ES2551014B1 - HEATING MEDIA SENSITIVE TO ELECTROMAGNETIC RADIATION BASED ON MOTT MATERIALS - Google Patents

Abstract

Heating medium sensitive to electromagnetic radiation based on Mott materials. # The present invention relates to heating means that allow optimum absorption of a non-ionizing electromagnetic radiation whose energy or associated information is transformed into heat. The object of the invention consists of a composition and an architecture based on micro and nanoparticles capable of resonating with external radiation. As radiation sensitive entities, said particles preferably comprise Mott materials. In terms of efficiency, Mott material particles must be dispersed in a spacer matrix whose maximum load will be determined exactly by the limit of electrical percolation of the nanocomposite. The invention can be used to generate heat selectively, for catalytic activation or as a material that prevents the propagation of an electromagnetic signal.

Description

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DESCRIPTION

HEATING MEDIA SENSITIVE TO ELECTROMAGNETIC RADIATION BASED ON MOTT MATERIALS

FIELD OF THE INVENTION

The present invention relates to means capable of dissipating energy through the absorption of non-ionizing electromagnetic radiation, capable of being used as heating means and / or as catalysts. More specifically, the invention falls within the technical field corresponding to nanoparticle-based heating media that have resonance when subjected to external radiation, said means, preferably comprising Mott materials sensitive to non-ionizing microwave and / or radiofrequency radiation.

BACKGROUND OF THE INVENTION

Mott materials are subject to a phenomenon of dissipation of single energy resulting from their interaction with electromagnetic radiation. The nature of the interaction is associated with relatively slow, free charge carriers, so that the greater electromagnetic absorption efficiency coincides with the transition from insulator or semiconductor to correlated metal (A-MC) of Mott material. In the range of temperatures where the A-MC transition takes place, the load carriers of the material adjust themselves to resonate with the external radiation applied. After the absorption of energy, the free electrons and the elementary vibrations of the crystalline network of the material interact, resulting in the deposition of electromagnetic energy.

The efficient conversion of non-ionizing electromagnetic radiation into heat on a micro and nanometric scale has been described for plasma-type metal nanoparticles. Photothermia driven by near infrared metal plasma nanoparticles is useful, for example, in biomedical applications. However, the response to electromagnetic radiation in Mott materials allows the spectrum to be extended to greater wavelengths, such as microwaves and radiofrequency. The response of these materials to electromagnetic radiation has been studied, traditionally, under the test principle, finding that certain Mott materials, mainly magnetite, Fe3O4, and vanadium oxide (VO2), can be used in applications

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technologies based on the use of non-ionizing electromagnetic radiation. There are no known, however, references in the state of the art that generalize and exemplify the requirements for optimal energy utilization of Mott nanometer particles for attenuation of incident radiation. In this sense, it is known, for example, the use of carbon nanostructures (for example, carbon nanotubes) dispersed in spacer matrices as a medium sensitive to electromagnetic radiation. However, such use does not distinguish between conductive or semiconductor nanotubes, and does not contemplate what are the conditions that allow a maximum dissipation of the absorbed electromagnetic radiation, such as the temperature of the material, the concentration of the nanoparticles or the conduction properties of The spacer matrices.

Other studies exemplify the particular use of certain compositions that fall within the Mott family of materials, such as perovskites. However, as with the uses related to carbon nanotubes, these studies do not specify which are the necessary physical properties or the architecture of the compound that guarantee optimum behavior associated with maximum energy efficiency.

The present invention proposes a solution to the aforementioned technical problem, through novel heating means, based on Mott materials.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is, therefore, a heating medium based on Mott materials, in combination with spacer matrices where micro or nanoparticles of said materials are deposited. The objective of this combination is to give rise to heating means that, due to their composition and architecture, are highly dissipative in the face of the absorption of non-ionizing electromagnetic radiation, preferably in the microwave and / or radiofrequency frequency band. Additionally, another object of the invention is to obtain means whose heating can be carried out selectively in its volume or surface, giving rise to heating foci at nanometric points or specific regions of the materials that compose them.

Said objects of the invention are achieved by means of a heating medium sensitive to electromagnetic radiation, comprising microparticles and / or nanoparticles of a Mott material, arranged on the surface and / or the volume of a spacer matrix of

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said particles, where the volume concentration of said Mott material in the spacer matrix is equal to or less than its percolation limit. Preferably, the Mott material has a concentration of between 1% and 30% by volume in said spacer matrix; depending on the shape and composition of the Mott material particles and the matrix. More preferably, the concentration of the material is between 5% and 20% (for example for spherical particles of metal oxides). This achieves an effective heating means, which also allows to obtain very high heating ramps.

Preferably but not necessarily, the Mott material of the heating medium is at a temperature Tt in the range XT = | (Tt-Ta.Mc) | /Ta.mc <20%, where the Ta-mc temperature is corresponding at the beginning of the progressive transition A-MC of the material. More preferably, XT <10% and, even more preferably, XT <5%. This is achieved by obtaining heating means whose Mott materials are in the region of transition A-MC, obtaining the optimal conditions to increase their dissipative capacities.

Preferably, the invention relates to heating means comprising Mott materials of different nature, both organic and inorganic. Among others, the most prominent Mott materials are: carbon semiconductors (nanotubes, fulerenes, graphene), metal oxides (vanadium, cobalt or nickel oxides) and complex oxides (perovskites). However, any Mott material has similar characteristics to a greater or lesser degree. The choice of the most suitable Mott support and material will depend on the specific use of the invention.

Mott materials of the invention can be, for example, one or more of the following: simple oxides, cobaltites, perovskites, magnetite, carbon nanostructures. More preferably, said materials can be NiO, VO2, CuO, LaCoO3, PrCoO3, NdCoO3, SmCoO3, EuCoO3, GaCoO3, La0,9Sr0,1CoO3, SmNiO3, EuNiO3, LaMnO3, La1- XSrXniOnly active substances in 3 phases, environment.

For its part, the spacer matrix of the heating medium of the invention may comprise a non-conductive material transparent to electromagnetic radiation, for example based on cordierite monoliths, KBr, TiO2, CeO2, a formally metallic metal oxide without dof electrons in the layer of Valencia, alumina, an acidic, calcium or zeolite

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exchanged with divalent cations, or a dielectric fluid. Alternatively, the spacer matrix may comprise a material capable of absorbing electromagnetic radiation, such as a sodium zeolite.

Another object of the invention relates to a method of heating materials comprising the application of non-ionizing radiation to a heating medium according to any of the embodiments described herein. Preferably, said radiation comprises microwave and / or radiofrequency radiation, optionally operated in a pulsed manner.

In a preferred embodiment of the method of the invention, Mott materials are disposed inhomogeneously in the spacer matrix, so that thermal gradients are generated before the application of radiation, to produce a selective surface or volumetric heating of said materials.

A third object of the invention relates to the use of a heating means according to any of the embodiments described herein.

The advantages of the proposed invention are mainly: high heating rate, high maximum temperature reached and selective heating, located in the regions where Mott materials are concentrated.

As possible applications of these heating systems, its use in SHS ("Self propagating High temperature Synthesis") synthesis for the generation of heat and obtaining certain compounds that require reaching high temperatures to be synthesized is considered. Other examples of application refer , for example, to the glass industry, which requires high temperatures to melt the sand (quartz) used, as well as the use of the proposed invention for use in waste disposal media present in gaseous effluents. case, the heat generated by the embodiments of the invention will serve to activate a catalyst.

DESCRIPTION OF THE FIGURES

Figures 1a-1b show graphs illustrating the selective heating of perovskite nanoparticles and carbon nanotubes for sheets. Figure 1a shows the evolution of temperature over time for 200 mg tablets with 2% of nanoparticles of the LaBO3 perovskite type (where B = Cr, Mn, Fe, Co, Ni), or nanotubes of

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Carbon (CNTs) on KBr as a binder. Figure 1b shows the behavior of sheets with 20% by weight of perovskite. The samples are located within a monomodal waveguide at 2.45 GHz and a power of 60W.

Figure 2 illustrates the selective heating of LaCoO3 nanoparticles dispersed in quartz plates, in two dimensions (2D), obtained by thermography. Figures 2a and 2b refer to a 0.18 mg sample of LaCoO3 deposited in two channels of a quartz plate, with a surface density of approximately 0.10 mg / cm2, where Figure 2a specifically refers to a stable system at 645 K at 65 W, and Figure 2b shows the temperature ramps working with power pulses at 150 W.

Figure 3 shows photographs illustrating the selective heating of LaMnO3 and LaCoO3 nanoparticles dispersed in cordierite monoliths, in three dimensions (3D). Specifically, Figures 3a and 3b show two scanning electron microscope images together with infrared thermography images for LaMnO3 and LaCoO3 nanoparticles (20 mg, approximately 5% by weight) dispersed in cordierite monoliths, with a stable temperature regime at 30 W.

Figure 4 shows a graph illustrating the combustion of hexane with nanoparticles of La0.95Ce0.05CoO3. In it, the combustion of hexane is compared in a conventional oven or in a waveguide at 2.45 GHz, where approximately 20 mg of La0.95Ce0.05CoO3 nanoparticles are deposited on a cordierite monolith, with 200 ppm in air and 4 ml / minmg.

Figure 5 illustrates the damping of an electromagnetic field from a monolayer of VO2 on alumina. The black dotted lines denote the faces of alumina on which the VO2 particles have been deposited. The experiment consists of two VO2 blocks deposited on alumina, where their order is altered (Figures 5a and 5b) with respect to the microwave source (whose direction is indicated by the arrow in the Figures). In both cases, the shielding of the electromagnetic field by the first surface of VO2 means that the second block does not dissipate energy in the thermographic image.

DETAILED DESCRIPTION OF THE INVENTION

As described in previous paragraphs, the present invention relates to a heating medium comprising a composition capable of attenuating in a manner

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Efficient non-ionizing electromagnetic radiation. The heating means of the invention thus allows energy dissipation by selective heating of the load carriers of a sensitive material, specifically of a Mott material. For this, it is required that the Mott material be in a temperature range that provides an increase in its electrical conductivity. As long as the compound as a whole does not become conductive, this allows, for percentages of Mott material load that are lower or substantially equal to the limit of electric percolation, an optimal and volumetric attenuation at micro and nanometric scale of external radiation . Thus, the heating medium of the invention preferably comprises micro or nanoparticles of a Mott material, which have an A-MC transition at a certain temperature range.

Likewise, said particles are preferably arranged superficially or volumetrically in a spacer matrix, for example a non-conductive matrix transparent to the electromagnetic radiation to be applied on the medium. The spacer matrix could thus be a two-dimensional, three-dimensional support or a shell on the particles of the Mott material. Likewise, a non-sensitive fluid, such as air, can be used as a spacer matrix. In a preferred embodiment of the invention, the spacer matrix is insulating, with a low and stable dielectric loss with the temperature in the radiation frequency to be used. However, in alternative embodiments of the invention, it is also possible to use other materials, such as electromagnetic radiation absorbing materials (eg, sodium zeolites), which serve as a concentrating agent and which can be rapidly heated under microwave irradiation. This architecture has the advantage that it allows to work with pulsed radiation, with low temperature stages in which the reactants would be adsorbed. The rapid heating of radiation sensitive zeolites and Mott materials is capable of producing concentrated dissipative currents, which favors their application as catalysts.

For each specific application of the invention, it is possible to make different choices of Mott material and the support matrix. Stop it, it is necessary to know, for example, the properties of Mott material, A-MC transition temperature, stability, and possible catalytic activity. If the application is directed to total oxidation reactions, it is possible to use compositions formed by mixed oxides, for example with perovskite base structure: A1-XA'XMnO3 for methane or A1-XA'XCoO3 for other hydrocarbons. In applications for dry reforming of methane, it is possible to start from nickel perovskites or NiO. In the general case of looking for a thermal response at room temperature, you can

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work with micro or nanostructures of carbon or vanadium oxides. At higher temperatures, mixed oxides can also be used in the composition. Finally, in applications for the use of radiation attenuation as a source of information, it is possible to start from graphene arranged in multilayers, as well! like other carbon nanostructures.

In addition, for applications directed to catalysis, the heating medium of the present invention may comprise micro or metallic catalytic nanoparticles, supported on the Mott material.

In order to maximize the phenomenon of electromagnetic dissipation volumetrically in a macroscopic sample, a percentage of Mott micro or nanoparticles is necessary in the limit of electrical percolation of the compound. Said limit is preferably found in a concentration of Mott material comprised between 1% and 30% in total volume and, more preferably between 5% and 20%. Above the percolating limit, when the A-MC transition arrives, the entire sample will become conductive, and behaves like a mirror that reflects electromagnetic radiation.

The heating medium of the invention, according to the compositions and applications described herein, allows the conception of structures that volumetrically absorb non-ionizing electromagnetic radiation. In any case, the energy, or information that carries the radiation, initially generates hot electrons. This type of compounds is of great interest for applications that require localized local thermal responses, in general for applications that demand a very efficient micro or nanocomposite for the absorption of non-ionizing radiation, or directly in catalysis, hot electrons. As a significant application, the improvement in the energy efficiency of chemical industry processes and in particular catallotic processes is highlighted. Given the scope and scales of consumption of the energy sector, such improvement in energy efficiency associated with particular chemical processes can have a considerable technological and economic impact.

The optimum temperature range in terms of energy efficiency in selective heating appears from those temperatures close to the region of transition A-MC in the micro or nanoparticles of the Mott material, but it is possible to obtain intense heating at other temperatures. Preferably, the temperature (Tt) of the Mott material of the present invention with which optimum dissipation utilization is achieved

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energy, is in a range XT = | (Tt-Ta_Mc) | / Ta-mc <20%, where the Ta-Mc temperature corresponds to the A-MC transition of the material. More preferably, XT <10% and, even more preferably, XT <5%. As reference of the Ta-Mc temperature values of different Mott materials, Table 1 of this document is provided.

 Material Temperature (A ^ MC) (K)

 Oxides

 NiO 525

 Simple

 VO2 340

 CuO 293

 LaCoO3 490

 PrCoO3 525

 Cobaltites

 NdCoO3 575

 SmCoO3 610

 EuCoO3 625

 GaCoO3 650

 Doped Cobaltite

 La0,9Sr0,1CoO3 400

 Perovskites

 SmNiO3 400

 Neither

 EuNiO3 420

 Mn

 LaMnO3 750

 Carbon nanostructures

 C 300

Table 1. Ta-Mc temperature of different Mott materials.

In other preferred embodiments of the invention, the Mott material and / or the support spacer matrix of the heating medium can be modified for a particular thermal and / or catalytic response, by introducing dopants or active phases. Such dopants or active phases, especially for mixed oxides, can improve the specific response of Mott's material, as long as the correlated character of the load carriers is not destroyed. The La1-XCeXCoO3 family is a suitable example of doped systems of this type. The introduction of Ce or other load donors allows to advance the A-MC transition. In turn, it is possible to combine several Mott materials in the spacer matrix, for example in order that the thermal dissipation occurs in phases.

Typically, to obtain a homogeneous energy absorption at the macroscopic level, the nanoparticles are distributed evenly over the matrix, to avoid the formation of macroscopic hot areas. However, as a complementary embodiment of the invention, the heating means of the invention may comprise a specific architecture, where the sensitive micro or nanoparticles are arranged according to a specific non-homogeneous spatial organization in the spacer matrix, in order to introduce gradients

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thermal controlled. In some preferred embodiments, Mott micro or nanopartlcuias may be distributed in a gradient, that is, in certain regions of the support matrix there is a greater number of nanoparticles and that this number decreases in other areas along any direction. , which would allow it to induce controlled thermal gradients in the heating medium.

In this way, it is achieved that the volumetric or surface response of the medium possesses a selective behavior, which allows to generate dissipation foci in the heating medium. This application can be very useful in certain applications of the invention in which it is required to create specific heat points within the medium.

In other embodiments of the invention, Mott materials can be arranged specifically forming multiple layers, in sufficient thickness to produce a barrier that prevents both the penetration of microwave radiation into the material, once the A-MC transition has occurred, ace! as the reflection of radiation.

- Examples of embodiments of the invention:

1. Heating of flat sheets. Samples arranged within a monomodal wave glutton, at 2.45 GHz and a power of 60W:

Figure 1a shows the evolution of the temperature obtained by thermography in sheets containing 2% by weight of Labo3 perovskite nanoparticles (B = Cr, Mn, Fe, Co, Ni) or carbon nanotubes (CNTs), on KBr as binder in a transparent matrix. In any of the cases shown, the percolation limit for the samples is not exceeded. It is observed how the energy dissipation for LaCoO3 grows exponentially from 500 K. Starting at 500 K the A-MC transition begins, and in view of the growth of the dielectric dissipation, it is exemplified that LaCoO3 is a very Mott material Efficient for applications above the transition temperature. In contrast, carbon-based materials are very useful examples for applications at room temperature. Figure 1b shows a sheet thermograph containing 20% by weight of Lavs3 perovskite nanoparticles. In the event of an A-MC transition, the charge limit necessary for electric percolation has been exceeded. In this case, LaMnO3 is the material that warms the most, since its metallic state manifests above 750 K. For the rest of the compositions, the samples behave like a mirror, causing the radiation / information to be reflected. And don't dissipate.

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2. Ultra-fast thermal response in microwave-assisted microreactors:

A second example of the heating means of the invention are LaCoO3 particles arranged on a quartz plate, to achieve selective heating in two dimensions. The example shows the stability of the LaCoO3 nanoparticles under microwave irradiation on several flat supports. For 2 x 0.2 cm quartz channels, stable systems with only 0.18 mg of LaCoO3 are achieved (Figure 2a). In addition, an ultra-rapid response of the system is observed under pulses with temperature ramps of approximately 500 K / s, see Figure 2b. The temperature gradient between the surface of the catalysts and the medium is very pronounced. In this particular case, the invention allows an ultra-rapid thermal response in microsystems, only accessible under electromagnetic excitation.

3. Perovskite nanoparticles with LaCoO3 and LaMnO3 base composition dispersed in cordierite:

As an example of catalytic applications of the invention, cordierite monoliths have been used on whose surface Mott nanoparticles have been dispersed, in this case LaCoO3 and LaMnO3. The loading of the MFC particles is less than the percolation limit, approximately 20% -30% of surface area (5% by weight). The seeded monoliths are introduced into the gluttony wave at 30W. The LaMnO3 material barely generates hot electrons, unlike the LaCoO3 sample (see Figure 3). Figure 4 shows a saving of around 10% for the combustion of hexane in the electromagnetic activation comparison, compared to conventional heating.

4. Shielding:

Figure 5 shows in profile two plates of alumina, 4 x 4 x 0.5 cm, where a layer of vanadium oxide particles, VO2, has been deposited. A plate is located inside the wave glutton, shielding the radiation path from the source, so that only the dissipation by the first of the VO2 layers is observed.

Claims (18)

5 10 fifteen twenty 25 30 35 1 Heating medium sensitive to electromagnetic radiation characterized in that it comprises microparticles and / or nanoparticles of a Mott material, arranged on the surface and / or the volume of a spacer matrix of said particles, where the volume concentration of said Mott material in the spacer matrix it is equal to or less than its percolation limit. 2. - Medium according to the previous claim, where Mott material has a concentration of between 1% and 30% by volume in said spacer matrix. 3. - Medium according to the previous claim, where Mott material has a concentration of between 10% and 20% by volume in said spacer matrix. 4. - Medium according to any of the preceding claims, wherein the Mott material is at a temperature Tt in the range | (Tt-Ta-mc) | / Ta-mc <20%, where the temperature Ta-mc is the corresponding to the A-MC transition of the material. 5. -Average according to the previous claim, where Mott material is at a temperature TT in the range | (TT-Ta-mc) | / Ta-mc <10%. 6. - Medium according to the preceding claim, wherein the Mott material is at a temperature TT in the range | (TT-Ta-mc) | / Ta-mc <5%. 7. - Medium according to any of the preceding claims, wherein the Mott material comprises one or more of the following: simple oxides, cobaltites, perovskites, magnetite, carbon nanostructures. 8. - Medium according to the preceding claim, comprising one or more of the following compounds: NiO, VO2, CuO, LaCoO3, PrCoO3, NdCoO3, SmCoO3, EuCoO3, GaCoO3, La0,9Sr0,1CoO3, SmNiO3, EuNi3, LaMnO3, LaMnO3, LaMnO3 , La1-XSrXNiO3. 9. - Medium according to any of the preceding claims, comprising doping substances or active phases in the environment of the Mott material. 5 10 fifteen twenty 25 30 10. - Medium according to any of the preceding claims, wherein the spacer matrix comprises a non-conductive material transparent to electromagnetic radiation. 11. - Medium according to any of the preceding claims, wherein the spacer matrix comprises a structure based on one or more of the following: cordierite monoliths, KBr, acidic and calcium zeolites, alumina, TiO2, CeO2, a formally electron-free metal oxide DOF in the Valencia layer, a dielectric fluid. 12. - Medium according to any of claims 1-9, wherein the spacer matrix comprises a material capable of absorbing electromagnetic radiation. 13. - Medium according to the preceding claim, wherein the material capable of absorbing electromagnetic radiation comprises a sodium zeolite. 14. - Method of heating materials comprising the application of non-ionizing radiation to a heating medium according to any of claims 1-13. 15. - Method according to claim 14, wherein the radiation is in the microwave and / or radiofrequency frequency range. 16. - Method according to any of claims 14-15, wherein the particles of the Mott material are arranged inhomogeneously in the spacer matrix, so that thermal gradients are generated before the application of radiation, to produce a surface or volumetric heating selective. 17. - Method according to any of claims 14-16, wherein the application of the radiation is operated pulsed by absorption-irradiation. 18. - Use of a heating medium according to any of claims 1-13 in a catalytic reactor.