ITRM20100407A1 - MEMBRANES AND PROCEDURE FOR THEIR REALIZATION. - Google Patents

Description

Membranes and procedure for their realization

The present invention relates to membranes and a process for their realization.

More particularly, the invention relates to ceramic and non-ceramic membranes, consisting of a plurality of superimposed layers with decreasing porosity, said layers possibly being made of different materials.

As is well known, ceramic membranes are often used in filtration separation processes, which consist of an active part, which allows the selective permeability of materials and, optionally, of auxiliary parts, for example for the mechanical support or the drainage. Compared to common filtration, membrane filtration allows to retain much smaller particles and ceramic membranes in particular allow to retain particles of micrometric or nanometric dimensions. A ceramic membrane, therefore, can be thought of as a barrier with selective permeability or as a sieve capable of separating, depending on its porosimetric distribution, macroparticles, dispersions, colloids up to molecules and ions.

Therefore, ceramic membranes are currently used in the industrial field in microfiltration, ultrafiltration, nanofiltration, reverse osmosis, separation of vapors and gas phases, pervaporation, dialysis, biochemical reactions, catalysis and contactors.

The two most important parameters that describe the performance of a membrane are its permeability and the separation factor. For porous membranes, these parameters are governed by the thickness of the membrane and its porosimetric distribution, while for dense membranes the variables involved are much more difficult to determine, as atomic phenomena occur which are not always quantifiable.

From a structural point of view, ceramic membranes are composed of one or more layers, which can also be made with different materials. The structurally load-bearing element is composed of a macroporous substrate or support, on which one or more mesoporous intermediate layers (or inter-layers) rest. The outer layer (or top layer) is the actual filter element and is often characterized by the fact that it has a finely dispersed porosity of micrometric or nanometric dimensions.

The macroporous support has the purpose of mechanically supporting the top layer while the inter-layers have the function of binding and giving structural continuity between the substrate and the top-layer. Generally, all the constituent elements of the membrane are metal oxides, such as AI2O3, TiO2, Zr02, Si02.

The porosimetric characteristics of a membrane are a function of the thickness of the different layers of which it is composed; the dimensions of the pores, in fact, range from a few nanometers (top layer) to a few microns (support) and this is possible only by inserting one or more inter-layers with pores of intermediate dimensions.

Currently, most commercially available membranes come as disc, plate or tube shaped cartridges. The various cartridges are assembled together to form a module; different modules can be installed in platforms (skids) and the union of several skids leads to the construction of modular systems of any size.

In order to increase the ratio between the exchange surface and the volume of the membrane, tubular monolithic elements of various sizes and shapes have been developed; in the case of tubular systems this ratio is about 30-250 m <2> / m <3>, while for monolithic multichannel systems it can reach up to 400 m <2> / m <3>.

Similarly, disc or plate-shaped cartridges can be assembled in a stack configuration, achieving high packing densities.

Potentially, ceramic membranes have the same filtering characteristics as polymeric membranes, however they differ in a series of peculiarities that make them indispensable in the case of special applications. First of all, they have a greater mechanical resistance, with the consequence that they can operate with pressures even higher than 200 bar. Furthermore, the high surface hardness limits the wear phenomena due to the use of fluids rich in abrasive dispersions. Metal oxides such as zirconia, alumina and silica also possess high refractory properties (possibility of working at very high temperatures) as well as a remarkable resistance to attack by strong acids and bases (operating environments with pH between 0 and 14).

According to the known art, the ceramic membranes are made through a process which involves a series of steps. The support is produced using the well-known sintering technologies, starting from a compact of powders (green) obtained by molding or extrusion. The green, which has an extremely plastic consistency, is sintered according to temperature and time ramps capable of providing a structurally acceptable component but with an open porosity communicating as high as possible. The next step is the deposition of the inter-layer and the top-layer, which are generally applied with successive phases by means of sol-gel or dip-casting technologies.

However, known ceramic membranes also have various disadvantages which are the direct consequence of the production technology. Firstly, the lack of automation of the process, due to the presence of numerous rather critical and difficult to automate phases. Furthermore, the ceramic membranes produced according to the known art have high costs, first of all due to the fact that this type of process has rather long production times and investments linked to economies of scale, so that profitability is achieved only in the case of high productions, secondly for the number of rejects and non-conforming due to the criticalities of the process / product chain.

A further limitation is the high fragility. In fact, the single ceramic cartridge produced is an element with a remarkable mechanical hardness but with an intrinsic fragility and very low resistance to dynamic loads, shocks or vibrations.

Furthermore, the known technique is applicable to a small number of materials, generally traditional ceramic oxides, due to the fact that the sintering process is based on the thermodynamics of stable equilibria. By sintering, for example, it would be impossible to produce a composite product of γ alumina and barium titanate, as the sintering time and temperature would lead to the formation of unwanted sub-phases, according to the laws described by the quaternary state diagram 0 , Ti, Ba, Al.

Finally, a further limitation of the solutions according to the known art is constituted by the operational criticalities. The ceramic cartridges obtained by sintering must be made to work in critical mechanical conditions, that is with the walls placed in traction, due to the need to maximize the exchange surface. Conversely, the optimal mechanical configuration would be with the cartridge operating under compression conditions.

In the light of the above, it is evident the need to have membranes with greater fracture toughness and which can be made with materials chosen from a wide range, as well as procedures for obtaining them that allow high automation and reduced costs.

In this context, the solution according to the present invention is inserted, which proposes the use of ceramic, metal or composite coatings deposited on porous ceramic, metal or composite supports by means of thermal deposition and vapor phase technologies.

The object of the present invention is therefore to produce membranes which allow to overcome the limits of the membranes according to the known technology and to obtain the previously described technical results, as well as a process for obtaining them.

A further object of the invention is that said membranes can be made with reduced production times and substantially low costs, both as regards production costs and as regards management costs.

Not the least object of the invention is to propose a process for manufacturing membranes which is repeatable, substantially simple, safe and reliable.

A first specific object of the present invention therefore forms a process for manufacturing a membrane which comprises the following steps:

- preparation of a substrate by sintering metal or ceramic powders or their composites;

covering of said substrate by thermodeposition of a layer of the same or different ceramic, metal or alloys and composites material.

Optionally, the process according to the invention can comprise a further step of covering said thermo-deposition layer by physical vapor deposition (PVD) or chemical vapor deposition (CVD) of a layer of the same or different ceramic or metallic material. or alloys and composites.

A second specific object of the present invention also forms a membrane consisting of a substrate of sintered material covered with a layer of material obtained by thermo-deposition, the microstructure of which is of the lamellar type, and optionally by a further layer of material, which covers said layer obtained by thermal deposition, obtained by physical vapor deposition (PVD) or chemical vapor deposition (CVD), the microstructure of said further layer comprising strongly packed columnar single crystals.

Preferably, according to the invention, said layer obtained by thermo-deposition has a thickness between 50 and 600 μπι, and more preferably between 100 and 400 μπι and has average pore sizes ranging from 10 to 1000 nm and more preferably from 50 to 300 nm, while said layer obtained by physical vapor deposition (PVD) or chemical vapor deposition (CVD) has a thickness between 10 and 1000 nm.

A third specific object of the present invention also forms a filtration cartridge with a membrane as previously defined which has a cylindrical or prismatic geometry with a hexagonal section, with a minimum diameter of 6 mm and a length between 300 and 1000 mm. By virtue of the process with which it is made, the membrane can adapt to any shape of the substrate.

The advantages of the membranes of the present invention and of the process for obtaining them, with respect to the solutions of the known art, are many and are presented below.

First of all, the high level of automation. It is in fact known that the production cycles of ceramic components by sintering are discontinuous and rather long, with thermal ramps of tens of hours that require the use of machines and kilns of considerable size and power. The use of thermal and vapor phase deposition, on the other hand, drastically reduces the energy, plant and time commitment. In fact, a single cartridge can be coated in less than 2 minutes and the processing cycle can be fully automated.

Another advantage of the solution according to the present invention is the cost reduction. The energy required for the production of a single cartridge by thermo-deposition is in fact significantly lower than that required for the same cartridge using sintering technologies. Furthermore, the drastic reduction of production times, the very high automation, the reduction of fixed costs due to smaller plants and the lower number of non-compliant rejects due to the criticalities of the process / product chain remarkably reduce the cost of the single component. increasing the quality standard and the production rate.

The membranes of the invention also have a higher fracture toughness than those according to the known art. The ceramic cartridges currently produced are elements with a considerable mechanical hardness but with an intrinsic fragility and very low resistance to dynamic loads, shocks or vibrations. According to the present invention it is possible to produce membranes, and in particular ceramic membranes, with an operational life that is certainly higher than any polymeric membrane and ceramic units currently on the market. In fact, consider that the ceramic membranes currently on the market are massive ceramic components with a very high probability of breakage during production and service (if only for the cyclical back-flush stresses to which they are subjected). A slight chipping of the module inevitably leads to its replacement. The membranes conceived in the form of coating, on the other hand, have the cartridge as the minimum unit of construction; more cartridges lead to the construction of the module. The breaking of a cartridge leads to the replacement of the single cartridge and not of the entire module (modularity of the cartridge system). Furthermore, while the classic ceramic membranes are entirely made up of extremely fragile ceramic elements, the cartridges produced by thermal deposition have an internal metal structure (extremely tough) on which the membrane is deposited. The component therefore has a mechanical reliability and a considerably higher fracture toughness.

Furthermore, the process according to the present invention leads to the formation of coatings which generally exhibit residual compressive stresses, which further increases the fracture toughness of the component.

The solution according to the present invention makes it possible to use membranes with a very wide range of ceramic, metal, alloy materials and their composites. Classical forming processes are based on the thermodynamics of stable equilibria for which the materials that can generally be processed are traditional ceramic oxides, while thermo deposition (cooling rate of the order of one million kelvins per second) allows to obtain a whole range of crystalline structures, from nanocrystals, amorphous structures as well as dispersions and alloys incompatible with the normal laws of thermodynamics. The discussion can be extended to the contrast between the sol-gel coating process and the vapor phase deposition.

A further advantage concerns operational criticalities. All the ceramic cartridges produced by sintering work with the external walls placed in traction. This operative choice is dictated by the need to maximize the exchange surface. The cartridges produced according to the present invention, on the other hand, work with the external surface in conditions of strong turbulence and above all in compression, with a consequent increase in the operating pressures and in the operating life.

The membrane manufacturing process of the present invention allows to realize membranes with a composite structure obtained by overlapping several layers. Consider, for example, the deposition of a layer of 600 μπι of zirconia on which a layer of 200 nm of gold is deposited by means of PVD for the separation of gaseous mixtures at high temperature.

According to the present invention it is then possible to obtain membranes with any shape (the shape is given by the substrate). In a particularly preferred form of the invention, the single cartridge is made in the shape of a prism with a hexagonal base, since the hexagonal structure with honeycomb order maximizes the surface / volume ratio by maximizing the exchanges and flows of permeate.

Furthermore, the fact of being able to use an unlimited quantity of materials (including semiconductors and variously doped composites) allows to create catalyst membranes or membranes with reactive surfaces. As an example, imagine the construction of a micro / ultrafiltration module for oxidized wastewater in which, during the filtration phase, the disinfection phase can also be carried out, thus making the use of specific disinfection lines using NaOCl superfluous. UV or ozone.

Finally, the possibility of being able to choose the materials with which modules, cartridges and membranes are built allows to operate at very high temperatures (even 1000 ° C), with pH ranges between 0 and 14, pressures above 200 bar, and with high flow velocity even with liquids in which abrasive particles are present (high resistance to erosion).

The invention will be described below for illustrative but not limitative purposes, with particular reference to some illustrative examples and the attached figures, in which:

Figure 1 shows a perspective view of the substrate of a filtration cartridge according to the present invention,

- figure 2 shows the filtration cartridge of figure 1, covered with a layer of material deposited by thermo-deposition,

- figure 3 shows the filtration cartridge of figure 2, covered by a layer of material deposited by vapor phase deposition,

Figure 4 shows a plurality of hexagonal section filtration cartridges packed in a honeycomb configuration,

- figure 5 shows a SEM micrograph of an AI2O3 + T1O2 membrane at 30% by weight APS 650x BSE,

- figure 6 shows a SEM micrograph of an AI2O3 T1O2 membrane at 30% by weight APS 5000x BSE, - figure 7 shows a SEM micrograph of a Cr203 + Al membrane at 20% by weight Lao, 5Sro, 5Mn0320% by weight 2000x BSE ,

- figure 8 shows a SEM micrograph of a membrane Cr203 + Al at 20% by weight Lao, sSro, 5Mn0320% by weight 6500x BSE,

- figure 9 shows a SEM micrograph of a TiO2 membrane deposited in an atmosphere of Ar (1200 mbar) 250Ox BSE,

- figure 10 shows a SEM micrograph of a TiO2 membrane deposited in an atmosphere of Ar (1200 mbar) 6500x BSE,

- figure 11 shows a SEM micrograph of a 17% by weight APS 1850x BSE WC-Co membrane,

- figure 12 shows a SEM micrograph of a 17% by weight APS 7500x BSE WC-Co membrane,

- figure 13 shows a SEM micrograph of sintered SiAlON 20000X SE,

- figure 14 shows a SEM micrograph of Cr203 + BaTi03 at 60% by weight 2500x BSE,

Figure 15 shows a micrograph of a silica solgel film produced on an alumina interlayer, e

- figure 16 shows a coating of YSZ grown on a substrate of Ce02 by means of the PVD technique.

According to the present invention, the production of ceramic membranes and filtration and separation cartridges based on ceramic membranes constructed with composite structures using composite materials is proposed.

The mechanical substrate (support) consists of a coarse-grained sinter (particles from 1 to 50 μπι) of metal powders (such as steels, brass, alloys based on nickel, cobalt, aluminum, or magnesium), ceramics (glass low fluxes, aluminosilicates, carbides, nitrides) or their composites. At a technical / commercial level, the simplest case is the use of pressed AISI 420 powders or brass, as they are easily available on the market at reduced costs.

The substrate can have any shape (discs, plates, tubes) even if, to maximize the exchange surface, cylindrical geometries or tubes with hexagonal section are preferred, as shown with reference to Figure 1. Furthermore, the substrate can have any compatible size with the mechanical stresses to which it must be subjected. Generally it can have a minimum diameter of 6 mm with lengths ranging from 300 up to 1000 mm.

After sintering the substrate, a ceramic coating is deposited on the substrate. This coating is carried out with one of the numerous thermal deposition techniques or their variants, using ceramic or metal materials or their alloys and composites. These technologies produce thick films, i.e. coatings with thicknesses that can range from 50 μπι up to several centimeters.

Coating technologies for thermo-deposition allow to obtain specific surface properties (in particular resistance to wear, corrosion and thermal stress, electrical and magnetic conductivity, optical properties) without altering the characteristics of the substrate (toughness and structural characteristics). The overlay must have good chemical compatibility with the substrate and high adhesion characteristics.

The thermo-deposition processes consist in the realization of a coating starting from metallic, ceramic or cermet type powders (dispersion of ceramic phases in metal matrices). The material to be deposited is melted inside an energy source and accelerated towards the substrate where it quickly solidifies, giving rise to overlapping lamellar structures. When the molten particle reaches the surface to be coated it possesses a high kinetic energy and, upon impact, it flattens giving rise to a lamella called splat. The structure of the overlay is therefore given by the overlapping of the solidified and anchored lamellae; in addition to the overlapping lamellae, porosities, non-fused particles and oxide inclusions are present, typical defects of such coatings. The oxide inclusions derive both from the interaction between the particles and the environment in which the spraying takes place, and from the heating of the surface of the coating being formed. Upon impact, the oxide film covering the particle breaks and remains trapped between the lamellae.

The coating produced on the substrate can have a thickness ranging from 50 μπι up to several millimeters, and, for reasons of head loss and economic, preferably between 100 and 400 μπι.

The coating process of a single support is fully automated and takes no more than a few minutes. The coating thus obtained generally has a porosity depending on the deposited materials, as well as on the process parameters used; the average pore size ranges from 50 to 300 nm.

The cartridge thus produced (figure 2) can be directly used in all industrial micro / ultrafiltration processes.

In case it is necessary to push the porosity levels to a few tens of nanometers and up to a few Angstroms (nanofiltration, reverse osmosis, separation of gas phases), the thermo-deposited coating is used as an inter-layer for a subsequent thin coating (by a few microns to tens of nanometers thick) produced by physical (PVD) or chemical (CVD) vapor deposition (Figure 3).

The top layers made by vapor phase deposition have remarkable adhesion and mechanical resistance. Furthermore, they can be lithographed to increase the surface physical properties of superhydrophobicity and ebullioscopicity (significant increase in the activation sites for the production of vapor / gas phases from liquid phases).

The cartridges thus produced, in order to be used, must be housed inside a metal or polymeric housing, forming a filtration module. The configuration that most maximizes the exchange surface value is a cartridge bundle system (very similar to the tube bundles of exchangers) arranged in a honeycomb pattern. With an average cartridge width of the order of 7 mm and an inter-cartridge distance of 0.2 mm, packing greater than 500 m <2> / m <3> can be achieved.

Figure 4 shows how 191 cartridges with a hexagonal section can be packed in a honeycomb configuration inside a cylindrical module with a radius of 50 mm. The ducts placed on the cylindrical surface of the module are the inlet (at high pressure) of the fluid to be filtered and the outlet of the concentrate. The permeate passes through the membranes through the cartridges and is collected and extracted

from the bottom and top of the module.

As previously observed, the cartridge inserts work in compression and not in traction like the classic elements.

Five examples are reported below which describe the construction of cartridges using materials and technologies according to different embodiments of the present invention.

Example 1. Production of a microfiltration cartridge by thermal deposition of AI2O3 + Ti02-x at 30% by weight with plasma spray technology The alumina-based composite coating was deposited by plasma spray technology in APS (Air Plasma Spray) configuration. The deposition substrate is a sintered in AISI 316 with a particle size of 10 μπι. The substrate has a hexagonal section with a hexagonal side dimension of 5 mm and a length of 300 mm.

The deposition parameters used are summarized in Table 1.

Table 1

(SLPM = standard liters per minute)

The torch used is a Sulzer-Metco model F4MB with an anode diameter of 6 mm.

The powders used (AI2O3 and TiO2-X at 30% by weight) were mixed by mechanical stirring for 1 hour and kept in an oven at 115 ° C for 24 hours before deposition.

The gas used to transport the powders from the feeder to the torch head is Ar with a flow rate of 3.5 SLPM.

The torch is managed by a 5-axis anthropomorphic manipulator, while the substrate is mounted vertically on a rotating table with a tangential deposition speed of 500 mm / sec.

The micrographs shown in figures 5 and 6 show, with different levels of magnification, a section of the membrane where the different phases are clearly highlighted in the form of lamellae of different colors. The open porosity, able to confer the ultrafiltration characteristics, is due to the multitude of microcracks that are generated between the lamellae due to the very fast cooling and consequent solidification.

Example 2. Production of an ultrafiltration cartridge with catalytic properties by thermal deposition of Cr203 + Al at 20% by weight Lag, sSrp, 5MnQ3 at 20% by weight with plasma spray technology The chromium-based composite coating was deposited by plasma spray technology in APS (Air Plasma Spray) configuration. The deposition substrate is a sintered in AISI 316 with a particle size of 10 μπι. The substrate has a hexagonal section with a hexagonal side dimension of 5 mm and a length of 300 mm.

The deposition parameters used are summarized in Table 2.

Table 2

(SLPM = standard liters per minute)

The torch used is a Sulzer-Metco model F4MB with an anode diameter of 6 mm.

The powders used (Cr203 + Al at 20% by weight Lao, sSro, 5Mn03 at 20% by weight) were mixed by mechanical stirring for 1 hour and kept in an oven at 115 ° C for 24 hours before deposition.

The gas used to transport the powders from the feeder to the torch head is Ar with a flow rate of 3.5 SLPM.

The torch is managed by a 5-axis anthropomorphic manipulator, while the substrate is mounted vertically on a rotating table with a tangential deposition speed of 500 mm / sec.

The micrographs shown in figures 7 and 8 show, with different levels of magnification, a section of the membrane in which the different phases are clearly highlighted in the form of lamellae of different colors. The open porosity, able to confer the ultrafiltration characteristics, is due to the multitude of microcracks that are generated between the lamellae due to the very fast cooling and consequent solidification.

The sub-stoichiometric color matrix and the semiconductor dispersion of LSM give the cartridge catalytic characteristics.

Example 3. Production of a self-disinfecting ultrafiltration cartridge for MBR water treatment systems by thermal deposition of Ti02 with plasma spray technology in a controlled atmosphere of Ar at 1200 mbar of pressure in the deposition chamber The titania coating TiO2-X was deposited by plasma spray technology in APS (Air Plasma Spray) configuration. The deposition substrate is a sintered in AISI 316 with a particle size of 10 μπι. The substrate has a hexagonal section with a hexagonal side dimension of 5 mm and a length of 300 mm.

The deposition parameters used are summarized in Table 3.

(table follows)

Table 3

(SLPM = standard liters per minute)

The torch used is a Sulzer-Metco model F4MB with an anode diameter of 6 mm.

The powders used were kept in an oven at 115 ° C for 24 hours before deposition.

The gas used to transport the powders from the feeder to the torch head is Ar with a flow rate of 3.5 SLPM.

The torch is managed by a 5-axis anthropomorphic manipulator while the substrate is mounted vertically on a rotating table with a tangential deposition speed of 500 mm / sec.

The micrographs shown in figures 9 and 10 show, with different levels of magnification, a section of the membrane in which the intricate and well-distributed network of communicating micropores capable of conferring ultrafiltration characteristics to the material is easily identifiable. The network is generated by the multitude of microcracks that are generated between the slats for the very fast cooling and consequent solidification.

The oxidizing power of the composite material of which the membrane is made gives it self-disinfecting characteristics. In fact, the substoichiometric titania TIO2-X is a high resistivity semiconductor capable of yielding electrons. Obviously, in order to function, everything must be closed on a circuit powered by direct voltage.

Example 4. Production of a microfiltration cartridge by thermal deposition of WC-Co at 17% by weight with high velocity oxy-fuel (HVOF) technology

The cobalt matrix composite coating was deposited using HVOF technology. The deposition substrate is a sintered in AISI 316 with a particle size of 10 μπι. The substrate has a hexagonal section with a hexagonal side dimension of 5 mm and a length of 300 mm.

The deposition parameters used are shown in table 4.

Table 4

(SCFH = standard cubic feet per hour; GPH = gallons per hour)

The torch used is a Tafa model JP-5000 with 8-inch bars.

The powders used (WC-Co at 17% by weight) were kept in an oven at 115 ° C for 24 hours before deposition.

The gas used to transport the powders from the feeder to the torch head is Ar with a rotation speed of the dosing screw equal to 330 rpm.

The torch is managed by a 5-axis anthropomorphic manipulator, while the substrate is mounted vertically on a rotating table with a tangential deposition speed of 400 mm / sec.

The micrographs shown in figures 11 and 12 show, with different levels of magnification, a section of the cermet membrane, where the different phases are clearly highlighted. The lamellae, in this particular case, are formed by a cobalt matrix enriched in different points of tungsten and carbon (different shades of gray) while the tungsten carbide is easily identifiable in the homogeneously dispersed clear reinforcement. The open porosity, capable of conferring the ultrafiltration characteristics, is due to the multitude of microcracks that are generated between the lamellae due to the very fast cooling and consequent solidification.

The data reported up to now allow to highlight a series of structural and microstructural differences between the membranes according to the present invention and those according to the known art; differences that are substantial, easily identifiable through laboratory investigations and directly related to the process technology.

In particular, according to the present invention, for the microfiltration, ultrafiltration and nanofiltration operations, membranes are proposed which, compared to the sintered ones commonly used for such purposes, are covered with a layer of material obtained by thermo-deposition.

The different microstructure can be highlighted by means of a simple optical or electron microscopy investigation. With reference to figures 13 and 14, in the case of a sintered ceramic (figure 13) it is possible to easily identify the starting particles and how, after a more or less prolonged residence at high temperature, following diffusion phenomena the particles have fused between them to form a porous structure. The size of the open porosity is directly dependent on the size of the starting particles, as well as on the combination of time / process temperature. The procedure follows the laws of the thermodynamics of equilibria and the phases that can be obtained are limited and easily identifiable by consulting the reference phase diagram.

With reference to figure 14, the thermal deposition technology instead provides for the rapid heating with deposition and subsequent instantaneous cooling (millions of Kelvin per second) of the particles that form the coating. The whole process is governed by kinetics, so it is possible to obtain an infinite series of phases and composites that are not compatible with the normal laws of thermodynamics. The microstructure of any heat-deposited coating (Figure 14) is of the lamellar type and directly depends on the process parameters used. The fact that these coatings have an open communicating porosity is not a foregone conclusion and, as far as we know, is linked to interlamellar interstices and defects such as inclusions and porosities produced during the deposition phases. In the particular case illustrated of a carryover of Cr203 + BaTiO3 at 60% by weight, the very fact that it exists is proof that it has been deposited by thermal deposition; in fact, no other process is able to create microstructures of this kind with two phases such as chromia and barium titanate packed in isolated lamellae. In the case of a sintering process, in fact, the result would have been a mixture of compounds that can be described with the quaternary diagram Cr, Ba, Ti and 0.

The two technologies therefore lead to the formation of two totally different and in no way interchangeable products.

The same can be said for the top layer produced by vapor phase deposition, in accordance with the present invention, or by solgel deposition, according to the known technique. Also in this case, the two different top layers can be easily distinguished by a simple microscopic investigation.

In fact, also in this case, the vapor phase deposition allows to obtain multiplayer alloys and composites otherwise not producible with the technologies of the known art, which foresee high temperature processes and therefore dependent on thermodynamic equilibrium conditions. As a result, the microstructure is also considerably different. The sol-gel and dip-coating processes lead, after heat treatment, to the formation of a compact, dense layer with porosity comparable to the size of the interstices left by the nano particles that form the layer (figure 15, taken from Ceramic Membrane for Separation and Reaction - Kang Li, John Wiley & Sons Ltd (2007) ISBN 978-0-470-01440-0 Great Britain). For densif ications close to the theoretical one, the porosity coincides with the lattice distances themselves.

The accretion via vapor phase, on the other hand, leads to the formation of columnar single crystals with strong packing (figure 16, taken from Handbook of Superconducting Materials: Superconduct ivity, Materials and Processes, David S. Ginley, ISBN 0750308982, 9780750308984). In this case, the porosity has a bimodal distribution, essentially due to the reticular distances inside the individual columns and to the intergranular defects which bind the various columns together.

The present invention has been described for illustrative, but not limitative purposes, according to its preferred embodiments, but it is to be understood that variations and / or modifications may be made by those skilled in the art without thereby departing from the relative scope of protection, as defined from the attached claims.

Claims (11)

CLAIMS 1) Process of making a membrane which includes the following steps: - preparation of a substrate by sintering metal or ceramic powders or their composites; covering of said substrate by thermodeposition of a layer of the same or different ceramic, metal or alloys and composites material. 2) Process for manufacturing a membrane according to claim 1, characterized in that it comprises a further step of covering said thermo-deposition layer by means of physical vapor deposition (PVD) or chemical vapor deposition (CVD) of a layer of the same or different material. 3) Membrane characterized by the fact that it consists of a substrate of sintered material covered with a layer of material obtained by thermo-deposition, whose microstructure is of the lamellar type. 4) Membrane according to claim 3, characterized in that it comprises a further layer of material, which covers said layer obtained by thermo-deposition, obtained by physical vapor deposition (PVD) or chemical vapor deposition (CVD), the microstructure of said further layer comprising strongly packed columnar single crystals. 5) Membrane according to claim 4, characterized in that said layer obtained by physical vapor deposition (PVD) or chemical vapor deposition (CVD) has a thickness ranging from 10 to 1000 nm. 6) Membrane according to any one of claims 3 - 5, characterized in that said layer obtained by thermo-deposition has a thickness comprised between 50 and 600 gm. 7) Membrane according to claim 6, characterized in that said layer obtained by thermo-deposition has a thickness comprised between 100 and 400 µm. 8) Membrane according to any one of claims 3 - 7, characterized in that said layer obtained by thermo-deposition has average pore sizes ranging from 10 to 1000 nm. 9) Membrane according to claim 8, characterized in that said layer obtained by thermo-deposition has average pore sizes ranging from 50 to 300 nm. 10) Filtration cartridge with a membrane as defined in claims 3 - 9, characterized by having a cylindrical or prismatic geometry with a hexagonal section. 11) Filtration cartridge according to claim 10, characterized in that it has a minimum diameter of 6 mm and a length of between 300 and 1000 mm.