SiOC membranes and methods of making the same

A method of making a porous SiOC membrane is provided. The method comprises disposing a SiOC layer on a porous substrate, and etching the SiOC layer to form through pores in the SiOC layer. A porous SiOC membrane having a network of pores extending through a thickness of the membrane is provided.

BACKGROUND

The invention relates to separation of fluids, and more particularly to SiOC membranes and methods of making the same.

Polymer derived ceramics (PDCs) membranes possess excellent thermal and chemical stability and can be used under critical process conditions where most of the polymeric membranes are unstable. PDCs have mainly two families: one is made of silicon, oxygen and carbon, commonly known as silicon oxycarbide (SiOC); while the other one is made of silicon, carbon and nitrogen, known as silicon carbonitride. Varieties of PDCs with multifunctional properties have evolved from these two families by adding different dopants. The composition of PDCs is non-stoichiometric, in general, and that determines their structure.

PDCs are obtained from the pyrolysis of polymer precursor. There exists strong bonding between silicon and carbon in the polymer that prevents carbon from volatilizing as a hydrocarbon during pyrolysis. The polymer pyrolysis process is a low-temperature route for making high-temperature ceramics as the pyrolysis is completed below about 1200° C. In recent years, ternary PDC systems made from SiCN and SiCO have attracted interest because of their unusual properties, such as the absence of steady state creep, presence of visco-elasticity at very high temperatures, oxidation resistance, corrosion resistance and optical properties.

Due to these properties PDCs are a suitable candidate for forming membranes. However, in many applications a major concern in the processing of membranes is to achieve nanopores with narrow pore size distribution. Apart from narrow pore size distribution, membranes need to have chemical and mechanical stability for efficient separation process. For example, lack of porous membranes with uniform pore size distribution makes the separation of bio-molecules like viruses and proteins relatively difficult.

Therefore, it would be desirable to provide membranes, which have the desired pore size distribution and that exhibit chemical and mechanical stability.

BRIEF DESCRIPTION

In one embodiment, a method of making a porous SiOC membrane is provided. The method comprises disposing a SiOC layer on a porous substrate, and etching the SiOC layer using an etching agent to form through pores in the SiOC layer.

In another embodiment, a method of making a porous SiOC membrane is provided. The method comprises applying a SiOC layer on a porous substrate, and exposing the SiOC layer to a hydrofluoric acid ambient to form through pores in the SiOC layer.

In yet another embodiment, a method of making a porous SiOC membrane is provided. The method comprises applying a SiOC layer on a porous substrate, and forming through pores in the SiOC layer by at least partial removal of silica from a bulk of the SiOC layer.

In another embodiment, a porous SiOC membrane having a network of pores extending through a thickness of the membrane is provided.

In another embodiment, a membrane assembly is provided. The membrane assembly comprises a substrate, and a porous SiOC membrane disposed on the substrate, wherein the porous SiOC membrane comprises at least one through pore.

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

DETAILED DESCRIPTION

Described herein are porous membranes made of SiOC. As used herein, the term “SiOC” refers to a polymer derived ceramic (PDC) material comprising silicon, oxygen and carbon and other materials such as, but not limited to, nitrogen and hydrogen. Further, the SiOC may also include one or more p-type or n-type dopants. Suitable dopants include, but are not limited to, boron, aluminum, and nitrogen. SiOC may or may not be electrically conductive. As used herein, the term “through pores” refer to a single pore or a network of pores that extend through a bulk of a SiOC layer.

FIG. 1is a ternary compositional diagram for SiOC. The plot10illustrating atomic percentages of elements silicon12, carbon14and oxygen16may be divided into compositional regions18,20and22. The vertices12,14and16represent 100% silicon, 100% carbon, and 100% oxygen, respectively. The compositional region18comprises excess silicon, whereas the compositional region20comprises excess carbon, and the compositional region22comprises excess oxygen. The points24and26represent silicon carbide (SiC) and silica compositions (SiO2), respectively. Stoichiometric SiOC composition falls on the tie-line28that connects silicon carbide24and silica26compositions. However, it is common to find materials that are a mixture of silica, silicon carbide and carbon, such materials are present in the region20. The domain size of the silica domain is determined by the surface to volume ratio of the domains, and therefore depends on the composition of the point S represented by the reference numeral30. For example, the domain size becomes smaller as the point S30moves closer to silicon carbide24. The free carbon content represented by c32determines the carbon chemistry of the interdomain boundary. In the illustrated embodiment, the overall composition of the SiOC material is represented by P35.

As illustrated inFIG. 2, amorphous SiOC is made of three different parts, silica tetrahedral or silica domains34, oxycarbide units36, and free graphitic carbon38. In certain embodiments, the silica domain34may extend upto a few nanometers. The clusters of silica domains34are surrounded by domain wall40that consists of mixed bonds of oxycarbide units36and graphitic carbon38. The oxycarbide units36in SiOC material are derivatives of silica tetrahedral where the oxygen atoms are replaced by carbon atoms and are represented by the formula SiCnO4-n. The structure or the building block of the SiOC consists of silica domain34surrounded by oxycarbide units36. These silica domains34encapsulated with oxycarbide units36are separated by graphitic carbon sheets38, which form a cage-like network. Typically, the size of the silica domains34as well as the thickness of the graphitic carbon sheets38is determined by the chemical composition of SiOC. The size of the silica domains34and the thickness of the graphitic carbon sheets38may be altered by changing the starting polymer precursors and processing parameters. Therefore, silica domain size34, starting from 2-3 nm to several tens of nanometers can be made by choosing proper starting precursors and by varying process parameters.

Out of these three constituents, both silicon oxycarbide units36and graphitic carbon38are resistant to hydrofluoric acid while silica domains34are susceptible to hydrofluoric acid. In the present technique, the structure of SiOC is utilized to make nanoporous SiOC membrane with uniform pore size. As illustrated inFIG. 3, at least a part42of the silica domains34is removed to obtain the porous SiOC structure. The pore size may be controlled by controlling the composition of SiOC, or etching conditions, or both.

Embodiments of the present technique disclose a method of making a porous SiOC membrane. The method of fabrication of the present technique provides an opportunity to control the pore size of the membrane. The tailoring of the pore size of the SiOC membrane is achieved by altering the composition of the starting polymer, processing parameters, and etching conditions. In certain embodiments, the pores are generated by chemical etching of silica from the nanodomains of silicon oxycarbide.

In certain embodiments, a method of making a SiOC membrane assembly comprises disposing a SiOC layer on a porous substrate. The SiOC layer may be formed separately and then disposed on the porous substrate. Alternatively, the SiOC layer may be formed directly onto the porous substrate. The porous substrate includes a material that is non-reactive with SiOC. Also, the porous substrate includes a material that may or may not have compatible thermal expansion with that of the SiOC layer. The porous substrate may include a ceramic substrate, a zirconia substrate, an alumina substrate, a silicon carbide substrate, a silicon nitride substrate, or combinations thereof. Further, in instances where the SiOC membrane is made separately and later disposed on the substrate, the substrate may be made of a porous polymeric material. The substrate may be chosen such that the average pore size of the substrate is greater than the average pore size of the SiOC membrane.

In some embodiments, the SiOC layer may be prepared using the sol-gel route. In these embodiments, the SiOC layer is prepared via hydrolysis and condensation of organically modified alkoxy silanes followed by the pyrolysis of the gel. In this process one or more precursors are mixed and diluted in solvent. Non-limiting examples of precursors include methyltrimethoxysilane, propyltrimethoxysilane, phenyltrimethoxysilane, or combinations thereof. Subsequently, water, is added for hydrolysis. In one embodiment, the molar ratio of water and silicon is about 4:1. Further, acid catalyst may be added to the solution to promote hydrolysis. In one embodiment, the solution thus formed is coated on a porous substrate and left for hydrolysis followed by condensation. The time needed for hydrolysis and condensation ranges from few minutes to several hours depending on the precursors. The hydrolyzed film is then pyrolyzed to convert the gel into ceramic.

In other embodiments, the SiOC layer may be prepared using the polymer pyrolysis route. In these embodiments, the SiOC layer is prepared by the cross-linking of thermoset polymers. In one example, the thermoset polymer includes different types of siloxanes, polycarbosilanes, or polydimethylsilane. The cross-linking is facilitated by the use of catalyst. Non-limiting examples of the catalyst include platinum containing polymeric catalyst, and dicumyl peroxide. The cross linked polymer is subsequently pyrolyzed to form SiOC. In one example, polydimethyl siloxane (PDMS) is used as a thermoset cross-linking polymer precursor with platinum catalyst. Since the viscosity of starting precursor (PDMS) is relatively high, in order to make thin film of SiOC on the substrate, PDMS precursor with cross-linking agent is diluted in xylene or hexane to lower the viscosity. The lower viscosity of the polymer solution facilitates thin coating of polymer on the substrate, thereby resulting in thin SiOC coating on the porous substrate after pyrolysis.

In certain embodiments, the SiOC layer is formed on a substrate, such as an alumina substrate. In one example, the alumina substrate is coated by dip coating in a polymer solution. The polymer solution includes a cross-linking thermoset polymer. Next, the coated substrate is heat-treated in air at a temperature in a range from about 150° C. to about 200° C. The heat treatment is carried out for a period of about 30 minutes to about 1 hour to cross link the coated polymer. Next, the heat-treated polymer is subjected to pyrolysis in an inert environment in a temperature range of about 600° C. to about 1200° C. In one example, the inert atmosphere may include argon, or nitrogen, or both. As will be appreciated, the cross linking or the heat treatment temperature range and the pyrolysis temperature range may vary depending on the selection of the polymer.

In certain embodiments, the SiOC layer may be deposited on the porous substrate by employing one or more of PECVD (plasma enhanced chemical vapor deposition), chemical vapor deposition, LPCVD (low pressure chemical vapor deposition), radio frequency (RF) sputtering and so forth. The thickness of the SiOC layer may be in a range from about 50 nm to about 500 nm, and preferably from about 50 nm to about 200 nm. In one embodiment, the thickness of the SiOC layer is about 100 nm.

Subsequent to depositing the SiOC layer on the porous substrate, the SiOC layer is made porous by at least partially removing silica tetrahedrals from the SiOC layer. The silica tetrahedrals are removed from the bulk of the SiOC layer forming a network of the pores throughout the thickness of the SiOC layer. Removal of the silica tetrahedrals leave behind a skeleton of carbon and oxycarbide units, with porosity derived from the removal of silica. Since the silica domain is generally uniform in size throughout the layer, the removal of silica tetrahedrals provides with narrow pore size distribution in the membranes.

In one embodiment, the silica tetrahedrals are chemically etched to form a network of pores. The etching agent may include greater than or equal to about 10 percent hydrofluoric acid dissolved in water. In one example, the etching agent comprises about 10 percent to about 50 percent, and preferably about 20 percent to about 50 percent of hydrofluoric acid dissolved in water. In one embodiment, the SiOC layer is exposed to a hydrofluoric acid ambient to form through pores in the SiOC layer.

In one embodiment, the etching is carried out at room temperature. The rate of etching may be increased by increasing the temperature. The etching is carried out at atmospheric pressure. Further, depending on the required pore density the etching is carried out for a time period in a range of about 1 minute to several hours. The average pore size tends to increase with the increase in concentration of the etching agent, etching time, or both.

FIG. 4illustrates an example of an X-ray diffraction pattern46for a SiOC material before and after partial removal of silica domains. In the illustrated embodiment, the abscissa48represents the angle of the diffraction pattern (2θ), and the ordinate50represents the intensity of the X-rays. The graph52represents SiOC that is not etched. The signature54of amorphous silica in the graph52is relatively more prominent as compared to the signature56of the silica for the SiOC58, which is etched to at least partially remove silica domains. The removal or decrease in the signature of silica in the etched SiOC58confirms that the silica domains are at least partially removed from the SiOC.

FIG. 5represents BET measurements for pore size distribution on a SiOC film. In the illustrated embodiment, the pore volume68is plotted with respect to the pore width70. The plot72shows a narrow distribution of the pore width as evident from the peak74of the plot72.

FIG. 6illustrates an example of a porous SiOC membrane assembly78. The assembly78comprises a porous membrane80having a network of pores82extending through a thickness84of the membrane80. The membrane80is supported on a patterned substrate86. In the illustrated embodiment, the substrate86includes regions88, such that in these regions88the membrane80is not coupled to the substrate86. That is, the membrane80is exposed from the substrate side90. In one embodiment, the regions88may be microfluidic channels. The substrate86may be patterned either before or after disposing the membrane80on the substrate86. In embodiment where the substrate86is patterned before disposing the membrane80on the substrate86, the pore size and/or the pore formation of the substrate86do not interfere with the pore formation in the layer having the SiOC membrane80. In one embodiment, the patterned substrate86may contain holes or microfluidic channels to deliver the analyte or buffer solutions.

FIG. 7is an alternate embodiment of the membrane assembly ofFIG. 6. In the illustrated embodiment, the membrane assembly92includes a porous SiOC membrane94having a network of pores96extending through the thickness98of the membrane94. The membrane94is disposed on an interfacial layer100, the layer100in turn, is disposed on a patterned substrate102having regions104where the membrane94is exposed. The interfacial layer100may protect the SiOC membrane during the fabrication of the substrate86. Non-limiting examples of the interfacial layer may include silicon dioxide, silicon nitride, and spin-on-glass.

The membrane assemblies78and92ofFIGS. 6 and 7respectively may be employed in on-chip, or inline applications where the membrane86is used for removing excess ions from the analyte or separating the bio-molecules based on their molecular weight or charges.

The SiOC membranes so prepared may be employed in separation processes. In separation processes, the pore size distribution and stability of membranes are critical factors for determining the efficiency of the process. SiOC exhibits excellent mechanical and thermo-mechanical properties, thus makes the membrane more durable. For example, the narrow pore size distribution and high mechanical stability facilitates efficient separation of bio molecules. In one example, selective separation of bio molecules such as proteins and viruses may be done using the SiOC membranes of the present technique that have narrow pore size distribution. In certain embodiments, the SiOC membrane has a pore size in a range of about 2 nm to several tenths of nanometers. In these embodiments, the pore size distribution is narrow. In one example, the pore size distribution is in a range from about 2 nm to about 15 nm. Also, the SiOC membranes exhibit mechanical and thermal stability to withstand relatively high-pressure differences needed for efficient separation processes. In one example, the SiOC membranes are adapted to withstand a pressure difference in a range from about 0.1 to about 1.5 atmospheric pressure.

In certain embodiments, SiOC is employed to fabricate high temperature membranes that are adapted to separate substances having sizes in the range from about 1 nm to 20 nm. In one example, the membrane is configured to filter fluids, both gas and liquid. Examples of fluids may include aromatic fluids, aliphatic fluids. Moreover, the pore size may be varied based on the requirements of the various applications where the SiOC may be employed. The membranes with average pore size of about 1 nm to about 2 nm may be used for separation of gaseous species. For example, when employed as a membrane to separate gaseous species, such as nitrogen, hydrogen, NOX sensors, the pore size of the SiOC membrane may be in a range from about 1 nm to about 2 nm. Whereas, higher pore size is required, when the SiOC membrane is applied as a separation membrane for the separation of bio-molecules. The pore size of the SiOC membrane may be in a range from about 2 nm to about 15 nm.