide gla~s is fully densified.
Mea~ured weight 105s during pyrolysis varied from ab3ut 17 to
54 percent. Part of the weight 105s can be attributed to
variations in the amount of solvents r~tained from the
productio~ of the resins.
- . :
2~2~3~:~
19- :
RD-19,549
The first, second, third and fourth MQ resins
described above were pyrolized according to the method of
this invention while weight loss ~rom the resins was measured
S by thermal g~avimetric analysis. Thermal gravimetric
analysis is a method for measuring weight loss from a sample
while i~ is being heated. The samples were heated in a hy-
drogen atmosphere at a rate of lO C~minute to a temperature
of 1250 C. The measured weight loss for each silicon-oxy-
carbide glass formed after pyrolysis is shown in Table :[.
Unexpectedly, the second, third and fourth precursor resins
consisting of M and Q units in a ratio of 1:1, 2:1, and 3:1
were found to form a translucent glass after pyrolysis. The
first resin having an M to Q ratio of about 0.5:1 was opaque
lS and black in appearance after pyrolysis. Therefore, it is
believed that precursor resins having an M to Q ratio of
about 0.7:1 or greater will form translcuent silicon-oxy-
carbide glasses, while precursor resins having an M to Q
ratio below about 0.7:1 will form opaque silicon-oxy carbide
glasses.
25 Example Sample Atmos- Weight Pyrolized
No. Precursor phere Lo ~ % Gla~s
Reisin Appearance
1 Mo,5Q H2 45 Black
2 MQ H2 17.5 Translucent
3 ~2Q H2 54 Tran~lucent
4 M3Q H2 Not Translucent
~easured
., .: .
2 ~ 2 ~
-20-
RD-19,549
The weight loss data ~rom Examp~.es lf2 and 3, as
det~rmined by thermal gravimetric analysis, is presented in
the graph of Figure l. In the graph of Figure 1, the percent
weight loss in each sample is plotted on the or~inate while
the increase in heating tempera~ure is plotted on tne ab-
scissa. The graph of Figure 1 shows that a significant por-
tion of the weight loss in each sample h s occurred at tem-
peratures as low as 900 C while weight loss was essentially
completed at 1200-C. Substantially no evidence of
crystallization was found by x-ray d~ffraction of the
pyrolized material, and substantially no bonding between
carbon and oxygen atoms was found by infra-red spectroscopy
of the same. Thè wei~ht loss in Example 4 followed the same
temperature pattern as in Examples 1,2 and 3.
lS The index of refraction was measured on the
silicon-oxy-carbide glass sample of Example 2 as 1.58, using
a sodium light frequency of 5,893 angstroms. Glasses, in
general, are known to have a refractive index between about
1.5 to 1.9 at the sodium frequency of 5,893 angstroms. The
refractive index is the phase veloc~ty of radiation in free
space di~ided by the phase velocity of the same radiation in
a specified medium.
The composition of different glasses can be broadly
defined by referring to the amount of each element in the
glass. However, it is the short-range ordering in glasses
that give them their different properties. Therefore, by
characteri~ing th~ short-range ordering in glasses different
glass compositions can be defined with respect to properties.
In Example 5, the short range ordering of the translucent
silicon-oxy-carbide glass of this invention is determined by
:
,. , . : .
2~2~
-21-
RD-lg,549
defining the percentage of each of the polyatomic units;
tetracarbosilicon, ~onocarbosiloxane, dicarbosiloxane, and
tetraoxysilicon that are present in the glass.
The 29Silicon solid state nuclea;r magnetic
resonance spectrum of 2 sample o~ translucent silicon-oxy-
carbide gl~sQ from Example 2 was recorded and is pr~sented in
Figure 2. Figure 3 is the 29Silicon nuclear magnetic
resonance spectrum from a sample of "Nicalon" silicon carbide
fibre. On the ordinate of Figures 2 and 3 is plotted the
intensity of radiation measured from the excited sample, and
on the abscissa is plotted the parts per million (ppm) in
chemical shift from a tetramethyl silicon standard that fixes
the zerc point on the abscissa. The chemical shift in ppm
are known for many polyatomic units, for example
tetraoxysilicon, dicarbosiloxane and monocarbosiloxane are
shown in; "NMR Basic Principles and Progress 29Si-NMR
Spectroscopic Results", Editors P. Diehl, R. Kosfeld,
Springer Verlag Berlin Heidelberg 1981 at pp. 186, 184 and
178. Therefore, each peak in Figures 2 and 3 defines the
short-range ordering of specific silicon polyatomic units.
In FIG. 2~ the spectrum of the silicon-oxy-carbide
glass prepared in Example 2 and containing peaks labeled 1
through 4 is shown. Peak 1 is ~etracarbo~ilicon, peak 2 is
dicarbosiloxane, peak 3 is monocarbosiloxane, and peak 4 is
tetraoxysilicon. By integrating the area under each peak,
the fraction of each of these polyatomic units that is
present in the glass can be dete~mined. A correction fo.r
background interference was made to the spectra in Figures 2
and 3 before determining the integrated area under each peak.
The integrated area under each peak in FIG. 2
reveals a composition for the silicon-oxy carbide glass of
. . . ~
~.
2 ~
-22-
RD-19,549
Example 2, comprising in weight percent about i 5 percent of
the following, about 33 percent tetracar~osilicon, about 17
percent dicarbosiloxane, about 26 percent monocarbosiloxane,
and about 23 percent tetraoxysilicon. ~na;Lysis of the
nuclear magnetic resonancA spectra and the translucent
appearance of the glass indicat~s that up to a trace amount
of elemental carbon of about 0.1 weight percen~ is dispersed
in the glass.
The spectrum in Figure 2 can be compared to the
silicon carbide spectrum in FIG. 3 measured from a "Nicalon"
silicon carbide fibre sample. The composition for "Nicalon"
in FIG. 3, in weight percent, is about 68 percent silicon
carbide, about 8 percent dicarbosiloxane, about 17 percent
monocarbosiloxane, and about 7 percent tetraoxysilicon. From
the spectrum in FIG. 3, it can be seen that "Nicalon" fibres
are comprised principally of silicon carbide with minor
amounts of dicarbosiloxane, monocarbosiloxane~ and
tetraoxysilicon. ln contrast, ~he spectrum of FIG. 2 shows
that silicon oxy-carbide glass is comprised of
tetracarbosilicon with substankial amounts of di-
carbosiloxane, monocarbosiloxane, and tetraoxysilicon. This
unique short range ordering of silicon-oxy-carbide glass that
bonds carbon to silicon in a heretofore unknown manner in
glasses, provides the increased devitri~ication and
decomposition resistance and characterizes the glasses of
~his invention.
The composition of the translucent silicon-oxy-
carbide glass sa~ple in Example 2 and the Nicalon sample can
also be described by referring to the mole percent of each
polyatomic unit. Table II below provides the conversion
between mole percent and weight percent for each of these
compositions. The compositions disclosed in Table II are
2~2~
-23-
RD-19,549
considered to be within ~5 weight percent or 5 mole percent
for each polyatomic unit.
~ TT
~ mpl~ 5 ~ampl~ 6
Silicon-Oxy-Carbide Gla3~ "Nicalon"
~ ~iC
1 0 ~
Tetraoxysilicon 23 2~ 7 5
MonocarboAil~xane 26 26 17 13
Dicar~03iloxane 17 17 ~ 7
Tetracarbo3ilicon 33 35 68 75
Because the mole unit is a molecular weight, the
mole percent gives the percentage of each polyatomic unit in
the samples on a molecular basis. The percentage of the
silicon atoms in the samples that is bonded to oxygen or
carbon can then be determined using the mole percent. The
silicon-oxy-carbide glass sample in Example 5, originally
prepared in Example 2, has about 73 to 83 percent of the
silicon atoms in the glass bonded to at least an in~ividual
carbon atom. The "Nicalon~' silicon carbide sample had about
90 to 100 percent of the silicon atoms in the ~ilicon carbide
sample bonded to carbon.
;
A sample of silicon-oxy-carbide glass was prepared
by pyrolizing, according to the method of this invention, a
methyl silicone resin consisting of five percent D groups and
95 percent T groups. The resin densified into a silicon-oxy-
carbide glass comprising, in weight percent, about 39 percent
tetraoxysilicon, about 24 percent monocarbosiloxane, about 22
- :` '
:, - . :., .
:,
-24-
RD-19,5~9
percent dicarbosiloxane, about 6 percen~ tetracarbosilicon,
with about 3 to 9 percent elemental carbon dispersed in the
glass. Silicon-oxy-carbide glass that is made from a DT type
methyl silicone resin is the sub~ect of co-pending
application serial number 359,619. The oxi.dation resistance
and structural stability or resistance to devitrification of
silicon-oxy-carbide glass made from the DT type methyl
silicone resin identified above was analyzed by hea~ing hot
pressed specimens of the glass for 240 hours at 1400-C and
1520-C in air. No weight loss from decomposition of silicon
or carbon in the glass was measured. X-ray diffraction of a
sectioned surface revealed no evidence of crystallization in
the bulk material of either specimen. X-ray diffraction of
exposed surfaces showed evidence of surface crystallization
to cristobalite in both specimens in about 0.002 inch of the
surface.
Although the silicon-oxy-carbide glass sample in
Example 7 is black in appearance, and has a different
composition from the translucent silicon-oxy-carbide glass,
it contains the chemical bonding between silicon and carbon
atoms with an absence of the chemical bonding between carbon
and oxygen atoms that characterize the glasses of this
invention. TherefQre, the silicon-oxy-carbide glass o
Example 1 and the translucent sllicon-oxy-carbide glasses of
Examples 2, 3 and 4 are expected to have substantially the
same resistance to devitrification and dPcomposition as the
silicon-oxy-carbide glass prepared for Example 7.
, ,"., , -:
.. . ..
: , : .
