Porous membranes and methods for making

A porous membrane produced by preparing a slurry made from at least one micropyretic substance and at least one liquid carrier. The slurry is dried into a green form having a desired geometric configuration. Combustion of the green form produces the porous membrane.

FIELD OF THE INVENTION 
The present invention relates to compositions and methods of making various 
types of porous membranes and to the porous membranes produced thereby. 
BACKGROUND OF THE INVENTION 
Porous membranes are used in a wide variety of engineering applications. 
Such membranes are often used in gas or vapor separation, reverse osmosis, 
electrochemical applications, hyperfiltration, ultrafiltration and 
microfiltration. Such membranes can even be used for the manipulation of 
chemical reactions including selective ion separation. The membranes are 
usually thin, two dimensional bodies which are normally less than 0.5 mm 
thick and contain upwards of 20% porosity. The pores in these membranes 
have radii ranging in size from a few nanometers to several microns. The 
flux of liquids or gas through the membrane is in most cases driven by a 
gradient of pressure or an electric field. 
Traditional methods of making such porous membranes have involved the 
deposition of adherent and highly continuous layers of materials such as 
alumina and aluminum. These materials are known to have been deposited 
using a number of techniques, such as tape casting, extrusion, vapor 
deposition. both physical vapor deposition (PVD) and chemical vapor 
deposition (CVD), and by electrochemical methods. Tape casting and 
extrusion are limited in the range of materials that can be used to make 
porous membranes. The degree of membrane porosity or pore density 
obtainable with either of these techniques is also limited. Vapor 
deposition is an extremely slow and costly process and does not lend 
itself to making large parts. Other techniques such as electrochemical 
methods are also known to suffer from such size limitations. In addition, 
the type of materials that may be applied by some electrochemical methods 
are severely limited. Sol-gel and hydrothermal techniques for making 
porous membranes are also known. The limitations of these techniques have 
been documented in a book on the subject by Ramesh Bhave (Inorganic 
Membranes, van Nostrand 1991). For instance, these techniques may be cost 
prohibitive and compatible with only a limited number of materials. In 
addition, the use of sol-gel techniques to form porous membranes has been 
discussed in an article by Y. S. Lin and A. J. Burggraaf, J.Amer. Ceram. 
Soc., Vol. 4, 1991, page 219. 
Several known materials have properties such as high temperature and 
corrosion resistance, which would be useful in some porous membrane 
applications. While some of these materials have been used with prior 
methods, like those mentioned above, to make such porous membranes, a 
number of these materials have not, for various reasons, including 
incompatibility with those prior methods. Those prior materials and 
methods which have proven compatible and which can produce useful 
membranes are typically involved and costly. 
Therefore, there is a need for a more cost effective method of making 
porous membranes not only with materials known for such use but also with 
materials having desirable properties but which previously have not been 
known for making such membranes. 
SUMMARY OF THE INVENTION 
The present invention is directed to a more cost effective method of making 
porous membranes. This invention is also directed to such a method which 
can be used with a wider variety of materials to produce porous membranes 
with a greater range of properties and applications. While theoretically 
complex, the present method is much simpler and less involved in practice 
than many prior methods for making porous membranes. The present method 
also lends itself to making multilayer as well as single layer monolithic 
membranes, and also layers of composite membranes. While mostly used to 
make inorganic membranes, inorganic and organic multilayer membranes can 
also be produced by the disclosed method directly or by secondary 
operations. 
The methods of the present invention utilize micropyretic (i.e., combustion 
synthesis) principles. As used herein, a micropyretic process is a process 
in which extremely high internal heat is quickly generated inside a green 
powder compact or form. The heat is of such a magnitude that sintering 
occurs between the powder particles. However, the high heat is not 
prolonged, thereby reducing densification. This rapid high temperature 
thermal process may involve propagation of a micropyretic front or wave, 
or the process may be initiated simultaneously at many points throughout 
the green form. 
Accordingly, the present method involves preparing a pliable slurry made 
from at least one micropyretic substance and at least one liquid carrier. 
The slurry is then dried into a green form having a desired geometric 
configuration. The green form is then fired or burned to produce a porous 
membrane. 
The micropyretic substances used in the slurry are combustible materials 
which react exothermically (i.e., provide heat) during the combustion of 
the green form and also add desired constituents to the membrane after 
combustion as well as clean and nascent products from the combustion. The 
micropyretic products (i.e., phases) are typically sub-micron in size even 
when the reacting particles have sizes in the tens of microns. Typical 
reactions could be, for example, Cr.sub.2 O.sub.3 +Al+B, Ni+Al or Ti+B or 
C+Al+SiO.sub.2, etc., which react spontaneously to give CrB.sub.2, 
Ni.sub.3 Al or TiB.sub.2 or SiC and Al.sub.2 O.sub.3, respectively, with a 
large release of heat. The adiabatic temperature of such a micropyretic 
reaction could be as high as 6,500.degree. K. When the product of the 
micropyretic reactions is electrically conductive, the final membranes 
could also find utility as thin and thick film resistors as well as 
heaters, electrodes in an electrochemical reaction, batteries or fuel 
cells. Mixtures of micropyretic substances are also possible. These 
slurries could also include the addition of combustible and 
non-combustible diluents or additives which could be the product itself or 
other materials in various forms, such as powder, toil, or fiber of a 
predetermined size. These diluents can be used to control the final 
membrane composition and properties, and the density and size of the 
membrane pores. These diluents may include refractory materials in 
addition to other ceramic materials, metallic materials (i.e., elemental 
metals or intermetallic compounds), metal organic compounds, pyrolizable 
organosilicon polymers and burnable materials in addition to other 
oxidizable constituents. The metallic and ceramic materials are typically 
in particle or powder form. Various liquids may be suitable as a carrier 
for the micropyretic substances and the diluents. The carrier could be 
aqueous or nonaqueous and a plasticizer. The carrier, like some 
plasticizers, may contain components which act as diluents. Possible 
plasticizers include metyl cellulose and related components, clays, 
silicates, borates, all types of lubricants including stearates, organic 
liquids, colloidal liquids or mixtures thereof. 
After preparing the slurry (which could range from very fluid to very 
powdery), the green or powder compact form of the membrane is produced 
with a desired geometric configuration. The viscosity of the slurry can 
have an effect on the final porosity (i.e., pore density) and pore size of 
the green form, and therefore the membrane. The green form can be 
produced, for example, by applying the slurry onto a surface of a 
substrate or an article using any one of a variety of well known coating 
techniques such as painting (by brush or roller), dipping, spraying, or 
pouring the slurry onto the surface. The applied slurry is then dried, for 
example by air drying, baking, etc. It may be desirable to soak the dried 
green form at an intermediate temperature above the drying temperature 
before combustion. Flat membranes are typically produced, however, other 
shapes such as cylinders, etc., are also contemplated. If multiple 
coatings are necessary to reach the required thickness or desired to 
produce a multilayer membrane, each coating should be allowed to at least 
partially dry before another coating is added. At least for some membrane 
materials, additional slurry coatings can be applied to already fired 
coatings for additional build up. In-situ repair of membranes during use 
is also contemplated without having to change to an entirely new membrane. 
In general, the micropyretic layers provide heat for the bonding of 
several layers and can also provide enough heat to bond the layers to the 
coated surface. Thus, the finished membrane can be made removable from the 
surface or permanently attached thereto. In addition to producing 
membranes with multiple micropyretic layers, membranes with one or more 
non-micropyretic layers can be produced, if desired. For example, a 
non-micropyretic layer having smaller pores can be sandwiched in between 
two micropyretic layers having larger pores. In this example, the 
non-micropyretic layer would be the membrane. 
Two modes can be used in the combustion of the green membrane, a wave mode 
or a thermal explosion mode. In the wave mode, the combustion is started 
at a discrete location and propagates in a wave-like manner across the 
entire green membrane. In the thermal explosion mode, the combustion 
reaction is initiated at several locations instead of being initiated at a 
single point by a concentrated heat source. Pore size can be controlled 
with the temperature of the combustion (Tc) and the velocity of the wave 
(u). Tc and u, in turn, can be controlled by particle size, atmosphere 
(including vacuum), the use of combustible and non-combustible diluents, 
the initial temperature (Ti) of the green form before combustion and, if 
used, the soaking temperature. 
Once it has been formed, combustion of the green membrane can be 
accomplished by any one of a number of firing or burning techniques, 
including by direct flame, concentrated sunlight, a plasma, a laser or an 
electron beam. In addition, if the substrate or article is conductive, 
combustion of the green form can occur by passing a current through the 
substrate or article. The coated substrate or article could also be placed 
inside a furnace at a predetermined temperature and time or heated by an 
induction method or by radiant heating. Solid-solid reactions, like 
Ni+Al.fwdarw.NiAl, are most commonly used, but solid-gas reactions are 
also used, such as in the synthesis of refractory nitrides like TiN where 
nitrogen gas is reacted with a refractory metal.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates to a wide variety of single layer and 
multilayer porous membranes used in diverse engineering applications. 
These membranes are usually thin, two dimensional bodies with a single 
layer membrane having a thickness within the range of 50 microns to 10 mm. 
It is believed that the pores in present membranes can range in size from 
0.1 micron to 500 microns. Because the pores have different shapes, pore 
size is usually determined by converting the pore area into an equivalent 
circular area. Pore densities can be within the range of 20 to 80%. 
The present membranes are synthesized by using a cost effective and 
versatile Method according to the principles of the present invention. 
These principles include the use of micropyretic technology (see U.S. Pat. 
No. 5,110,688 for an early application of this technology). The present 
principle of micropyretic synthesis generally involves local ignition of a 
green compact or form of a mixture of reactant powder to produce a 
product. Heat released from the product formation, if exothermic enough, 
enables the reaction to propagate through the compact in the form of a 
combustion wave. Combustion can be solid to solid (e.g. Ni+Al.fwdarw.NiAl 
or Ni.sub.3 Al or NiAl.sub.3) or solid to aas (e.g. Ti+N.fwdarw.TIN). The 
resulting micropyretic products in both cases will be porous. While the 
high heat generated is enough to quickly fuse the solid powder together, 
it does so with little densification because the high heat is short lived. 
In order to obtain products with the required properties and 
microstructure, it is important to understand the parameters that control 
the process. 
The two most important parameters that are characteristic of the reaction 
are the velocity of wave propagation u and the maximum temperature 
obtained during combustion Tc. These parameters depend upon the 
thermophysical properties like heat of the reaction, specific heat 
capacity, thermal diffusivity, activation energy, as well as the 
processing variables like reactant size, compact density and the thermal 
environment including the initial temperature. The above method of 
combustion is referred to as the wave mode of combustion. In addition, a 
thermal explosion mode is also possible when a combustion reaction is 
initiated at several locations. For example, ignition can be carried out 
in a furnace instead of being initiated at a single point or along a line 
by a concentrated heat source. 
An understanding of the thermodynamics of the process can be used to 
theoretically predict the maximum temperature Tc, obtained during 
combustion. This is also called the adiabatic temperature or combustion 
temperature. The adiabatic temperature, may be calculated theoretically by 
assuming the total enthalpy change (.DELTA.H.degree.) during the process 
is given by: 
##EQU1## 
The variable T is the temperature, T.sub.i is the initial temperature and 
T.sub.c is the combustion temperature. .DELTA.H.sub.i .degree. is the 
change in the enthalpy of reaction at the initial temperature. Since the 
process is adiabatic, .DELTA.H.degree.=0 and the rise in temperature (T) 
is equal to the change in the reaction enthalpy .DELTA.H.degree. divided 
by the specific heat of the products Cp. This enthalpy can be converted to 
useful purposes such as for sintering or the generation or control of 
porosity because of the temperature gain. The values of the maximum 
temperatures measured experimentally (Tmax), are usually lower than the 
theoretical values due to heat losses. The theoretical values provide an 
upper limit and are useful tier predicting the occurrence of phase 
transformation. The temperature may also be controlled by the addition of 
non-reacting constituents, by changing the initial temperature and, if 
used, the soaking temperature. 
Micropyretic kinetics can be homogeneous or heterogenous. In homogeneous 
combustion, the system is uniform throughout. Therefore, transport of any 
solute and mixing are not limiting steps. In case of heterogeneous 
combustion, the system is non-uniform when combustion is initiated or 
becomes non-uniform once combustion is initiated. Transport of solute in 
such cases becomes the rate limiting step. In the case of membrane 
formation, both such reactions are important for the control and 
distribution of porosity. The propagation of the wave could be under 
steady state conditions or it could deviate from the steady state 
conditions. Under steady state conditions the wave propagates with a 
constant velocity, and the temperature and the concentration across the 
wave remain constant. The propagation of the combustion wave can also take 
place in a pulsating manner, depending on the values of the thermophysical 
constants of the reactants and the products. This kind of behavior has 
been attributed to the excess enthalpy which may be a characteristic 
feature of condensed phase combustion or gasless combustion. Pulsating 
propagation consists of an alternate increase and decrease in temperature 
and velocity during wave propagation. Another kind of unsteady propagation 
is also possible where the reaction zone proceeds in the form of a spiral. 
Both steady and unsteady combustion may be used to synthesize membranes 
according to the present invention. 
The basic method for making or synthesizing a porous membrane according to 
the present invention includes preparing a slurry having at least one 
micropyretic substance and at least one liquid carrier for the 
micropyretic substance. The slurry is preferably applied to the surface of 
a substrate or article and allowed to dry on the surface into a green form 
of the membrane. The green form of the membrane is then fired or burned 
according to the present micropyretic principles in order to form a porous 
membrane. 
It is often desirable to modify the slurry with the addition of other 
substances, referred herein as diluents. The slurry could have a 
consistency ranging from very fluid to very powdery. Such slurries 
according to the present invention can include various combinations of the 
following constituents: 
a.) Micropyretic substances or agents. These agents are typically 
particles, fibers, or foils of materials such as Ni, Al, Ti, B, Si, Nb, C, 
Cr.sub.2 O.sub.3, Zr, Ta, Mg, Zn, MgO, ZnO.sub.2, ZrO.sub.2, TiO.sub.2, 
B.sub.2 O.sub.3, Fe or combinations thereof which may react to yield both 
heat as well as clean and nascent products from the combustion. Typical 
reactions could be for example Cr.sub.2 O.sub.3 +Al+B, Ni+Al or Ti+B or 
C+Al+SiO.sub.2, etc., which react spontaneously to give CrB.sub.2, 
Ni.sub.3 Al or TiB.sub.2 or SiC and Al.sub.2 O.sub.3, respectively, with a 
large release of heat. The adiabatic temperature of such a micropyretic 
reaction could be as high as 6,500.degree. K. Tables I, II, and III give a 
partial listing of micropyretic reactions (reactants and products) and the 
approximate amount of heat released in each reaction. .DELTA.H(KJ/mole) is 
the enthalpy release for the reaction and T.sub.ad K is the adiabatic 
temperature which is expected to be reached in such reactions. The 
enthalpy release and the adiabatic temperature are not precisely known for 
all the reactions in Tables I-III. However, all of the reactions listed 
are believed to be sufficiently exothermic. Table IV gives a list of some 
micropyretic reactions and stoicbiometrics. It is believed that mixtures 
of the constituents of Table I-IV are also possible along with the 
addition of diluents which could often be the product itself or other 
materials in powder, foil, fiber or other form of a predetermined size. It 
is also believed that each of the reactants and products of the reactions 
listed in Tables I-III could function as diluents. 
TABLE 1 
______________________________________ 
FORMATION OF REFRACTORY COMPOUNDS 
REACTION .DELTA.H(KJ/mole) 
T.sub.ad K 
______________________________________ 
Ti + 2B = TiB.sub.2 -293 3190 
Zr + 2B = ZrB.sub.2 -263.75 3310 
Nb + 2B = NbB.sub.2 -207.74 2400 
Ti + B = TiB -158.84 3350 
Hf + 2B = HfB.sub.2 -310.15 3520 
Ta + 2B = TaB.sub.2 -193.53 3370 
Ti + C = TiC -232 3210 
3B.sub.2 O.sub.3 + 10Al + 3TiO.sub.2 = 3TiB.sub.2 + 5Al.sub.2 O.sub.3 
4000 
B.sub.2 O.sub.3 + 5Mg + TiO.sub.2 = TiB.sub.2 + 5MgO 
B.sub.2 O.sub.3 + 5Zn + TiO.sub.2 = TiB.sub.2 + 5ZnO 
2B.sub.2 O.sub.3 + 5Zr + 2TiO.sub.2 = 2TiB.sub.2 + 5ZrO.sub.2 
Si + C = SiC -65.3 1800 
W + C = WC -40.6 1000 
V + C = VC -102 2400 
Nb + C = NbC -140 2800 
2Nb + C = Nb.sub.2 C -186 2600 
Zr + C = ZrC -202.9 3440 
Hf + C = HfC -218.6 3900 
Ta + C = TaC -142.9 2700 
2Ta + C = Ta.sub.2 C -202.7 2660 
4Al + 3C = Al.sub.4 C.sub.3 
-208.8 1670 
2Mo + C = Mo.sub.2 C -50 1000 
4B + C = B.sub.4 C -71 1000 
V + 2B = VB.sub.2 2670 
La + 6B = LaB.sub.6 2800 
W + B = WB 1700 
2W + B = W.sub.2 B -87 1400 
Cr + 2B = CrB.sub.2 -94.1 2470 
U + 4B = UB4 1770 
Mo + 2B = MoB.sub.2 1800 
Mo + B = MoB -112.4 1800 
Al + 12B = AlB.sub.12 -200.6 
Ti + 1/2N.sub.2 = TiN -336.6 4900 
3Ti + NaN.sub.3 = 3TiN + Na 
3Si + 2N.sub.2 = Si.sub.3 N.sub.4 
-738.1 4300 
3Si + 4NaN.sub.3 = Si.sub.3 N.sub.4 + 4Na 
Hf + 1/2N.sub.2 = HfN -368.7 5100 
B + 1/2N.sub.2 = BN -254.1 3700 
Zr + 1/2N.sub.2 = ZrN 4900 
Ta + 1/2N.sub.2 = TaN -252.1 3360 
2Ta + 1/2N.sub.2 = Ta.sub.2 N 
-272.5 3000 
V + 1/2N.sub.2 = VN -216.9 3500 
Al + 1/2N.sub.2 = AlN -302.5 2900 
La + 1/2N.sub.2 = LaN -299.4 2500 
3Be + N.sub.2 = Be.sub.3 N.sub.2 
-564.0 3200 
U + 1/2N.sub.2 = UN -286.8 3000 
3Mg + N.sub.2 = Mg.sub.3 N.sub.2 
-416.1 2900 
Nb + 1/2N.sub.2 = NbN -237.8 3500 
2Nb + 1/2N.sub.2 = Nb.sub.2 N 
-248.3 2670 
______________________________________ 
TABLE II 
______________________________________ 
FORMATION OF INTERMETALLICS 
REACTION .DELTA.H(KJ/mole) 
T.sub.ad K 
______________________________________ 
Ti + Ni = TiNi -278.2 1773 
Ti + Pd = TiPd -103.4 1873 
Ni + Al = NiAl -118.4 1911 
Ti + Al = TiAl -72.8 1654 
Ti + Fe = TiFe -40.6 1110 
5Ti + 3Si = Ti.sub.5 Si.sub.3 
-578.9 2500 
Ti + 2Si = TiSi.sub.2 -134.2 1800 
Ti + Si = TiSi -130 2000 
Mo + 2Si = MoSi.sub.2 -131.7 3190 
W + 2Si = WSi.sub.2 -92.9 1500 
5V + 3Si = V.sub.5 Si.sub.3 
-461.9 3190 
Ta + 2Si = TaSi.sub.2 -119.1 1800 
Zr + Si = ZrSi -155 2100 
Zr + 2Si = ZrSi.sub.2 -153.8 2100 
5Zr + 3Si = Zr.sub.5 Si.sub.3 
-147.6 2800 
Nb + 2Si = NbSi.sub.2 -137.9 1900 
2Ni + SiC = Ni.sub.2 Si + C 
-76 
3Ni + 2SiC = Ni.sub.3 Si.sub.2 + 2C 
-98 
Cd + S = CdS -149.2 2000 
Mn + S = MnS -213.2 3000 
Mo + 2S = MoS.sub.2 -275 2300 
Ni + 2S = NiS.sub.2 
Ni + P = NiP 
Nb + P = NbP 
3Ni + Al = Ni.sub.3 Al -153.2 
3Ni.sub.3 Al.sub.2 + 9Ni = 6Ni.sub.3 Al 
Ni + 3Al = NiAl.sub.3 -162 
3Ni + 2Al = Ni.sub.3 Al.sub.2 
-282.6 
Ti + 3Al = TiAl.sub.3 -142.1 1517 
Cu + Al = CuAl 899 
Cu + 2Al = CuAl.sub.2 
4Cu + 3Al = Cu.sub.4 Al.sub.3 
3Cu + 2Al = Cu.sub.3 Al.sub.2 
9Cu + 4Al = Cu.sub.9 Al.sub.3 
Fe + Al = FeAl -18 
3Fe + Al = Fe.sub.3 Al -31.8 
Zr + Al = ZrAl.sub.2 1918 
Pd + Al = PdAl 2579 
Ti + Ni = TiNi -66.5 1552 
Ti + Pt = TiPt -159.5 
Ti + Co = TiCo -47.7 1723 
Co + Al = CoAl -110.4 1901 
50Ti + (50-x)Ni + xPd = Ti.sub.50 Ni.sub.50-x Pd 
50Ti + (50-x)Ni + xFe = Ti.sub.50 Ni.sub.50-x Fe 
50Cu + (50-x)Al + xNi = Cu.sub.50 Al.sub.50-x Ni 
50Cu + (50-x)Al + xMn = Cu.sub.50 Al.sub.50-x Mn 
______________________________________ 
TABLE III 
______________________________________ 
FORMATION OF COMPOSITES 
REACTION .DELTA.H(KJ/mole) 
T.sub.ad K 
______________________________________ 
Fe.sub.2 O.sub.2 + Al = Al.sub.2 O.sub.3 + 2Fe 
-836 3753 
Cr.sub.2 O.sub.3 + Al = Al.sub.2 O.sub.3 + 2Cr 
-530 2460 
3Cr.sub.2 O.sub.3 + 6Al + 4C = 2Cr.sub.3 C.sub.2 + 3Al.sub.2 O.sub.3 
6500 
0.86Ti + 1.72B + 1.48Al = 0.86TiB.sub.2 + 
-293 1450 
1.48Al 
Ti + C + 0.68Ni = TiC + 0.68Ni 
-232 1370 
Zr + 2B + Cu = ZrB.sub.2 + Cu 
-263.75 1100 
4Al + 3SiO.sub.2 + 3C = 2Al.sub.2 O.sub.3 + 3SiC 
3Fe.sub.3 O.sub.4 + 8Al = 4Al.sub.2 O.sub.3 + 9Fe 
-816 3509 
3NiO + 2Al = 2Al.sub.2 O.sub.3 + 3Ni 
-928 3546 
3MnO.sub.2 + 4Al = 2Al.sub.2 O.sub.3 + 3Mn 
-878 4123 
3SiO.sub.2 + 4Al = 2Al.sub.2 O.sub.3 + 3Si 
3TiO.sub.2 + 4Al = 2Al.sub.2 O.sub.3 + 3Ti 
Fe.sub.2 O.sub.3 + 3Mg = 3MgO + 2Fe 
-323 3076 
Fe.sub.3 O.sub.4 + 3Mg = 4MgO + 3Fe 
-316 3184 
Cr.sub.2 O.sub.3 + 3Mg = 3MgO + 2Cr 
-221 2181 
NiO + Mg = MgO + Ni -353 2579 
3MnO.sub.2 + 2Mg = 2MgO + Mn 
-337 3665 
2Fe.sub.2 O.sub.3 + 3Si = 3SiO.sub.2 + 4Fe 
-311 2626 
Fe.sub.3 O.sub.4 + 2Si = 2SiO.sub.2 + 3Fe 
-298 1808 
2NiO + Si = SiO.sub.2 + 2Ni 
-373 2602 
2MnO.sub.2 + Si = 2SiO.sub.2 + Mn 
-339 3024 
2Fe.sub.2 O.sub.3 + 3Ti = 3TiO.sub.2 + 4Fe 
2Fe.sub.2 O.sub.3 + 3Zr = 3ZrO.sub.2 + 4Fe 
2Cr.sub.2 O.sub.3 + 3Zr = 3ZrO.sub.2 + 4Cr 
Ti + 2B + aTiB2 + bCu = (a+1)TiB.sub.2 + 
bCu 
CrO.sub.3 + Cr.sub.2 O.sub.3 + 6Al + 2C + 3NiO = 
Cr.sub.3 C.sub.2 + 3Al.sub.2 O.sub.3 + 3Ni 
Nb.sub.2 O.sub.5 + Al.sub.2 Zr + vAl.sub.2 O.sub.3 = 
2Nb + ZrO.sub.2 + Al.sub.2 O.sub.3 + vAl.sub.2 O.sub.3 
Nb.sub.2 O.sub.5 + 2Al + Zr + vAl.sub.2 O.sub.3 = 
2Nb + ZrO.sub.2 + Al.sub.2 O.sub.3 + vAl.sub.2 O.sub.3 
Nb.sub.2 O.sub.5 + 10/3Al + .phi.ZrO.sub.2 + vAl.sub.2 O.sub.3 = 
2Nb + .phi.ZrO.sub.2 + 5/3Al.sub.2 O.sub.3 + vAl.sub.2 O.sub.3 
B.sub.4 C + (x+5)Ti = xTiB + 4TiB + TiC 
2Ti + C + 2B = TiB.sub.2 + TiC 
38B + TiAl.sub.3 = TiB.sub.2 + 3AlB.sub.12 
3TiO.sub.2 + 4Al + 3C = 3TiC + 2Al.sub.2 O.sub.3 
2300 
9TiO + 11C + 2TiAl.sub.3 = 11TiC + 3Al.sub.2 O.sub.3 
3SiO.sub.2 + 4Al + 3C = 3SiC + 2Al.sub.2 O.sub.3 
3ZrSiO.sub.4 + 4Al + 3C = 3ZrO.sub.2 + 3SiC + 
2Al.sub.2 O.sub.3 
WO.sub.3 + 2Al + C = WC + Al.sub.2 O.sub.3 
2B.sub.2 O.sub.3 + 4Al + C = B.sub.4 C + 2Al.sub.2 O.sub.3 
2ZrO.sub.2 + 4Al + C = ZrC + 2Al.sub.2 O.sub.3 
2MoO.sub.3 + Al + 3C = 3Mo.sub.2 C + 2Al.sub.2 O.sub.3 
3B.sub.2 O.sub.3 + 10Al + 3TiO.sub.2 = 3TiB.sub.2 + 5Al.sub.2 O.sub.3 
4000 
6B + 4Al + 3TiO.sub.2 = 3TiB.sub.2 + 2Al.sub.2 O.sub.3 
4000 
10B.sub.2 O.sub.3 + 3TiO.sub.2 + 2B + 8TiAl.sub.3 = 
11TiB.sub.2 + 12Al.sub.2 O.sub.3 
9TiO.sub.2 + 26B + 4TiAl.sub.3 = 13TiB.sub.2 + 6Al.sub.2 O.sub.3 
3Ti + 3B.sub.2 O.sub.3 + 2TiAl.sub.3 = 3TiB.sub.2 + 3Al.sub.2 O.sub.3 
B.sub.2 O.sub.3 + ZrO.sub.2 + 10/3Al = ZrB.sub.2 + 
2400 
5/3Al.sub.2 O.sub.3 
MoO.sub.3 + 2Al + B = MoB + Al.sub.2 O.sub.3 
-1117.3 4000 
3HfO.sub.2 + 4Al + 6B = 3HfB + 2Al.sub.2 O.sub.3 
3V.sub.2 O.sub.5 + 10Al + 3N.sub.2 = 6VN + 5Al.sub.2 O.sub.3 
4800 
3TiO.sub.2 + 2Al + NaN.sub.3 = 3TiN + Al.sub.2 O.sub.3 + 
Na 
3TiO.sub.2 + 4Al + 1.5NaCN = 
3TiC.sub.0.5 N.sub.0.5 + 2Al.sub.2 O.sub.3 + 1.5Na 
Ti + 0.5C + 0.167NaN.sub.3 = TiC.sub.0.5 N.sub.0.5 + 
0.167Na 
MoO.sub.3 + 2Al + 2Si = MoSi.sub.2 + Al.sub.2 O.sub.3 
3300 
2Si.sub.3 N.sub.4 + 4B.sub.2 O.sub.3 + 9Al.sub.2 O.sub.3 
4800 
8BN + 3(3Al.sub.2 O.sub.3 +2SiO.sub.2) 
TiO.sub.2 + 2Mg + C = TiC + 2MgO 
SiO.sub.2 + 2Mg + C = SiC + 2MgO 
2300 
2B.sub.2 O.sub.3 + 6Mg + C = B.sub.4 C + 6MgO 
B.sub.2 O.sub.3 + 5Mg + TiO.sub.2 = TiB.sub.2 + 5MgO 
MoO.sub.3 + 3Mg + B = MoB + 3MgO 
MoO.sub.3 + Mg + 2Si = MoSi.sub.2 + 3MgO 
TiO.sub.2 + Zr + C = TiC + 2ZrO.sub.2 
SiO.sub.2 + Zr + C = SiC + ZrO.sub.2 
2B.sub.2 O.sub.3 + 5Zr + 2TiO.sub.2 = 2TiB.sub.2 + 5ZrO.sub.2 
2MoO.sub.3 + 3Zr + 2B = 2MoB + 3ZrO.sub.2 
2MoO.sub.3 + 3Zr + 4Si = 2MoSi.sub.2 + 3ZrO.sub.2 
1/2V.sub.2 O.sub.5 + 11/3B = VB.sub.2 + 5/6B.sub.2 O.sub.3 
2700 
1/2Cr.sub.2 O.sub.3 + 3B = CrB.sub.2 + 1/2B.sub.2 O.sub.3 
1900 
2MoO.sub.3 + 5B = Mo.sub.2 B + 2B.sub.2 O.sub.3 
3000 
1/2Fe.sub.2 O.sub.3 + 2B = FeB + 1/2B.sub.2 O.sub.3 
2400 
1/2Fe.sub.2 O.sub.3 + 4B + 2Fe = 3FeB + 1/2B.sub.2 O.sub.3 
1800 
2MoO.sub.3 + 10Mo + 24B = 11MoB.sub.2 + B.sub.2 O.sub.3 
2200 
PbO + MoO.sub.2 = PbMoO.sub.3 1340 
PbO.sub.2 + WO.sub.2 = PbWO.sub.4 2000 
BaO.sub.2 + SiO = BaSiO.sub.3 1880 
BaO.sub.2 + TiO = BaTiO.sub.3 1980 
PbO.sub.2 + TiO = PbTiO.sub.3 1440 
MnO.sub.2 + TiO = MnTiO.sub.3 1630 
MnO.sub.2 + TiO = MnSiO.sub.3 1540 
Si + N.sub.2 + Si.sub.3 N.sub.4 + (SiO.sub.2).sub.z + AlN 
2673 
Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z 
______________________________________ 
TABLE IV 
______________________________________ 
SAMPLE MICROPYRETIC REACTIONS 
AND STOICHIOMETRIC WEIGHTS 
REACTION WEIGHT % 
______________________________________ 
Ni + Al = NiAl Ni: 68.5, Al: 31.5 
3Ni + Al = Ni.sub.3 Al 
Ni: 86.7, Al: 13.3 
3Cr.sub.2 O.sub.3 + 6Al + 4C = 2Cr.sub.3 C.sub.2 + 3Al.sub.2 O.sub.3 
Cr.sub.2 O.sub.3 : 69, Al: 24, C: 7 
MoO.sub.3 + 2Al + B = MoB + Al.sub.2 O.sub.3 
MoO.sub.3 : 69, Al: 25.9, 
B: 5.1 
MoO.sub.3 + 2Al + 2Si = MoSi.sub.2 + Al.sub.2 O.sub.3 
MoO.sub.3 : 57, Al: 21, 
Si: 22 
Ti + 2B = TiB.sub.2 Ti: 68.9, B: 31.1 
5Ti + 3Si + Ti.sub.5 Si.sub.3 
Ti: 74, Si: 26 
Nb + 2Al = NbAl.sub.2 Nb: 63.3, Al: 36.7 
Zr + 2B = ZrB.sub.2 Zr: 80.8, B: 19.2 
Nb + 2B = NbB.sub.2 Nb: 81.1, B: 18.9 
Fe.sub.2 O.sub.3 + 2Al = Al.sub.2 O.sub.3 + 2Fe 
Fe.sub.2 O.sub.3 N: 74.7, Al: 25.3 
Cr.sub.2 O.sub.3 + 2Al = Al.sub.2 O.sub.3 + 2Cr 
Cr.sub.2 O.sub.3 : 73.8, Al: 26.2 
0.86Ti + 1.72B + 1.48Al = 0.86TiB.sub.2 + 
Ti: 41.3, B: 18.7, 
1.48 Al Al: 40 
Ti + B = TiB Ti: 81.6, B: 18.4 
Hf + 2B = HfB.sub.2 Hf: 89.2, B: 10.8 
Ta + 2B = TaB.sub.2 Ta: 89.3, B: 10.7 
Ti + C = TiC Ti: 80, C: 20 
Ti + Ni = TiNi Ti: 44.9, Ni: 55.1 
Ti + Pd + TiPd Ti: 31.0, Pd: 69.0 
Ti + Al = TiAl Ti: 64, Al: 36 
Ti + Fe = TiFe Ti: 46.2, Fe: 53.8 
Ti + C + 0.68Ni = TiC + 0.68Ni 
Ti: 48, C: 12, Ni: 40 
Ni + 3Al = NiAl.sub.3 Ni: 42, Al: 58 
4Al + 3SiO.sub.2 + 3C = 2Al.sub.2 O.sub.3 + 3SiC 
Al: 33.29, SiO.sub.2 : 55.59, 
C: 11.2 
______________________________________ 
b) A liquid carrier (i.e. liquid suspending medium) which could be aqueous 
or non-aqueous and have either a low or high viscosity. The carrier is 
most often chosen from a group of plasticizers (i.e., binders in 
suspension) which may include clays of various sorts such as bentonite, 
fused silica, kaolinite and related compounds; silicates; borates; 
stearates and other lubricants including MoS.sub.2 and PbS; methyl 
cellulose and related compounds; organic liquids such as acetone. 
polyvinyl butyryl, polyvinyl alcohol, polyethylene glycol, oils of various 
kinds. tetraisoamyloxide, and water. The plasticizer may also be a 
colloidal liquid such as colloidal alumina, colloidal ceria. colloidal 
yttria, colloidal silica, colloidal zirconia, mono-aluminum phosphate, 
colloidal cerium acetate or mixtures thereof. Colloidal binders can also 
be derived from a suspension containing colloid precursors and reagents 
which are solutions of at least one salt such as chlorides, sulfates, 
nitrates, chlorates, perchlorates or metal organic compounds. Colloidal 
binders will usually be relatively dilute aqueous or non-aqueous 
suspensions, but the use of concentrated colloids or partly or fully 
precipitated colloids is also possible. Alternatively, the colloidal 
binder can be derived from a suspension containing chelating agents such 
as acetyl acetone and ethylacetoacetate. Various mixtures of different 
carriers are possible. 
When using colloids, three types of colloidal processing are possible. The 
first involves the gelation of certain polysaccharide solutions. The other 
two involve colloids and metal organic compounds. These last two involve 
the mixing of materials in a very fine scale. Colloids may be defined as 
comprising a dispersed phase with at least one dimension between 0.5 nm 
(nanometer) and about 10 microns (micrometers) in a dispersion medium 
which in the present case is a liquid. The magnitude of this dimension 
distinguishes colloids from bulk systems in the following way: (a) an 
extremely large surface area and (b) a significant percentage of molecules 
reside in the surface of colloidal systems. Up to 40% of molecules may 
reside on the surface. The colloidal systems which are important to this 
invention are both the thermodynamically stable lyophilic type (which 
include macro molecular systems such as polymers) and the kinetically 
stable lyophobic type (those that contain particles). In the formation of 
the slurry, new materials and other agents or diluents may be mixed in 
with the plasticizers. 
c.) One diluent may be a powder additive containing carbides, silicides, 
borides, nitrides, oxides, carbonitrides, oxynitrides and combinations 
thereof. When choosing combinations of powder additives, the particle size 
selection is important. It is preferable to choose particle sizes below 
100 microns and when employing combinations of powder additives, to choose 
particle sizes which are varied such that the packing of particles is 
optimized. Generally, the ratio of the particle sizes will be in the range 
from about 2:1 to about 5:1. Sometimes packing is optimized by choosing 
one constituent size three times smaller than the other constituent. i.e. 
having a particle ratio size of about 3:1. 
d. ) Metallic particles, intermetallic particles or a combination thereof, 
for example Ni, Pt, Al, CrZr, Zn, Mg, NiAl, NiAl.sub.3, CrSi, CrB, etc. 
The sizes of these particles are also preferably varied to achieve optimum 
packing, like with the above powder additives. 
e.) Metal organic compounds principally metal alkoxides of the general 
formula M(OR).sub.z, where M is a metal or a complex cation made up of two 
or more elements, R is an alkyl chain and z is a number usually in the 
range from 1 to 12. Alternatively, these metal alkoxides can be described 
as solutions in which molecules of organic groups are bound to a metal 
atom through oxygen. Examples of metal alkoxides are silicon 
tetraisoamyloxide, aluminum butoxide, aluminum isopropoxide, tetraethyl 
orthosilicates, etc. The organic portions of other metal organic compounds 
may include formates, acetates and acetylacetonates. 
f.) Pyrolizable chlorosilanes, polycarbosilanes, polysilazanes and other 
organosilicon polymers may be used as binders which pyrolize to useful 
products for oxidation prevention. Such compounds are expected to 
participate in the micropyretic reaction in a beneficial but complex 
manner to increase the yield of useful products with a morphology and size 
useful for the membrane. Organosilicon polymers typically dissolve in 
water and therefore should be avoided when producing membranes for 
filtering aqueous solutions. 
g.) Alkaline or acidic solutions may be needed to modify the pH of the 
slurry. Standard laboratory grade alkalines and acids are used. 
h.) Burnable and/or oxidizable liquid or solid constituents such as 
polymers (e.g., polyurethane, polyester) or carbonaceous materials may be 
added to the slurry to be eventually burned off leaving behind a 
predetermined pore size and pore volume (density) in the membrane. 
Considering the above defined constituent groups (a) to (h), the slurries 
used in the invention are made up of at least one of the constituents from 
groups (a) and (b), optionally together with one or more constituents from 
groups (c) to (h). The constituents from groups (c) to (h) are generally 
considered diluents. Constituents From groups (c) to (h) are added to the 
basic slurry of constituents (a) and (b) (i.e., the micropyretic 
substance(s) and the liquid carrier) for a number of reasons, including: 
(1) to control the final composition of the membrane, (2) to control the 
rate of the micropyretic reaction (i.e., to control the velocity of wave 
propagation u and the combustion temperature Tc), (3) to add additional 
phases to the membrane structure, and/or (4) to control the pore size and 
pore density in the membrane. The carriers of group (b) may contain 
components which act as diluents. In addition, some materials may be 
present under more than one heading. For instance, silica or aluminum in 
colloidal form can be included in the carrier, and in powder form as an 
additive. Also, particulate nickel and aluminum can be present as a 
micropyretic reactant, but in excess of the stoichiometric amount, whereby 
the excess forms a particulate additive. It is also possible for the 
powder additive to be the same as the reaction product of the micropyretic 
reaction. 
Tables V and VI give examples of typical micropyretic slurry compositions 
and non-micropyretic slurry compositions, respectively. 
TABLE V 
__________________________________________________________________________ 
EXAMPLES OF MICROPYRETIC SLURRY COMPOSITIONS 
POWDER/ 
SOLID POWDER COMPOSITION LIQUID CARRIER 
CARRIER 
SAMPLE 
(Wt %/Particle Size) (Vol. %) (g/ml) 
__________________________________________________________________________ 
1 SiC (60%/3 Microns), Si.sub.3 N.sub.4 (10%/1 Micron), 
Colloidal - Silica 50% 
10/6 
Ti (17%/-325 Mesh)*, and TiB.sub.2 (5%/-325 Mesh)* 
Colloidal - Alumina 50% 
2 SiC (72.5%/1-3 Microns), Si.sub.3 N.sub.4 (2.5%/0.1-1 
Colloidal - Silica 50% 
10/5 
Y.sub.2 O.sub.3 (5%/0.1-1 Micron), Ti (15%/-325 Mesh)*, 
Colloidal - Alumina 50% 
Si (5%/-325 Mesh)* 
3 SiC (50%/1 Micron), Zr (4%/1 Micron), B (5%/ 
**Colloidal Yttria, 
10/6 
1 Micron), C (7%/1 Micron), Al (3%/-325 Mesh)*, Ti 
Polycarbosilane, Mono- 
(27%/-325 Mesh)*, Al.sub.2 O.sub.3 (2%/0.3 Microns), 
aluminum phosphate, 
MoSi.sub.2 (0.5%/0.5 Microns), Cr.sub.2 O.sub.3 (0.5%/0.5 
Methyl Cellulose, 
and TiB.sub.2 (1%/1 Micron) 
Polyvinyl Alcohol, 
Colloidal Ceria, 
Colloidal Zirconia 
__________________________________________________________________________ 
*-325 Mesh .congruent. 44 microns 
**any of these liquid carriers may be used alone or in combination. 
TABLE VI 
__________________________________________________________________________ 
EXAMPLES OF NON-MICROPYRETIC SLURRY COMPOSITIONS 
SOLID POWDER COMPOSITION 
LIQUID CARRIER 
SAMPLE 
(Wt %/Particle Size) 
(Vol. %) 
__________________________________________________________________________ 
1 TiB.sub.2 (25 gms/1 Micron) 
Colloidal Alumina (10 ml) 
2 CrB.sub.2 (25 gms/10 Microns) 
Colloidal Alumina (10 ml) 
__________________________________________________________________________ 
Once the desired slurry mixture is prepared, the slurry is then dried into 
a green form having a desired geometric configuration. Preferably, the 
slurry, is applied to the surface of a substrate or article. The applied 
slurry is then dried, such as by air drying or being baked at relatively 
low temperatures, for example, in an oven, usually so as not to start the 
micropyretic reaction. There are various methods of applying the slurry 
including painting (by brush or roller), dipping, spraying, or pouring the 
liquid onto the surface. Typically, each coating of the slurry is allowed 
to dry before another coating is added. However, the underlying coating 
does not necessarily need to be entirely dry before the next coating is 
applied. If one or more coatings with micropyretic constituents are 
present, then it is preferable to dry these coatings completely prior to 
firing (i.e., the combustion step). Multiple coatings may be necessary in 
order to obtain the desired layer thickness. Depending upon the slurry 
composition, additional coatings may be added to already fired layers 
either for repair or for additional build up. Even when micropyretic 
constituents are absent, it is preferred to heat the green membrane with a 
suitable heat source, such as a torch (e.g., butane or oxyacetylene), a 
laser, a furnace, etc., if improvement in the densification of the 
membrane is required. Such heating takes place preferably in air but could 
be in other oxidizing atmospheres or in inert or reducing atmospheres. 
In general, the micropyretic layers provide heat for the bonding of several 
layers as well as bonding to the substrate or article. While membranes 
with multiple micropyretic layers can be produced according to the 
invention, multilayer membranes with one or more non-micropyretic layers 
can also be produced, if desired. These non-micropyretic layers could for 
example be made of polymers. 
If desired, bonding of the coatings to the surface of the substrate or 
article can be enhanced by treating the surface. The surface may be 
treated by sandblasting or pickling with acids or fluxes such as cryolite 
or other combinations of fluorides and chlorides prior to the application 
of the coating. Similarly, the substrate may be cleaned with an organic 
solvent such as acetone to remove oily products or other debris prior to 
the application of the coating. 
After the substrate or article surface is coated and the coating dried, a 
final coat of a liquid containing one or more of the constituents in 
paragraphs (a)-(e) described above may be applied lightly prior to use. 
Such light coatings may be applied by clipping or painting. As an 
additional step, chemically or surface active compounds may be added to 
the membrane layers or may be impregnated into the membrane after 
combustion. 
In the case of micropyretic coatings, an additional step alter the drying 
of the slurry coating(s) will be the firing or combustion of the slurry 
constituents (i.e., the membrane in its green state). Combustion of the 
green membrane can be performed by direct flame, concentrated sunlight, a 
plasma, a laser or an electron beam. In addition, if the substrate or 
article is conductive, the green form can be combusted by passing a 
current through the substrate or article. The coated substrate or article 
could also be placed inside a furnace at a predetermined temperature and 
time or heated by an induction method or by radiant heating. The applied 
slurry contains particulate substances which sinter above a given 
temperature, in particular reactant and/or non-reactant substances that 
reaction sinter at a temperature greater than about 0.5 Tm, where Tm is 
the melting point of the lowest melting reaction product. 
A porous membrane produced according to the invention will have the 
additional property of being one-time switchable as excess enthalpy from 
the heat source can be used to change the nature and content of the 
membrane porosity. That is, additional heat can be applied to the membrane 
in order to reduce the pore size and pore density in the membrane. The 
present method is good for obtaining membranes with pore sizes ranging 
from 10 nanometers to 100 microns. In-situ repair, rather than replacement 
of membranes by using the principles of the present invention is also 
contemplated. 
The present invention is particularly applicable to producing porous 
membranes which will encounter high temperatures and corrosive 
environments. In addition, while the present method is used mainly to 
synthesize inorganic monolithic and composite single layer and multilayer 
membranes, multilayer membranes with inorganic interlayers are also 
possible. In general, the inorganic membranes are superior to organic ones 
because of their low reactivity to organic and corrosive liquids and to 
high temperature liquids and gases. 
After it has been produced, the porous membrane may be further impregnated 
with organic materials or other slurries, colloids, etc., including 
micropyretic slurries in order to alter the properties of the initially 
produced membrane. 
The present invention will be further described with reference to the 
following examples: 
EXAMPLE 1 
A micropyretic slurry was coated on high density graphite blocks and copper 
blocks such that a non-adherent membrane could be synthesized thereon. 
Reactant powders of elemental titanium (99.5% pure) and boron (92% pure), 
both -325 mesh in size (less than 44 microns), in equal molar proportions 
were mechanically blended for about 15 minutes and, by adding a carrier of 
30% by volume colloidal silica and 70% by volume mono-aluminum phosphate, 
were formed into a slurry. The powder/carrier ratio was 10 gms./1 ml. The 
carrier could be 0-50% by volume of colloidal Silica and 100-50% by volume 
of mono-aluminum phosphate. The powder/carrier ratio could be 10 g/10 ml 
or 20 g/10 ml. The slurry was applied by brushing, but rolling, spraying, 
dipping or pouring could be acceptable. The layer of applied slurry was 
allowed to air dry in an atmosphere of 50% relative humidity for about 
15-30 minutes, forming a layer having a thickness of about 200 
micrometers. Additional layers could be applied after each previous layer 
had dried. The rate of drying was controlled such that no cracks appeared. 
After this drying step, the green membrane was ignited with an 
oxy-acetylene torch. Ignition could be with a laser, an electron beam, a 
propane torch, an oxy-acetylene torch or even concentrated sunlight. The 
micropyretic front propagated through the sample which popped up after 
completion of the synthesis. The synthesized material was TiB/TiB.sub.2 
which is known to be highly refractory. The membrane may be cooled with 
cold air jets after the passage of the micropyretic reaction front or wave 
in order to maintain pore size. 
EXAMPLE 2 
Several membranes were prepared by mixing powders of Ni and Al, in 
compositions which yielded Ni.sub.3 Al, NiAl and/or NiAl.sub.3. The Ni 
particles were about 3 microns and the Al particles were about 1 micron in 
size. Three slurries were made by mixing the Ni and Al particles in 
colloidal alumina, ceria and zirconia (10 g of powder/3 ml of carrier). A 
layer of each of the slurries was deposited on a metal substrate and a 
very porous alumina substrate. After ignition, each membrane popped off 
the metal substrate but remained adherent to the ceramic substrate. Each 
of the membranes had 40% porosity, an average pore size of about 4 
microns, and were about 200-300 microns thick. 
EXAMPLE 3 
B.sub.2 O.sub.3, TiO.sub.2, and Al powders, each having particle sizes less 
than 40 microns, were combined in a molar ratio of (3B.sub.2 
O.sub.3)+(10Al)+(3TiO.sub.2). These powders were mixed with polyurethane 
paint thinner and applied to a tantalum substrate by painting. Care was 
taken in each application of the slurry not to remove previously applied 
coatings. The layer was built up to a thickness of 550 microns. The layer 
was then ignited to obtain a porous membrane which could be removed from 
the tantalum substrate. The final composition of the membrane was 
TiB.sub.2 /Al.sub.2 O.sub.3. 
EXAMPLE 4 
A slurry was prepared containing 17.3 weight percent aluminum powder (1-44 
microns), 78.9 weight percent carbon powder (amorphous or crystalline), 
and 3.8 weight percent colloidal alumina (optional). The slurry was 
brushed onto a number of porous silica substrates to a thickness of about 
500 microns. The coated substrates were either heated in a furnace to 
1000.degree. C. for about 5 minutes or heated with a torch flame. The 
coated substrates were then cooled in a reducing atmosphere of dry ice 
(CO.sub.2). 
EXAMPLE 5 
A micropyretic powder mixture was prepared containing 26.29 wt. % Al. 20.36 
wt % B.sub.2 O.sub.3, 23.35 wt. % TiB.sub.2, and 30 wt. % carbon powder. 
This micropyretic powder was mixed with an amount of colloidal alumina and 
more carbon powder as follows: 38.5 wt. % of the micropyretic powder plus 
38.5 wt. % carbon powder pills 23 wt. % colloidal alumina (or 2 wt. % 
methyl cellulose aqueous solution). The above slurry was brushed onto the 
surface of a number of porous alumina substrates. The coated substrates 
were either heated in a furnace to 1100.degree. C. or with a torch flame 
until combustion was complete. The coated substrates were then cooled in a 
reducing atmosphere of dry CO.sub.2. A gas of Ar, He, N, etc. could also 
be used. 
In Examples 4 and 5 above, predominantly carbonaceous materials are used to 
make the porous membrane. 
The following processing approach has been found to be particularly useful 
in collecting the porous membranes after the combustion or micropyretic 
step. In the green state, these membranes may tend to stick to the 
substrate on which they are initially laid, whether in the monolayer state 
or multilayer state. If the substrate is non-porous metal, graphite or 
even ceramic, and with a mismatch between the coefficients of thermal 
expansion of the membrane material (typically at least about 10.sup.-6 
/.degree.K.) and the substrate, the thermal shock of the combustion front 
and corresponding expansion or contraction of the membrane is enough to 
neatly peel the membrane from the substrate. In an example of a 200 micron 
thick layer containing 38 grams of boron, 162 grams of titanium and 10 
ml/gm of polyurethane applied to a graphite substrate and combusted with a 
propane torch, the resulting membrane self detached after the firing 
process and was fully recovered without any damage. The membrane pore size 
ranged from about 0.2 micron to about 500 microns and pore density was 
about 60%. The membrane was made of TiB.sub.2 formed from the micropyretic 
reaction between Ti and B. In addition, the detachment process can be 
enhanced by waxing the substrate surface prior to application of the 
membrane. 
In another embodiment of the present process, the membrane slurry is 
applied between two bulk porous materials of large pore size prior to 
combustion in order to obtain a robust (i.e., thicker and stronger) body 
containing the membrane. Similarly, the slurry may be applied to one/race 
of a porous body prior to combustion to allow robustness. The porous 
bodies are chosen in either case so that the membrane remains adherent to 
the porous body during and after combustion. This embodiment is further 
described with reference to Examples 6-9 and FIGS. 1-3. 
EXAMPLE 6 
An integrated membrane 10, as shown in FIG. 1, with a composite membrane 
layer 11 having an alumina (Al.sub.2 O.sub.3) phase and silicon carbide 
(SiC) phase on a porous support layer (i.e., substrate 12), was prepared 
by the micropyretic method of the present invention. Both a support layer 
12 of porous alumina and of silica (SiO.sub.2) were used. Two micropyretic 
slurry samples were produced. The composition of each were as follows: 
______________________________________ 
SLURRY COMPOSITION (Wt. %) 
CONSTITUENTS SAMPLE A SAMPLE B 
______________________________________ 
Micropyretic Source 
43% 35% 
33.29 wt. % Al, 
55.59 wt. % SiO.sub.2, 
and 11.2 wt. % Graphite 
Polyvinyl Alcohol 
2% -- 
Colloidal Silica -- 10% 
Solvent 55% 55% 
Distilled Water 
______________________________________ 
A brush was used to coat each slurry sample onto each type of support layer 
12 to form specimens of integrated membranes 10 with green composite 
membrane layers 11. These specimens were dried in an oven at 50.degree. C. 
for about 12 hours and then at 100.degree. C. for about 3 hours. The dried 
specimens were next ignited in a furnace at 1000.degree. C. to obtain 
integrated membranes 10 with the micropyretic composite membrane layer 11 
being adherent to the porous support layer 12. The membrane layer 11 of 
each specimen had a porosity of about 55%, pore sizes ranging from about 
0.5 microns to about 2 microns, and grain sizes ranging from about 1 
micron to about 4 microns. 
The pore size distribution of micropyretic membranes have been controlled 
by adding various diluents. These additives can be large particles or 
small particles. Nanometer size particles 20 of silica are obtained from 
the colloid after the drying step. These silica particles 20 act as 
diluents in filling up large size pores 22 formed between the micropyretic 
reaction products, silicon carbide particles 24 and alumina particles 25, 
in order to produce smaller size pores 28. 
EXAMPLE 7 
Integrated membranes with a composite membrane layer of Cr.sub.3 C.sub.2 
and Al.sub.2 O.sub.3 was produced from two slurries, as in Example 6, 
except the slurry compositions were as follows: 
______________________________________ 
SLURRY COMPOSITION (Wt. %) 
CONSTITUENTS SAMPLE A SAMPLE B 
______________________________________ 
Micropyretic Source 
43% 43% 
24.31 wt. % Al, 
68.48 wt. % Cr.sub.2 O.sub.3, 
and 7.21 wt. % C 
Polyvinyl Alcohol 
2% 2% 
Colloidal Silica -- 10% 
Solvent 55% 45% 
Distilled Water 
______________________________________ 
EXAMPLE 8 
Another integrated membrane 30, as shown in FIG. 2, has a micropyretic 
monolithic membrane layer 31 on a porous alumna support layer or substrate 
32. Two integrated membranes 30 were prepared by the micropyretic method 
of the present invention. Each integrated membrane 30 had a monolithic 
membrane layer 31 produced from a different micropyretic slurry 
composition. One slurry (Sample A) produced a membrane layer 31 of 
titanium diboride (TiB.sub.2), and the other slurry (Sample B) produced a 
membrane layer 31 of titanium carbide (TiC). The compositions of the two 
slurry samples are as follows: 
______________________________________ 
SLURRY COMPOSITION (Wt. %) 
CONSTITUENTS 
SAMPLE A SAMPLE B 
______________________________________ 
Micropyretic 
40% 40% 
Source (68.90 wt % Ti and 
(79.95 wt. % Ti and 
31.10 wt % B) 20.05 wt. % Graphite) 
Polyvinyl Alcohol 
2% 2% 
(PVA) 
Polethylene Glycol 
3% 3% 
Solvent 55% 55% 
Distilled Water 
______________________________________ 
For each slurry sample, a brush was used to coat the slurries onto the 
surface of each substrate 32 to form about a 300 micron thick layer. The 
slurry layer on each substrate 32 was dried in an oven at a controlled 
temperature of 50.degree. C. for 12 hours. Once dried, the green membrane 
layer of each specimen was ignited to react micropyretically in a 
controlled furnace at 800.degree. C. to obtain two integrated membranes 30 
with the micropyretic monolithic membrane layer 31 described above being 
adherent to the porous substrate 32. 
EXAMPLE 9 
A different kind of integrated membrane 40, as shown in FIG. 3, can be 
produced having a micropyretic monolithic or composite membrane layer 41 
on a filter 42. Integrated membranes 40 having monolithic membrane layers 
41 were produced by coating the surface of conventional molten metal type 
filters 42 made of alumina, alumina-silicate, zirconia, and silicon 
carbide with either of the slurry compositions mentioned in Example 8 
above. These molten metal type filters 42 can be made by conventional 
means or according to the inventive micropyretic principles of U.S. Pat. 
No. 5,279,737 which is incorporated in its entirety, herein by reference. 
Either slurry composition can be coated onto the surface of the filter 42 
to a desired layer thickness with a brush. The green membrane layer is 
dried and then ignited to produce the integrated membrane 40. 
It is believed that the formation of pressure assisted membranes is also 
possible. The pressure may be gas pressure of approximately two 
atmospheres during micropyretic synthesis involving a gas. Additionally, 
in solid-solid combustion, the placement of a weight uniformly over the 
green membrane may be used as a technique to control surface quality and 
pore size uniformity. In-situ formed membranes are also contemplated with 
multiple layers which can include one or more layers of organics, 
polymers, glass, metals, or intermetallic compounds along with 
micropyretic layers. The application of several coatings of colloidal 
liquids onto micropyretic layers, both in a symmetric as well as a 
non-symmetric fashion is also envisioned. 
From the above disclosure of the general principles of the present 
invention and the preceding derailed description, those skilled in the art 
will readily comprehend the various modifications to which the present 
invention is susceptible. Therefore, the scope of the invention should be 
limited only by the following claims and equivalents thereof.