Synthesis of microporous ceramics

The present invention provides for microporous ceramic materials having a surface area in excess of 50 m.sup.2 /gm and an open microporous cell structure wherein the micropores have a mean width of less than 20 Angstroms and wherein said microporous structure comprises a volume of greater than about 0.015 cm.sup.3 /gm of the ceramic. The invention also provides for a preceramic composite intermediate composition comprising a mixture of a ceramic precursor and finely divided particles comprising a non-silicon containing ceramic, carbon, or an inorganic compound having a decomposition temperature in excess of 400.degree. C., whose pyrolysis product in inert atmosphere or in an ammonia atmosphere at temperatures of up to less than about 1100.degree. C. gives rise to the microporous ceramics of the invention. Also provided is a process for the preparation of the microporous ceramics of the invention involving pyrolysis of the ceramic intermediate under controlled conditions of heating up to temperatures of less than 1100.degree. C. to form a microporous ceramic product.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to open pore, microporous ceramic materials 
and their method of manufacture. 
2. Description of Related Art 
Porous materials play a particularly important role in a number of chemical 
processing industries and applications. Separations based on membranes are 
critical in such fields as chemical recovery, purification and 
dehumidification. Porous oxides (e.g., clays, silica, alumina and zeolite) 
are the materials of choice as catalysts or catalyst supports in chemical 
and petroleum processing reactions such as hydrocracking, 
hydrodesulfurization, reforming, and polymerization. 
With respect to membrane technology, inorganic membranes offer a number of 
advantages over polymeric membranes which are typically limited to uses at 
temperatures below about 250.degree. C. These include: i) higher operating 
temperatures, ii) greater structural integrity and hence the ability to 
withstand higher pressure differentials and backflushing and iii) improved 
resistance to corrosion. Porous oxide, (e.g., aluminum oxide) and carbon 
membranes offer some of these characteristics, but advanced materials are 
still required for improved strength, toughness, structural integrity, 
temperature stability, water and oxygen resistance, thermal shock 
resistance, molecular selectivity to small molecules and gases, and high 
flux. 
Similar considerations apply to clay and metal oxide type catalysts or 
catalyst supports, particularly as relates to stability and thermal shock 
resistance at temperatures above about 500.degree. C. 
Ceramic materials of the Si--C, Si--N, Si--C--N, Si--B--C, Si--BN, Al--N, 
Si--Al--N, B--Al--N and related types appear to offer many of the 
properties set forth above. However, the solgel synthesis methods 
typically used to prepare porous oxide membranes or catalyst supports are 
incompatible with the preparation of ceramics of the type described above 
because of the need to use water in their preparation. Sintering or 
reactive sintering of these ceramics likewise produces materials with pore 
sizes of from about 0.1 to about 1000 microns which are non-uniform and 
generally too large for effective molecular separation and other uses 
described above. Chemical vapor deposition can produce microporous ceramic 
layers, but this tends to be an expensive, high temperature process with 
limited ability to tailor complex ceramic compositions. 
Recently, researchers have discovered improved methods for preparing 
ceramics using ceramic precursors as starting materials. A ceramic 
precursor is a material, a chemical compound, oligomer or polymer, which 
upon pyrolysis in an inert atmosphere and at high temperatures, e.g., 
above about 700.degree.-1000.degree. C., preferably above 1000.degree. C., 
will undergo cleavage of chemical bonds liberating such species as 
hydrogen, organic compounds and the like, depending upon the maximum 
pyrolysis temperature. The resulting decomposition product is typically an 
amorphous ceramic containing Si--C bonds (silicon carbide), Si--N bonds 
(silicon nitride) or other bond structures which will vary as a function 
of the identity of the ceramic precursor, e.g., Si--C--N, Si--N--B, B--N, 
Al--N and other bond structures, as well as combinations of these 
structures. Crystallization of these amorphous ceramic products usually 
requires even higher temperatures in the range of 
1200.degree.-1600.degree. C. 
The pyrolysis of various ceramic precursors, e.g., polycarbosilanes, 
polysilanes, polycarbosiloxanes, polysilazanes, and like materials at 
temperatures of 1200.degree. C. and higher to produce ceramic products, 
e.g., silicon carbide and/or silicon nitride, is disclosed, for example, 
in M. Peuckert et al., "Ceramics from Organometallic Polymers", Adv. 
Mater. 2, 398-404 (1990). 
During pyrolysis, preceramic precursors such as described above liberate 
various gaseous decomposition species such as hydrogen and organic 
compounds, including methane, higher molecular weight hydrocarbon 
molecules, and lower molecular weight precursor fragments. These gases 
tend to coalesce within the preceramic matrix as they form, resulting in a 
bulking or swelling to form a voluminous mass of low bulk density. These 
entrained gases can also lead to the formation of smaller gas bubbles 
within the developing ceramic mass as the preceramic precursor crosslinks 
and hardens, resulting in a reduced density ceramic having a mesoporous or 
macroporous closed-cell structure, without development of a significant 
amount of open celled micropores. 
Where dense, non-porous ceramic materials are sought using ceramic 
precursors yielding high gas volumes, it is often necessary to conduct the 
pyrolysis over a very long period of time with very gradual incremental 
temperature increases and/or under vacuum to assist in removal of these 
gaseous species at temperatures where they are formed. 
SUMMARY OF THE INVENTION 
The present invention provides for amorphous, microporous, ceramic 
materials having a surface area in excess of 50 m.sup.2 /gm, preferably in 
excess of 100 m.sup.2 /gm, and an open-pore microporous cell structure 
wherein the micropores have a mean width (diameter) of less than 20 
Angstroms and wherein said microporous structure comprises a volume of 
greater than about 0.015 cm.sup.3 /gm, preferably greater than 0.05 
cm.sup.3 /gm, of the ceramic. The invention also provides for a preceramic 
composite intermediate composition comprising a mixture of a ceramic 
precursor and finely divided particulate material selected from the group 
consisting of non-silicon containing ceramics, carbon, inorganic compounds 
having a decomposition temperature greater than 1000.degree. C. and 
mixtures thereof, whose pyrolysis product in inert atmosphere or in an 
ammonia atmosphere at temperatures of up to less than about 1100.degree. 
C. gives rise to the microporous ceramics of the invention. As used in 
this application, the term "non-silicon containing ceramics" is defined to 
exclude oxide phases. Also provided is a process for the preparation of 
the microporous ceramics of the invention comprising: a) forming an 
intimate mixture comprising from greater than 30 up to about 99 parts by 
weight of a ceramic precursor oligomer or polymer having a number average 
molecular weight in the range of from about 200 to about 100,000 g/mole 
and from about 1 to less than 70 parts by weight of particulate material 
selected from the group consisting of non-silicon containing ceramics, 
carbon, inorganic compounds having a decomposition temperature greater 
than 1000.degree. C. and mixtures thereof, said particles having a mean 
particle size of less than about 10 microns, b) gradually heating said 
mixture in the presence of an inert gas or ammonia gas and in sequential 
stages with hold times at intermediate temperatures to a maximum 
temperature in the range of from about 400.degree. C. up to less than 
about 1100.degree. C. and over a period of total heating and hold time of 
from about 5 to about 50 hours to form a microporous ceramic product, and 
c) cooling said microporous ceramic product. 
The microporous ceramics prepared in accordance with this invention 
generally exhibit a surface area within the range of from about 50 to 
about 400 m.sup.2 /gm based on the combined weight of amorphous phase and 
particles, and amorphous phase micropore volumes of greater than 0.015 up 
to about 0.17 cm.sup.3 /g, wherein the volume fraction of micropores in 
the ceramic product ranges from about 5% to about 32%. 
Ceramics produced in accordance with this invention are particularly useful 
in bulk sorbent applications, as active layers in membrane separation 
structures and as catalyst supports. 
DETAILED DESCRIPTION OF THE INVENTION 
As used herein, the term microporous ceramic refers to a ceramic having a 
porous structure wherein the pores have a mean width (diameter) of less 
than 20 Angstroms. This definition and the physical and chemical 
adsorption behavior of microporous materials are disclosed in S. J. Gregg 
and K. S. W. Sing, "Adsorption, Surface Area and Porosity", Academic 
Press, New York, 1982; and S. Lowell and J. F. Shields, "Powder Surface 
Area and Porosity", Chapman and Hall, New York, 3rd Edition, 1984. This 
term is to be contrasted with the term "mesoporous" which refers to pores 
having a mean width of greater than 20 Angstroms up to about 500 Angstroms 
and the term "macroporous" which refers to pores having a mean width 
greater than about 500 Angstroms. More specifically, the term microporous 
refers to such structures wherein essentially all of the pores have a 
width of from about 2 to about 20 Angstroms. The surface area and 
micropore volume are calculated from the nitrogen adsorption isotherm, 
which is measured at 77.degree. K using an automated continuous flow 
apparatus. The total surface area is calculated using the BET method, and 
the micropore volume and mesopore/macropore surface area are calculated 
using the T-plot method, as described in the Gregg reference above. 
Subtraction of the mesopore/macropore surface area from the total surface 
area gives an estimate of the micropore surface area. 
Ceramic precursor materials which are preferred for the purposes of this 
invention include oligomers and polymers such as polysilazanes, 
polycarbosilazanes, perhydro polysilazanes, polycarbosilanes, 
vinylicpolysilanes, amine boranes, polyphenylborazanes, 
carboranesiloxanes, polysilastyrenes, polytitanocarbosilanes, alumanes, 
polyalazanes and like materials, as well as mixtures thereof, whose 
pyrolysis products yield ceramic compositions containing structural units 
having bond linkages selected from Si--c, Si--N, Si--C--N, Si--B, 
Si--B--N, Si--B--C, Si--C--N--B, Si--Al--N--C, Si--Al--N, Al--N, B--N, 
Al--N--B and B--N--C, as well as oxycarbide and oxynitride bond linkages 
such as Si--O--N, Si--Al--O--N and Ti--O--C. The preferred precursors are 
those oligomers and polymers having a number average molecular weight in 
the range of from about 200 to about 100,000 g/mole, more preferably from 
about 400 to about 20,000 g/mole. The chemistry of these oligomeric and 
polymeric precursors are further disclosed in the monograph "Inorganic 
Polymers", J. E. Mark, H. R. Allcock, and R. West, Prentice Hall, 1992. 
Particularly preferred polysilazanes are those materials disclosed in U.S. 
Pat. Nos. 4,937,304 and 4,950,381, the complete disclosures of which are 
incorporated herein by reference. These materials contain, for example, 
recurring --Si(H)(CH.sub.3)--NH-- and --Si(CH.sub.3).sub.2 --NH-- units 
and are prepared by reacting one or a mixture of monomers having the 
formula R.sub.1 SiHX.sub.2 and R.sub.2 R.sub.3 SiX.sub.2 in anhydrous 
solvent with ammonia. In the above formulas, R.sub.1, R.sub.2 and R.sub.3 
may be the same or different groups selected from hydrocarbyl, alkyl silyl 
or alkylamino and X.sub.2 is halogen. The preferred polysilazanes are 
prepared using methyldichlorosilane or a mixture of methyldichorosilane 
and dimethyldichlorosilane as monomer reactants with ammonia. The primary 
high temperature pyrolysis product (&gt;1300.degree. C.) of this precursor 
are silicon nitride (Si.sub.3 N.sub.4) and silicon carbide (Sic). These 
precursors are commercially available from Chisso Corporation, Japan under 
the trade designations NCP-100 and NCP-200, and have a number average 
molecular weight of about 6300 and 1300 respectively. 
Another class of polysilazane precursors are polyorgano (hydro) silazanes 
having units of the structure 
EQU [(RSiHNH).sub.x (R.sub.1 SiH).sub.1.5 N].sub.1-x 
where R.sub.1 is the same or different hydrocarbyl, alkylsilyl, alkylamino 
or alkoxy and 0.4&lt;X&lt;1. These materials are disclosed in U.S. Pat. No. 
4,659,850, the complete disclosure of which is incorporated herein by 
reference. 
Another preferred ceramic precursor is a polysilastyrene having the 
structure [(phenyl)(methyl)Si--Si(methyl).sub.2 ].sub.n available under 
the trade designation "Polysilastyrene-120" from Nippon Soda, Japan. This 
material has a number average molecular weight of about 2000 and the 
primary pyrolysis products silicon carbide and carbon. 
Other preferred ceramic precursors are polycarbosilanes having units of the 
structure (Si(CH.sub.3).sub.2 CH.sub.2).sub.n and/or 
(Si(H)(CH.sub.3)CH.sub.2).sub.n having a number average molecular weight 
in the range of about 1000 to 7000. Suitable polycarbosilanes are 
available from Dow Corning under the trade designation PC-X9-6348 (Mn=1420 
g/mol) and from Nippon Carbon of Japan under the trade designation 
PC-X9-6348 (Mn=1420 g/mol). The main pyrolysis product of these materials 
in an inert atmosphere are silicon carbide and excess carbon. 
Vinylic polysilanes useful in this invention are available from Union 
Carbide Corporation under the trade designation Y-12044. These yield 
silicon carbide together with excess carbon as the main pyrolysis product. 
Suitable polyalazane (alumane) preceramic precursors are those having 
recurring units of the structure R--Al--N--R', where R and R', are the 
same or different hydrocarbyl groups (particularly C.sub.1 -C.sub.4 
alkyl), and are described in an article "Polymer Precursors For Aluminum 
Nitride Ceramics", J. A. Jensen, pp. 845-850, in "Better Ceramics Through 
Chemistry", MRS Symposium Proceedings, Vol. 271. The main pyrolysis 
product of these materials is aluminum nitride. 
Other suitable preceramic precursors will be evident to those skilled in 
the art, particularly those yielding amorphous or crystalline phases such 
as SiC, Si.sub.3 N.sub.4, Si--C--N, B--N, Si--B--N, B.sub.4 C--BN--C, 
Si--B--C, Si--Al--N, B--Al--N and Al--N pyrolysis products. 
The solid particulate material which is mixed with the ceramic precursor 
material may be in the form a powder having a mean particle size of less 
than 10 microns or in the form of finely chopped fibers less than 1 mm 
long and having a mean diameter of less than 10 microns. These particles 
may comprise non-silicon containing ceramics such as the nitride of 
aluminum, the nitrides or carbides of boron, molybdenum, manganese, 
titanium, zirconium, tungsten and other refractory or rare earth metals, 
as well as ceramics containing a combination of bond linkages such as 
B--Al--N, B--N--C and Al--N--B. These particles may have either a 
crystalline or amorphous atomic structure. 
Other types of particles which may be used include carbon in various forms 
such as carbon black, carbon fibers, natural or synthetic diamond and 
tubular fullerenes. 
Still other types of particles which may be employed include non-ceramic 
inorganic compounds having a decomposition temperature in excess of 
400.degree. C., preferably in excess of 500.degree. C. to 1100.degree. C. 
These include Periodic Table Group II, III, IV, V, VI, VII and VIII metal 
and non-metal oxides, hydroxides, sulfides and like compounds such as 
alumina (aluminum oxide), silica, iron oxides, copper oxides, nickel 
oxides, titanium dioxide, zinc oxide, magnesium oxide, chromium oxide, 
calcium oxide and like materials, as well as crystalline silica-aluminates 
such as clay, silicalite and zeolites such as Zeolite X, Zeolite Y, beta 
Zeolite, Zeolite L, Zeolites ZSM-5, ZSM-11, ZSM-25 and like materials. 
The surface area and the degree of microporosity which can be achieved in 
the microporous ceramics prepared in accordance with this invention has 
been found to vary inversely with the mean particle size or mean diameter 
of the particles which are blended with the ceramic precursor to form the 
composite intermediate. Where the mean particle size or diameter is large, 
i.e., 20 microns or greater, the particles tend to settle within the 
preceramic matrix giving rise to two distinct phases, i.e., a dense phase 
and a voluminous non-microporous phase containing high and low 
concentrations of particles respectively. Preferably the particles will 
have a mean particle size or diameter of less than 10 microns, preferably 
less than 5 microns and more preferably from about 0.1 to about 2 microns. 
Where necessary, commercially available materials of larger particle size 
can be ground by any suitable means, including cryogenic grinding below 
minus 100.degree. C., to achieve non-aggregate, mean particle sizes within 
these preferred ranges. 
Although the factors underlying the development of the microporous, 
open-celled ceramic structure achieved in accordance with this invention 
are not completely understood, it is believed that the individual solid 
particulates dispersed within the molten or glassy preceramic polymer 
matrix serve to prevent nucleation of large bubbles of decomposition gases 
which form within the matrix as the temperature increases. The 
decomposition gases thus more readily escape from the matrix by diffusion, 
thereby avoiding the development of a voluminous swelling of the ceramic 
mass. The elimination of molecular species from the ceramic precursor 
molecules, accompanied by crosslinking, provides a templating effect which 
thus entrains a significant volume of microporosity and contributes to 
enhanced surface area of the resulting solidified ceramic mass. 
Another factor which has been found to influence both the total surface 
area and degree of microporosity achieved in the pyrolyzed ceramic of this 
invention is the amount of ceramic precursor mixed with the additive 
particles to form the composite intermediate. This level will vary within 
the range of from greater than 30 parts by weight up to about 99 parts by 
weight of ceramic precursor and correspondingly from about 1 to less than 
70 parts by weight of the particles. Microporous ceramics having a 
post-pyrolysis surface area in excess of about 150 m.sup.2 /gm and a 
micropore volume in excess of 0.03 cm.sup.3 /gm, preferably in excess of 
0.05 cm.sup.3 /gm, can be achieved when the amount of ceramic precursor 
mixed with the particles to form the composite intermediate is in excess 
of 40 parts by weight up to about 80 parts by weight precursor and the 
balance to 100 parts by weight of particles. The most preferred range is 
from about 50 to about 70 parts by weight ceramic precursor per 
corresponding about 30 to about 50 parts by weight of additive particles, 
since composite intermediates containing this latter ratio of components 
can yield post-pyrolysis surface areas of greater than 200 m.sup.2 /gm and 
micropore volumes of greater than 0.08 cm.sup.3 /gm. 
The microporous ceramic compositions of this invention are prepared by 
first forming an intimate mixture of the ceramic precursor and the 
additive particles to provide a composite intermediate, followed by 
pyrolysis of the composite intermediate under an inert atmosphere or 
ammonia in sequential stages with hold times at intermediate temperatures 
to a final temperature in the range of from about 400.degree. C. to less 
than 1100.degree. C. 
The composite intermediate mixture may be formed by any suitable process 
which will provide for a uniform dispersion of the particles within the 
ceramic precursor matrix. Thus, the components may be ground, ball milled 
or pulverized together in dry form, or the components may be slurry 
blended by forming a very fine suspension of the additive particles in an 
organic liquid which is a solvent for the ceramic precursor, dissolving 
the precursor in the solvent to form a slurry and evaporating the solvent 
at temperatures of 30.degree. to 80.degree. C. at atmospheric pressure or 
under vacuum to obtain a composite intermediate composed of the preceramic 
precursor having the particles uniformly dispersed therein. The composite 
may then be comminuted to provide a particulate molding powder. 
Suitable solvents for the solution blending process include aromatic and 
aliphatic hydrocarbons such as benzene, toluene, and hexane, as well as 
ethers such as tetrahydrofuran, diethyl ether and dimethyl ether. Where 
the slurry blending technique is used, the ceramic precursor and particles 
are preferably added to the solvent at a combined weight ratio within the 
range of from about 20% to 50% by weight solids. Ultrasonic mixing 
techniques and/or a suitable dispersant can be used to facilitate the 
formation of a very fine suspension of the particles in the organic 
solvent. 
Prior to pyrolysis, the composite intermediate may be formed into any 
desired shape such as a pellet, disc, fiber, thin membrane or other three 
dimensional shape. The dry composite may be shaped using an extruder or a 
hydraulic press, with or without heat being applied, or by conducting the 
pyrolysis in a suitable mold cavity containing the composite intermediate. 
Fibers may be prepared by extruding or spinning a melt or a solvent slurry 
of the composite intermediate, while thin separation membranes may be 
formed by applying a melt or solvent slurry of the composite intermediate 
to the surface of a suitable substrate, such as another ceramic, and 
subjecting the structure to well known spin or whirl coating techniques to 
form a uniform, thin coating of the composite intermediate on the surface 
of the substrate, followed by heating to evaporate off the solvent where 
solvent is present. 
As indicated above, pyrolysis of the composite intermediate is next 
conducted by heating it under inert flowing gas, e.g., argon, helium or 
nitrogen, or under flowing ammonia gas, at a controlled rate of 
temperature, with preferred hold times at intermediate temperatures to 
maintain uniformity of the ceramic product, and a final hold time at the 
maximum heating temperature, followed by gradual cooling of the ceramic 
end product to room temperature. The heating rate may range from about 
0.5.degree. to 10.degree. C. per minute, more preferably from about 
0.5.degree. to 6.degree. C. per minute and most preferably from about 0.5 
to less than 3.degree. C. per minute. Generally speaking, microporous 
ceramics are formed by gradually heating the composite intermediate to a 
maximum temperature (T.sub.max) in the range of from about 400.degree. C. 
to less than about 1100.degree. at a heating rate in the range of from 
about 30.degree. C. to 400.degree. C. per hour, with various holding times 
of about 0.5 to about 5 hours at selected temperatures between about 
200.degree. C. and T.sub.max. Total combined heating/holding time may 
range from about 5 to about 50 hours, more preferably from about 8 to 
about 24 hours. Holding times and temperatures are dictated by ceramic 
precursor decomposition and reaction kinetics. Hence, they depend on 
precursor composition and the rate of evolution of specific molecular 
species at or about the holding temperature, e.g., CH.sub.4, H.sub.2, 
higher molecular weight hydrocarbon or H--C--N species or precursor 
fragments, as reflected by sample weight losses at or about these 
temperatures. The flow rate of the inert gas or ammonia gas may range from 
about 100 to about 1000 cc per minute. 
In the more preferred embodiment of the invention, pyrolysis is carried out 
in a heat treating furnace or muffle oven using the following schedule and 
using flowing inert gas or ammonia throughout: 
i) after flushing the furnace with inert gas, e.g., helium, the temperature 
is first increased to about 200.degree..+-.25.degree. C. over a period of 
0.5-3 hours, held at that temperature for a period of 0.5-5 hours, 
preferably 1-2 hours and the temperature then increased; 
ii) in the second step, the temperature is increased to about 
300.degree..+-.25.degree. C. over a time of from about 0.5 to 5 hours, 
preferably from 1-2 hours and held at that temperature for 0.5-5 hours, 
preferably 1-2 hours, and the temperature again increased; 
iii) in the third step the temperature is increased to T.sub.max or about 
500.degree..+-.25.degree. C., whichever is less, over a time period up to 
about 5 hours, preferably up to 2 hours, and held at that temperature for 
0.5-5 hours, preferably 1-2 hours; 
iv) in a fourth step where T.sub.max is above 500.degree. C., the 
temperature is increased to T.sub.max or about 
700.degree..+-.25.degree.C., whichever is less, over a time period up to 
about 5 hours, preferably up to 2 hours, and held at that temperature for 
0.5-5 hours, preferably 1-2 hours; 
v) in a subsequent step where T.sub.max ranges between 700.degree. C. and 
1100.degree. C., the temperature is increased to T.sub.max over a time 
period of up to 5 hours, preferably 1-3 hours, and held at T.sub.max for 
0.5-5 hours, preferably 1-2 hours. 
In the most preferred embodiment of the invention, the composite 
intermediate is heated as above with a 1-2 hour hold at about 200.degree. 
C., 300.degree. C., 500.degree. C. and 700.degree. C. (and T.sub.max if 
T.sub.max is greater than 700.degree. C.), and the pyrolyzed ceramic then 
allowed to return from T.sub.max to room temperature while continuing the 
flow of inert gas or ammonia during the cooling period. 
In addition to particle size and quantity of particles present in the 
composite intermediate, another factor which influences both surface area 
and the degree of microporosity which can be achieved in the microporous 
ceramic is the final temperature to which the ceramic is heated. It has 
been found with respect to most composite intermediates pyrolyzed under 
inert or ammonia gas that the surface area and degree of microporosity 
tends to diminish as T.sub.max approaches 1100.degree. C. and tends to be 
at maximum levels at T.sub.max of up to about 700.degree..+-.150.degree. 
C. For these reasons, a more preferred heating schedule is such that 
T.sub.max ranges from about 500.degree. C. to about 850.degree. C., more 
preferably from about 550.degree. C. to about 750.degree. C.

The following examples are illustrative of the invention. As used in the 
examples and tables, the following designations have the following 
meanings: 
NCP-100--A polysilazane polymer available from the Chisso Corporation of 
Japan having a number average molecular weight of about 6300 g/mole and a 
melting point of about 200.degree. C. 
NCP-200--A polysilazane polymer available from the Chisso Corporation of 
Japan having a number average molecular weight of about 1300 g/mole and a 
melting point of about 100.degree. C. 
PCS--A polycarbosilane preceramic polymer available from Nippon Carbon 
Company of Japan (U.S. distribution Dow Chemical Company) having a number 
average molecular weight of about 2000 g/mole and a melting point of about 
100.degree. C. 
PSS--A polysilastyrene preceramic polymer available from Nippon Soda 
Corporation of Japan having a number average molecular weight of about 
2000 g/mole and a melting pointing of about 200.degree. C. 
EXAMPLE 1 
(D23-2-D25-3) 
Mixtures of 1.8 grams of the preceramic polymer (PCP) identified in Table 1 
and 1.2 grams of alumina (Al.sub.2 O.sub.3) having a particle size of 
about 1 micron or 0.05 microns as set forth in Table 1 were prepared by 
grinding each mixture in an agate mortar and pestle. The ground mixtures 
were then each placed in a 40 cc polystyrene jar together with 0.6 cm 
alumina balls and mixed on a rolling mill for 48 hours. The mixtures were 
then each transferred to an aluminum oxide boat and inserted in the steel 
liner of a heat treating furnace. 
The material was then heated in helium (He) to a final temperature of 
500.degree. C., 600.degree. C., 700.degree. C., 850.degree. C. or 
1000.degree. C. The general temperature-time sequence used was as follows: 
Purge with He at a flow rate of 300 cc/min for 30 minutes. Heat in 60 
minutes to 200.degree. C. and hold at that temperature for 240 minutes. 
Then heat in 120 minutes to 300.degree. C., followed by a hold for 300 
minutes at that temperature. Next, heat to 400.degree. C. in 120 minutes, 
followed by a hold for 300 minutes at that temperature. Then heat in 120 
minutes to 500.degree. C. If this if the final temperature, heat for 120 
minutes, followed by cool to room temperature. For 700.degree. C. run 
schedule, heat from 500.degree. C. to 700.degree. C. in 120 minutes, and 
hold for 120 minutes before cooling to room temperature. For 850.degree. 
C. run schedule, heat from 700.degree. C. to 850.degree. C. in 120 
minutes, and hold for 120 minutes before cooling to room temperature. For 
1000.degree. C. run, heat from 850.degree. C. to 1000.degree. C. in 120 
minutes and hold at that temperature for 120 minutes before cooling to 
room temperature in 480 minutes. 
The resulting compact or granular products all exhibited Type 1 nitrogen 
adsorption isotherms. Surface area and micropore volume analysis on each 
sample is shown in Table 1. 
TABLE 1 
______________________________________ 
Al.sub.2 O.sub.3 Micro- 
Particle Surface 
pore Maximum 
Sample Size Area Volume Temp. 
Number (micron) PCP (m.sup.2 /gm) 
(cm.sup.3 /gm) 
(.degree.C.) 
______________________________________ 
D23-2 0.05 NCP-200 323 0.1186 500 
D23-1 0.05 NCP-100 331 0.1214 500 
D24-2 0.05 NCP-200 372 0.1441 600 
D24-1 0.05 NCP-100 321 0.1196 600 
D18-4 1 NCP-200 198 0.0793 700 
D18-5 0.05 NCP-200 212 0.0834 700 
D26-2 0.05 NCP-200 260 0.1021 850 
D26-1 0.05 NCP-100 158 0.0590 850 
D26-3 0.05 PCS 164 0.0652 850 
D25-2 0.05 NCP-200 130 0.0507 1000 
D25-1 0.05 NCP-100 66 0.0232 1000 
D25-3 0.05 PCS 87 0.0328 1000 
______________________________________ 
The data in Table 1 show that at a 60-40 PCP/Al.sub.2 O.sub.3 blend ratio, 
the surface area and micropore volume present in the resulting ceramic 
product tends to vary inversely as a function of the maximum pyrolysis 
temperature. 
EXAMPLE 2 
(D18-3) 
A mixture of 1.8 gm of NCP-200 polysilazane and 1.2 gm of a silicalite 
powder having an average particle size of about 1 micron was prepared and 
processed as in Example 1. The mixture was pyrolyzed in flowing helium as 
set forth in Example 1 except that heating was discontinued at a T.sub.max 
of about 700.degree. C. The resulting product exhibited a Type 1 nitrogen 
adsorption isotherm, a surface area of 190 m.sup.2 /gm and a micropore 
volume of 0.0799 cm.sup.3 /gm. 
EXAMPLE 3 
(D18-1) 
A polysilazane-ceramic mixture was prepared in the manner described in 
Example 2 except that the silicalite was substituted by a typical carbon 
black powder. The resulting product sample exhibited a Type 1 nitrogen 
adsorption isotherm, and had a surface area of 260 m.sup.2 /gm and a 
micropore volume of 0.1002 cm.sup.3 /gm. 
EXAMPLE 4 
(D18-2) 
A polysilazane-ceramic mixture was prepared in the manner described in 
Example 2 except that the silicalite was substituted with a powder of &lt;1 
micron filamentous carbon. The resulting product sample exhibited a Type 1 
nitrogen adsorption isotherm, and had a surface area of 149 m.sup.2 /gm 
and a micropore volume of 0.0534 cm.sup.3 /gm. 
EXAMPLE 5 
(D34-1) 
A polysilazane-ceramic mixture was prepared in the manner described in 
Example 2 except that the silicalite was substituted by an aluminum 
nitride ceramic powder having an average particle size of about 1.6 
micron. The resulting product sample exhibited a Type 1 nitrogen 
adsorption isotherm, and had a surface area of 192 m.sup.2 /gm and a 
micropore volume of 0.0769 cm.sup.3 /gm. 
EXAMPLE 6 
A series of four different mixtures of 3 gms each of polysilazane 
preceramic polymers identified in Table 2 were mixed with 2 grams of 
powder materials also identified in Table 2 by a two step solvent blending 
process. The two materials were mixed and then added to about 15 cc of 
toluene in a glass beaker. The polysilazane dissolved, and upon stirring a 
slurry was formed with the powdery additive. Next the toluene was 
evaporated on a hot plate. The integrally mixed samples of polysilazane 
polymer and powder were then placed in an aluminum oxide boat, and 
inserted into a steel heat treating furnace. The samples were then heated 
to 700.degree. C. in He, using the schedule described in Example 2. Each 
of the resulting pyrolyzed samples exhibited a Type 1 nitrogen adsorption 
isotherm, demonstrating microporosity. The surface area and micropore 
volume on each of these product samples are shown in Table 2. 
TABLE 2 
______________________________________ 
Micropore 
Surface 
Sample T.sub.max 
Volume Area 
Number PCP 2nd Phase (.degree.C.) 
(cm.sup.3 /gm) 
(m.sup.2 /gm) 
______________________________________ 
5 D11-4 
NCP-100 5.mu. diamond 
700 0.0144 45 
D12-5 NCP-100 1.mu. Zeolite 
700 0.0349 108 
"LZ 210" 
D12-6 NCP-100 1.mu. Zeolite 
700 0.0459 120 
"L" 
D16-5 NCP-100 0.6.mu. TiO.sub.2 
700 0.0257 81 
______________________________________ 
EXAMPLE 7 
Effect of heating to 700.degree. C. for PCP/Metal Oxide Mixtures 
A series of five different mixtures of 1.8 gms each of polycarbosilane 
preceramic polymer, PC-X9-6348 from Dow Corning, identified in Table 3 
were mixed with 1.2 gms of metal oxide powder materials, also identified 
in Table 3, by a two-step blending process. The metal oxide powders 
obtained commercially from CERAC were first seived to &lt;20 .mu.m particle 
size and then mixed with the polycarbosilane material in a mortar and 
pestle. Then each of the five mixtures in Table 3 was placed in a 40 cc 
polystyrene plastic jar together with 0.6 cm alumina balls and mixed on a 
rolling mill for 2 hours. The resulting material was then heated in He to 
700.degree. C., using the schedule described in Example 2. Each of the 
resulting ceramic product samples exhibited a Type 1 nitrogen adsorption 
isotherm, demonstrating that they are microporous. The surface area and 
micropore volume of each of the pyrolyzed samples are shown in Table 3. 
TABLE 3 
______________________________________ 
Micropore 
Sample 2nd Volume Surface Area 
Number PCP Phase T.sub.max (.degree.C.) 
(cm.sup.3 /gm) 
(m.sup.2 /gm) 
______________________________________ 
D46-1 PCS ZnO 700 0.0551 148 
D46-2 PCS MgO 700 0.0991 244 
D46-3 PCS FeO 700 0.0154 51 
D46-4 PCS CuO 700 0.0144 52 
D46-5 PCS NiO 700 0.0566 149 
______________________________________ 
EXAMPLE 8 
Effect of Heating to 1300.degree. C. for PCP/2nd Phase Mixtures 
Two samples described in Table 4 were prepared as follows. A mixture of 1.8 
gm of NCP-200 preceramic polymer (PCP) was made with 1.2 gm of Al.sub.2 
O.sub.3 or AlN to provide a 60/40 mixture. Then the mixtures were placed 
in a 40 cc polystyrene jar together with 0.6 cm alumina balls and mixed on 
a rolling mill for 48 hours. The resulting material was then heated in 
flowing He to a final temperature of 1300.degree. C. The schedule from 
room temperature to 1000.degree. C. was similar to that previously 
described in Example 1. The heating from 1000.degree. C. to 1300.degree. 
C. was done in 120 minutes, with a hold at 1300.degree. C. for 60 minutes, 
followed by cooling to room temperature in 480 minutes. It can be seen 
from the micropore volumes in Table 4 that the microporosity obtained by 
heating all the way to 1300.degree. C. is quite small. The relatively high 
surface areas are the surface areas of fine particles, as evidenced by the 
fact that Type 1 isotherms were not obtained. 
TABLE 4 
______________________________________ 
Samples of Example 8, Heated to 1300.degree. C. 
Micropore 
Surface 
Sample T.sub.max 
Volume Area 
Number PCP 2nd Phase (.degree.C.) 
(cm.sup.3 /gm) 
(m.sup.2 /gm) 
______________________________________ 
D31-1 NCP-200 0.5.mu. Al.sub.2 O.sub.3 
1300 0.0028 62 
D31-2 NCP-200 1.6.mu. AlN 
1300 0.0003 9 
______________________________________ 
EXAMPLE 9 
Microporosity of Thermally Decomposed PCP/2nd Phase Mixtures Pyrolyzed in 
Ammonia Gas 
A series of samples described in Table 5 were prepared as follows. A 
mixture of 1.8 gm or preceramic polymer (PCP) was made with 1.2 gm of the 
second phase (2nd Phase), using 1.6 micron AlN powder as the second phase. 
Each mixture was placed in a 40 cc polystyrene jar together with 0.6 cm 
alumina balls and mixed on a rolling mill for 2 hours. Each of the 
resulting mixtures was placed in an aluminum oxide boat and inserted in 
the steel linear of a heat treating furnace, and was then heated in 
flowing ammonia at 300 cc/min to a maximum temperature of 700.degree. C. 
according to the heating schedule described in Example 2. Each of the 
resulting product samples exhibited a Type 1 nitrogen adsorption isotherm, 
demonstrating that they are microporous. The surface area and micropore 
volume of each of the pyrolyzed samples are shown in Table 5. 
TABLE 5 
______________________________________ 
Thermal Decomposition in Ammonia Gas 
Micro- 
Surface 
pore Maximum 
Sample 2nd Area Volume Temp. 
Number Phase PCP (m.sup.2 /gm) 
(cm.sup.3 /gm) 
(.degree.C.) 
______________________________________ 
38-4 1.6.mu. AlN 
PCS 318 0.1283 700 
39-3 1.6.mu. AlN 
NCP-200 141 0.0573 700 
39-6 1.6.mu. AlN 
NCP-100 179 0.0719 700 
39-9 1.6.mu. AlN 
PSS 113 0.0447 700 
______________________________________ 
EXAMPLE 10 
(D35-3)--Control 
A sample of 3 gm of NCP-200 polysilazane was ground in a mortar and pestle 
and heated by itself in an alumina crucible using the heating schedule 
described in Example 1. 
After heating to 700.degree. C. the resulting product sample exhibited a 
very low surface area (&lt;1 m.sup.2 /gm), with no microporosity observed in 
the nitrogen adsorption and no micropore volume. 
EXAMPLE 11 
(D35-2)--Control 
A sample of 3 gm of NCP-100 polysilazane was ground in a mortar and pestle 
and heated by itself in an aluminum crucible using the heating schedule 
described in Example 1. After heating to 700.degree. C., the resulting 
product sample exhibited a very low surface area (&lt;1 m.sup.2 /gm), with no 
microporosity observed in the nitrogen adsorption. 
EXAMPLE 12 
(D35-5)--Control 
Example 11 was repeated except that PSS polysilastyrene was substituted for 
NCP-100 polysilazane. After heating to 700.degree. C., the resulting 
product exhibited a surface area of less than 1 m.sup.2 /gm with no 
observable microporosity. 
EXAMPLE 13 
(D36-1)--Control 
Example 11 was repeated except that PCS polycarbosilane was substituted for 
NCP-100 polysilazane. After heating to 700.degree. C., the resulting 
product exhibited a surface area of less than 1 m.sup.2 /gm with no 
observable microporosity. 
Control Examples 10-13 demonstrate that heating the various ceramic 
precursors which do not contain the particulate additives in inert helium 
gas does not provide ceramic products having the microporous structure 
which is the subject matter of this invention.