WO2025017138A1 - A method for magnetic conversion from paramagnetic to ferromagnetic of microporous metal materials, converted ferromagnetic microporous metal material and uses thereof - Google Patents

A method for magnetic conversion from paramagnetic to ferromagnetic of microporous metal materials, converted ferromagnetic microporous metal material and uses thereof Download PDF

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Publication number
WO2025017138A1
WO2025017138A1 PCT/EP2024/070425 EP2024070425W WO2025017138A1 WO 2025017138 A1 WO2025017138 A1 WO 2025017138A1 EP 2024070425 W EP2024070425 W EP 2024070425W WO 2025017138 A1 WO2025017138 A1 WO 2025017138A1
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Prior art keywords
microporous
stainless
steel material
ferromagnetic
filters
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French (fr)
Inventor
Josep NOGUÉS SANMIQUEL
Borja SEPÚLVEDA MARTÍNEZ
Maria Jose de Monserrat Esplandiu Egido
Arnau Fons
Aritz Lafuente
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Consejo Superior de Investigaciones Cientificas CSIC
Institucio Catalana de Recerca i Estudis Avancats ICREA
Institut Catala de Nanociencia i Nanotecnologia ICN2
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Consejo Superior de Investigaciones Cientificas CSIC
Institucio Catalana de Recerca i Estudis Avancats ICREA
Institut Catala de Nanociencia i Nanotecnologia ICN2
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
    • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
    • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
    • H01F1/14—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys
    • H01F1/147—Alloys characterised by their composition
    • H01F1/14708—Fe-Ni based alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1244—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties characterised by the heat treatment
    • C21D8/1272—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite

Definitions

  • the present invention relates to methods for magnetic conversion of microporous metal materials from paramagnetic to ferromagnetic.
  • the present invention relates to a conversion method for changing (converting) magnetic behavior of microporous austenitic stainless-steel materials from paramagnetic to ferromagnetic.
  • the invention also relates to a ferromagnetic microporous stainless-steel filter obtainable thereby as well as to their uses, particularly in applications where an efficient heater/heat generator or heat exchanger for fluids, gas or liquid, is required.
  • a widespread method to obtain magnetic (martensitic) steel from non-magnetic (austenitic) steel is by a quenching process of the later from high temperatures.
  • the process is usually based on heating the austenitic steel to high temperature, typically around 1000 °C, and then rapidly cooling the material (i.e., quenching) to room temperature, or below, usually using a liquid, like water, oil or liquid nitrogen.
  • austenitic stainless steel is known for its excellent corrosion resistance and formability, but it has relatively low strength compared to other steel types.
  • quenching By subjecting the austenitic stainless steel to a specific heat treatment known as quenching, it can be transformed into a harder and stronger phase called martensite.
  • this transformation has not been applied to enhance the magnetic heating efficiency.
  • the large size of the microstructures in porous stainless makes that the optical heating efficiency is also low.
  • the inert Argon atmosphere in such conditions (combined mechanical compression and high annealing temperature) is needed to prevent oxidation.
  • this cycle of annealing is responsible of enabling to delubricate/debind, reduce of oxides, remove impurity in the sample and provide the uniform temperature distribution throughout heating process. Mechanical features of treated stainless-steel samples are examined.
  • US11478761 B2 discloses an annealing of long stainless-steel fibers, “long fibers” is emphasized, of diameter 25-30 microns in a mesh that are woven (as in a textile) to leave very small gaps between fibers, through which the fluids can penetrate.
  • the magnetic transformation is not studied.
  • the purpose of these meshes is not for magnetic or optical heating.
  • the surface mesh is superhydrophilic and used to separate oil and water in an oil and water mixture or for photocatalytic degradation of methylene blue and other organic contaminants.
  • a wavelength or range of wavelengths of light is selected based on the absorbance of the specific contaminant to be degraded. No thermal treatment for this purpose is mentioned therein.
  • CN107164704 discloses again an annealing at controllable atmosphere. This patent describes porous material, mechanical properties (elastic young modulus) close to bone. The purpose of CN’s patent is a microstructured stainless steel with mechanical properties close to those of bone. The stainless-steel powder and a porogen agent are sintered at high temperatures and under a reductive H2 atmosphere to prevent the oxidation of the sintered grains and keep metallic structure. CN107164704 does not describe wetting capabilities or optical or magnetic heating capabilities of the filter by self-heating for applications of the present invention.
  • the present invention is also concerned with providing improved heat generators that are capable of improving heat exchange between fluids.
  • the present invention was made in view of the prior art described above, and a first aspect of the invention is to provide a method for magnetic conversion of a microporous metal material from paramagnetic to ferromagnetic.
  • the microporous metal material comprises a microporous austenitic stainless-steel material as raw material
  • the method comprises the steps of: i) thermal annealing the microporous austenitic stainless-steel material as raw material, wherein thermal annealing comprises heating the microporous austenitic stainless- steel material to an annealing temperature comprised of between 500°C and 1 ,200°C, and maintaining the microporous stainless-steel material at the annealing temperature for a period, and then, ii) cooling down the annealed microporous stainless-steel material to room temperature, wherein the cooling is carried out by allowing the annealed microporous stainless-steel material to cool under room temperature, so the annealed microporous stainless-steel material is slowly cooled, or alternatively wherein the cooling down is carried out by immersing the annealed microporous stainless-steel material in an organic solvent, prefer
  • the annealing process of step i) is carried out under atmospheric conditions. This means that in this step no gas treatment is carried out, i.e. , no inert gases or other components are used during annealing. Compared to Nur Acar et al., annealing in accordance with the method of the present invention does not require mechanical compression nor inert atmosphere. Additionally, the annealing is performed in a single step, no cycles are carried out, only heating and holding over a period of time. In addition, lower annealing temperatures and a shorter total time can be used without the characteristics of the filter obtained deviating from its purpose.
  • the microporous austenitic stainless-steel filter converted to ferromagnetic of the invention has the ability to be self-heated by magnetic induction, which is not possible with the material described by Nur Acar et al.
  • the annealing of the invention does not require controllable atmospheres, it is performed under atmospheric conditions. Additionally, the austenitic stainless-steel microporous filter converted to ferromagnetic of the invention is capable of functioning as a heat generator (self-heating) by magnetic induction or optical absorption and/or at the same time as a heat exchanger for fluids, gases or liquids, which pass through the heated filter.
  • microporous metal material as raw material consists of a microporous austenitic stainless-steel filter. Still more preferable, it is a microporous austenitic stainless-steel filter, for example, an Amespore® disc filter that is commercially available in the state of the art. More data about the Amespore® disc filters can be found at the web page https://amespore.com/en/find-out-your-amespore-filter/stainless- steel-disc-filters/, whose content is briefly included in Appendix of this invention.
  • the method of the first aspect is capable of: changing the magnetic behavior from paramagnetic to ferromagnetic of a microporous austenitic stainless-steel material; increasing the surface area due to the nanostructuring of the surface during the thermal annealing (step i) and the cooling (quick and slow) down the annealed material (step ii); enhancing the magnetic specific absorption rate (SAR) and/or optical absorption efficiency in a wide light spectral range.
  • SAR magnetic specific absorption rate
  • Preferable annealing temperature (step i) is comprised of between 800°C and 1 ,100°C, more preferable between 940°C and 960°C.
  • the annealing is performed in a conventional furnace, such as a tube or tubular furnace.
  • the thermal annealing (step i) of the invention can be also carried out in an inductive furnace.
  • the annealing is much faster heating and much lower energy consumption than in a conventional tubular furnace.
  • preferable thermal annealing step i) is carried out in an inductive furnace.
  • the filter or metal powder In this annealing embodiment by an inductive furnace, the filter or metal powder must be sintered. Metal powder or filter not been sintered have very low efficiency of inductive heating and not suitable for inductive furnace.
  • Preferable organic solvent (step ii) consists of isopropanol.
  • Other pure organic solvents can be used as well.
  • the microporous austenitic stainless-steel material as raw material includes an average pore diameter equal to or lower than 80 pm, preferably an average pore diameter equal to or lower than 25 pm, more preferable equal to or lower than 10 pm.
  • the microporosity can be alternatively measured by the average particle size of the particles delimiting the pores in the microporous material. Therefore, the microporous austenitic stainless-steel material as raw material includes an average particle size lower than 100 pm, preferably an average particle size lower than 50 pm.
  • microporous austenitic stainless-steel material as raw material can be initially cleaned by immersing the microporous austenitic stainless-steel material in an organic solvent bath under ultra-sonication and then drying with a nitrogen flow.
  • organic solvent is isopropanol.
  • the microporous austenitic stainless-steel material as raw material can be further initially chemically treated to remove metals and oxides that can be present at their surface, whereby the availability of the iron at the surface of the raw material be assured.
  • the chemical treatment is carried out before the thermal annealing of step i), and comprises chemically treating the microporous austenitic stainless-steel material as raw material in an acidic mixture comprising hydrochloride acid and nitric acid in a volume ratio from 3:1 to 1 :3.
  • Preferable acidic mixture is in an equimolar ratio 1 :1.
  • the present invention also provides a ferromagnetic microporous stainless- steel material obtainable by the method defined in the first aspect of the invention.
  • microporous stainless-steel material is ferromagnetic, and the surface nanostructured.
  • the microporous filters obtained according to the first aspect of the present invention are composed of a 3D matrix of stainless-steel microparticles.
  • the authors of the present invention have found that, in addition to converting from paramagnetic to ferromagnetic, the obtainable microporous stainless-steel material is superhydrophilic or superhydrophobic in accordance with the way of cooling (slow or quick). In fact, when the cooling step is carried out by allowing to slow cool down under room temperature, then the obtainable ferromagnetic microporous stainless-steel material is also hydrophilic.
  • the cooling step is carried out at by immersing in a pure organic solvent bath that is at room temperature, then the obtainable ferromagnetic microporous stainless-steel material is also hydrophobic.
  • the surface of the obtainable ferromagnetic microporous stainless-steel material is also varied in accordance with the way to cool down, slow or quick.
  • the surface of the ferromagnetic microporous stainless-steel material is all of iron oxides
  • the surface of the ferromagnetic microporous stainless- steel material is of carbon.
  • the material is a filter.
  • the ferromagnetic microporous stainless-steel filter of the second aspect compositionally, the filter’s surface is either all of iron oxides, or alternatively, is of carbon.
  • the present invention is directed to the use of the ferromagnetic microporous stainless-steel material obtainable by the method defined in the first aspect as a heat generator.
  • the filter working as a heat generator is also capable of exchanging the heat generated to the fluids that pass through it with improved efficiency. Therefore, its use is also as a fluid heat exchanger.
  • Stainless steel microporous ferromagnetic material of the invention as a heat generator and as a heat exchanger is capable of saving energy.
  • the temperature reached is around 45% higher, and close to 75% higher using optical heating. Heating can be done wirelessly, that is, without direct contact between the ferromagnetic material and the energy source, either magnetic or optical.
  • the heat generator is able to be heated by magnetic induction and/or by optical absorption to act as a very efficient heat exchanger in fluids flowing therethrough.
  • the method in accordance with the first aspect of the present invention can be also an alternative to fabricate ferromagnetic microporous stainless-steel materials.
  • the method of the first aspect of the present invention encompasses a method to fabricate ferromagnetic microporous stainless-steel filters, in which iron powder compacted as raw material is subjected to the annealing treatment described in step i), and then cooling down as described in step ii) to obtain a ferromagnetic microporous stainless-steel filter.
  • the annealing temperatures of step i) may also allow compacted iron powder to be sintered as a raw material, so that the obtainable ferromagnetic microporous stainless-steel material may also be sintered.
  • the ferromagnetic microporous stainless-steels material can be used in applications where an efficient heat exchanger for fluids is required. Applications such as air disinfection, thermo- catalytic processes, or water heating among others are susceptible to be performed using a ferromagnetic microporous stainless-steel material or filter obtainable in accordance with the first aspect of the present invention as an efficient heat exchanger.
  • annealing has the common meaning in the art that is a heat treatment process which alters the microstructure of a material to change its mechanical or electronic properties.
  • annealing is used to reduce hardness, increase ductility, and help eliminate internal stresses.
  • a typical annealing process includes three main stages. During the annealing process, the metal is heated to a specific temperature where recrystallization can occur. At this stage, any defects caused by deformation of the metal are repaired. The metal is held at that temperature for a fixed period, then cooled down to room temperature.
  • austenitic stainless steel means a specific type of stainless-steel alloy.
  • Stainless steels can be classified by their crystalline structure into four main types: austenitic, ferritic, martensitic, and duplex.
  • Austenitic stainless steels possess austenite as their primary crystal structure (face-centered cubic). This austenite crystal structure is achieved by sufficient additions of the stabilizing elements of austenite: nickel, manganese, and nitrogen. Their crystalline structure makes the austenitic steels essentially non-magnetic.
  • microporous austenitic stainless-steel material is a filter, sintered or not, or metal powder.
  • an average size of less than 50 pm is defined herein as the average particle size, as measured by any conventional means such as dynamic light scattering or microscopy, of a sampling of particles wherein the average is less than about 50 micrometers in diameter, assuming for purposes of the calculation that the irregular particles have an approximate diameter, that is, that they are approximately spherical. This assumption is purely for the calculation of average particle size, due to the particles often being non-spherical in shape. Methods used to measure particle size include dynamic light scattering, scanning electron microscopy or transmission electron microscopy (see also Appendix)
  • the expression “atmospheric conditions” means the atmospheric pressure and the ambient or room temperature, usually 1 atmosphere of pressure and 23°C of temperature.
  • the ambient or room temperature includes values from 15°C to 35°C.
  • Figure 1 depicts the morphology of microporous austenitic stainless-steel filters (Amespore®) commercially available in the art.
  • Figure 2 depicts the morphology of a ferromagnetic microporous stainless-steel filter obtained in Example 4 using slow cooling (blue surface filter).
  • Figure 3 depicts the morphology of a ferromagnetic microporous stainless-steel filter obtained in Example 4 using rapid cooling, in two different magnification scales, (a) 8,000 and (b) 60,000 (black surface filter).
  • Figure 4 depicts a bar graph of the effect on porosity of magnetic induction heating (132 kHz, 500 Oe) of 20 L/min constant airflow through treated filters (striped columns), treated with an annealing temperature of 950°C and with rapid or slow cooling, versus untreated filters, porosity varies for average pore diameters of 5 pm, 10 pm, 25 pm, 40 pm, 60 pm and 80 pm.
  • Figure 5 depicts a bar graph of the effect of the annealing temperature, between 650°C and 1 ,000°C, on the magnetic induction heating (132 kHz, 500 Oe) after 2 minutes (100 W) in a constant airflow of 20 L/min flowing through filters of average pore diameter of 10 pm.
  • the filters were treated with annealing at different temperatures during 1h and thereafter rapid or slow cooled.
  • Figure 6 depicts the obtained data of the effect of the period maintaining to the annealing temperature on the magnetic induction heating (132 kHz, 500 Oe) after 2 minutes (100 W) in filters of average pore diameter of 10 pm.
  • the filters were treated with an annealing temperature of 950°C at different periods of time and thereafter rapid or slow cooled.
  • Figure 7 depicts four graphs of the magnetic induction heating effect for four treated filters of different average pore diameters (5 pm, 60 pm), subjected to different magnetic frequencies (78 kHz, 404 kHz) and magnetic field amplitude of 125 Oe.
  • the squares show the data obtained with thermally treated filters and with rapid or slow cooling, and the circles show the data obtained with untreated filters.
  • Figure 8 depicts four graphs of the magnetization reversal loop effect for four treated filters of different average pore diameters (5 pm, 60 pm), subjected to different magnetic frequencies (78 kHz, 404 kHz). Black cycles show the data obtained with thermally treated filters using an annealing temperature of 950°C and with rapid or slow cooling, and grey circles show the data obtained with untreated filters.
  • Figure 10 depicts a graph of the effect of optical heating (917 nm wavelength and 15 W power) on treated filters (black line) versus untreated filters (grey line).
  • Figure 11 depicts a graph that compares the magnetic heating efficiency for untreated filters versus treated filters with a thermal annealing in a tubular furnace and then slow or quick cooling down.
  • Figure 12 depicts several pictures of disc filters Amespore® to comparatively show the untreated filters versus treated filters of Example 6.
  • Figure 13 depicts a graph of the comparative results obtained by thermal annealing (step i) using tubular furnace versus inductive furnace.
  • Figure 14 depicts a graph of the effect of optical heating that compares filters obtained by slow cooling versus quick cooling and also versus untreated filters.
  • Figure 15 depicts SEM images by SEM QUANTA 650 FEG, with a voltage of 5kV of untreated and treated filters with slow and quick cooling: untreated filters (first column); with slow cooling (second column) and fast cooling (third column).
  • Figure 16 depicts SEM images by SEM QUANTA 650 FEG, with a voltage of 5kV, of treated filters using an inductive furnace at the thermal annealing and, then, slow and quick cooling down.
  • Figure 17 depicts the XPS spectra (X-ray photoemission spectroscopy) obtained with SPECS PHOIBOS 150 hemispherical energy analysers, under ultra-high vacuum conditions. It shows the comparative of the C1s, O1s y Fe 2p spectra for the quick and slow cooled filters versus untreated filters.
  • Figure 18 depicts the Contact angles determined with the Drop Shape Analyzer DSA25S (KRUSS) of untreated filter that is hydrophilic (final contact angle es 0°), first image, slowly cooled filter that is superhydrophilic, the water droplet is absorbed very fast by the filter network (final contact angle es 0°), second image, and the fast cooled filter (I PA) that shows superhydrophobic behavior (144.1°), third image.
  • KRUSS Drop Shape Analyzer DSA25S
  • the present invention provides a method for magnetic conversion of microporous metal materials from paramagnetic to ferromagnetic.
  • the present invention relates to a conversion method for changing magnetic behavior of microporous austenitic stainless-steel materials from paramagnetic to ferromagnetic.
  • the conversion method is suitable for changing the austenitic stainless steels which is paramagnetic to a ferromagnetic behavior.
  • preferable thermal annealing step i) is carried out in an inductive furnace.
  • the magnetic conversion method of the first aspect transforms their magnetic properties from paramagnetic to ferromagnetic and generates micro/nano-structuration of the pore surface ( Figures 2 and 3 (a), (b)).
  • the cooling step ii) (quick cooling) in an organic solvent (preferably, isopropanol) results in the deposition of a carbon layer on the material (filter) surface.
  • an organic solvent preferably, isopropanol
  • the ferromagnetic behavior of the converted filter enables a high improvement of the magnetic induction heating efficiency. Furthermore, the nanostructured surface and the deposited carbon layer enable boosting the optical heating efficiency.
  • the conversion method also provides a powerful protection for corrosion in both air and water conditions, even at very high temperatures.
  • the potential applications include air disinfection, methane pyrolysis for generation of H2 without CO2 emission, other thermo-catalytic processes in gas o liquid phase, and even efficient decentralized water heating.
  • the obtainable ferromagnetic microporous stainless-steel reveals an efficient heating of fluids by magnetic induction and/or optical absorption.
  • the filters obtainable using the method of the first aspect of the present invention were surprisingly improved compared to the untreated filters, as it will be discussed below.
  • the effect of the porosity on the magnetic induction heating was surprisingly improved using an average pore diameter equal to or lower than 25 pm.
  • the filters with smaller average pore diameters, that is of about 5 pm reached a still higher temperature in the filter, even above of 400°C, compared to filters having a higher average pore diameter, particularly higher than the threshold of 25 pm.
  • heating of the porous material by magnetic induction is further improved with an average pore diameter equal to or lower than 25 pm.
  • the maximum temperature, expressed in Celsius degrees, achieved in filters heated by magnetic induction is obtained by using an annealing temperature range between 900°C and 960°C.
  • heating of materials by magnetic induction is further improved when materials are thermally treated (step i) using annealing temperatures from 900°C to 960°C, independent of the benefits derived from cooling down (step ii) by a slow or quick way.
  • the effect of the period maintaining the annealing temperature on the magnetic induction heating of the annealed filter does not change too much after the first 15 minutes, and does not affect or even goes down after 1 hour maintaining the annealing temperature.
  • the filters obtained by thermal annealing at 950°C and either slow or quick colling were subjected to magnetic inductive heating (power 100W).
  • the maximum temperature reached at the filters was collected after 2 minutes heating by magnetic induction at these conditions.
  • the preferable period maintaining the annealed material to the annealing temperature (step i) is equal to or lower than 1 hour, preferably equal to or lower than 30 min.
  • the optimal magnetic field frequency for magnetic induction heating depends on the pore size and, therefore on the size of the particles that form the porous structure.
  • ferromagnetic microporous stainless-steel material obtainable by the method defined in the first aspect of the invention as a heat generator as well as in use as a heat exchanger in fluids is preferable having a small average pore diameter, preferably equal to or lower than 25 pm, still more preferable below 10 pm, and combined with low magnetic frequencies, preferably from 70 kHz to 200 kHz.
  • the heating efficiency increases with the frequency.
  • higher magnetic fields can be obtained at lower frequencies using similar electric energy consumption, and the magnetic heating efficiency drastically increases with the amplitude of the magnetic field.
  • the magnetization reversal loops of the four filters confirmed the conversion from paramagnetic to ferromagnetic of the austenitic microporous stainless-steel filters.
  • the ellipsoidal magnetization reversal loops with positive slope reflect the effect of the ferromagnetism in the treated filters (black circle), compared to the ellipse with negative slope of the untreated samples (grey circle), which is sign of the Eddy currents and diamagnetic-like behavior under alternating magnetic fields.
  • the difference in amplitude of the loops is correlated with the higher heating efficiency in the treated samples.
  • the magnetic induction heating efficiency increases as long as the magnetic field amplitude is increased.
  • the induced temperature change in the treated filter subjected to an optical heating is increased compared with the untreated filter using a laser light beam with 917 nm wavelength and 15 W power.
  • the initial step was cleaning the filters in isopropanol for 5 minutes under ultra-sonication and dried with a nitrogen flow.
  • Example 1 The cleaned filters of Example 1 were submitted to a chemical treatment by immersing the filters in a mixture of HCkHNCh (1 :1), then rinsed with isopropanol for 5 minutes and dried with a nitrogen flow.
  • the time (seconds) in the acid immersion was varied.
  • the time in the acid mixture increases as the average pore diameter is reduced.
  • the filters were immersed for 45 seconds for the average pore diameter of 5 pm, 10 seconds for the average pore diameter of 10 pm, and 5 seconds for the average pore diameters of 25 pm, 40 pm, 60 pm and 80 pm.
  • the chemical treatment removes the chromium and nickel oxide layers usually present in the stainless-steel surface of the filters.
  • a tube furnace (Lindberg Blue Mini-MiteTMTF55030A-1) was used for the high temperature treatment.
  • the filters were introduced in a quartz tube.
  • the thermal annealing step i) was performed by heating the filters of the above average pore diameters (with the ends of the tube uncapped) at a rate of 1 °C/s until an annealing temperature of 950°C was reached. The filters were then kept at this annealing temperature for 1 hour.
  • Example 3 The filters of Example 3 were cooled down to room temperature.
  • step ii) The cooling of step ii) was carried out in two different ways.
  • the first way of cooling down was done by removing the quartz tube from the furnace including the filters inside it and allowing to cool it to room temperature, yielding dark blue filters of morphology shown in Figure 2.
  • Example 5 Surface composition of ferromagnetic microporous stainless-steel filter
  • the surface composition of the filters obtained in Example 4 were compared by SEM-EDX. The results showed that the surface composition of the slowly cooled down filters became all iron oxides as shown in Table 1 , second column, below. The results also showed that the surface composition of the rapidly cooled down filters in the isopropanol bath caused carbon incorporation into the surface composition as shown in Table 1 , third column, below.
  • the blue filters obtained by slow cooling were highly hydrophilic, whereas the black carbon coated filters obtained by quick cooling were highly hydrophobic.
  • Example 6 Method for magnetic conversion from paramagnetic to ferromagnetic using a tubular furnace in the thermal annealing
  • Thermal annealing The filters were heated with a heating ramp of 60°C/min until reaching 950°C, then holding at such temperature for 1 hour. The thermal annealing was performed under atmospheric conditions, air at standard conditions.
  • Cooling down Two different ways of cooling down were performed using the half of the filters annealed in each way of cooling down.
  • the first way of cooling down was done by removing the quartz tube from the furnace including the filters inside it and slowly cooled with a ramp of -60°C/min, allowing to cool they to room temperature, yielding dark blue filters of morphology shown in Figure 15 (second column).
  • the untreated filters “Virgin”, were microporous austenitic stainless steel sintered filters having an average pore size of 10 pm.
  • the treated filters are the filters obtained in Example 6.
  • Example 6 the quick cooled by immersion was assayed using different organic pure solvents: pure IPA, MetOH, EtOH, or acetone, all at room temperature and standard conditions.
  • Comparative assays were performed to compare the effect of optical heating when the filters were obtained by cooling down using slow cooling versus quick cooling and also the optical heating of untreated filters.
  • the untreated filters “Virgin” and treated filters were illuminated with a collimated laser beam at a wavelength 808 nm and power of 15W for 2 min to induce the optical heating under atmospheric conditions (static conditions, no air flow).
  • the SEM images show that the surface of the untreated filters (first column) have stainless steels grains with smooth surface. Contrarily, the annealing treatment in conventional furnace and slow cooling (second column) increases the surface roughness yielding submicron oxidized structures with sharp angles and rather flat sides. And, in contrast, the annealing and fast cooling (third column) in organic solvent provides a carbon coating formed of submicron (100-500 nm) carbon nanoparticles homogeneously distributed on the surface.
  • the microporous filters according to the present invention are composed of a 3D matrix of stainless-steel microparticles, which are sintered to achieve a mechanically robust and electrically connected microporous 3D structure, having tunable porosity by means of the size of the microparticles.
  • the authors of the present invention have guantified the hydrophilicity (superhydrophilic behavior of the filters). In the “virgin” filter, it takes 0.5 seconds for the water droplet to be absorbed (hydrophilic), while after slow cooling treatment (superhydrophilic), it takes 0.25 seconds.
  • the filters were microporous austenitic stainless steel sintered filters with average pore size of 10 pm, either untreated “Virgin” or treated in tubular furnace and then either slowly cooled in atmosphere or fast cooled by immersion in IPA).
  • Example 7 Method for magnetic conversion from paramagnetic to ferromagnetic using a inductive furnace in the thermal annealing - Raw material- Microporous austenitic stainless-steel sintered filters with average pore size of 10pm.
  • the filters were heated to reach a temperature of 950°C, then holding at such temperature for 1 hour.
  • the first way of cooling down was done by removing the filters from the inductive furnace and allowing to cool they to room temperature in atmospheric conditions with a rate of -90°C/min, yielding filters of morphology shown in Figure 16 (first column).
  • step i the thermal annealing (step i) in an inductive furnace has advantages with respect to the thermal annealing in a tubular furnace. It is much faster heating and much lower energy consumption than in a conventional tubular furnace.
  • FIG. 16 shows that when the thermal annealing treatment is performed in a magnetic induction furnace and slow cooling, the surface of stainless-steel grains that form the filters is modified yielding a 3D nanoporous structure of oxidized metal (pores size 100-600 nm). Using the same thermal annealing treatment condition but with quick cooling down in IPA, yields also a porous surface but with higher porosity (ca. 1 micron), that is covered by pyrolytic carbon.
  • the microporous filters according to the present invention are composed of a 3D matrix of stainless-steel microparticles, which are sintered to achieve a mechanically robust and electrically connected microporous 3D structure, having tunable porosity by means of the size of the microparticles.
  • the material used to produce these disc filters is AISI 316L or AISI 316 stainless steel. They are made (SSU) by uniaxial compaction of powder in a rigid tool with the negative shape of the part, and then sintered. The pore size is adjusted by changing the compaction pressure and/or the particle size of the powder.
  • the diameter or width of these filters is determined by the tooling, while their height is adjustable from approximately 1 ,5mm to the maximum length indicated in the tables.
  • the material used to produce these filters is AISI 316L or AISI 316 stainless steel. They are made by uniaxial compaction of powder in a rigid tool with the negative shape of the part, and then sintered. The pore size is adjusted by changing the compaction pressure and/or the particle size of the powder.
  • Porous AISI 316L stainless steel Characteristics in uniaxial compaction
  • Intermediate grades can be manufactured on demand.
  • AmesPore® filters The characteristics and properties of AmesPore® filters are determined in the laboratory by the following tests:
  • Bubble point according to ISO 4003: 1977 determined as the appearance of the first bubble or continuous flow.
  • Chemical composition including carbon, oxygen and nitrogen content. Corrosion resistance.
  • Mechanical properties such as radial tensile strength and shear strength.
  • Metallography and fractography including an electron scanning microscope. Advanced measurement equipment.
  • AmesPore® porous metallic filters and components are sintered stainless steel or bronze parts with high porosity (between 25% and 60% by volume).
  • the preform may be created by gravity filling, by uniaxial or isostatic pressing or by extrusion, depending on the material to be shaped, the desired porosity and the component geometry.
  • Sintering consists in heating the preform to a temperature lower than the melting point of the base metal (between 700°C and 1300°C) under carefully controlled conditions of atmosphere and time. The temperature causes the powder particles to weld together.
  • the result of this process is a structurally functional metallic part with a controlled microporosity level.

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Abstract

The present invention is directed to a new method for magnetic conversion from paramagnetic to ferromagnetic of a raw material being composed of microporous austenitic stainless-steel. The method comprises an annealing treatment, and then a cooling down treatment. The invention is also directed to a converted ferromagnetic microporous stainless-steel material obtainable thereby with hydrophilic or with hydrophobic properties, as well as to their uses as heat generator for use in applications where an efficient heat exchanger for fluids is required. The heat generator is provided by means of magnetic field and/or optical absorption.

Description

A METHOD FOR MAGNETIC CONVERSION FROM PARAMAGNETIC TO FERROMAGNETIC OF MICROPOROUS METAL MATERIALS, CONVERTED FERROMAGNETIC MICROPOROUS METAL MATERIAL AND USES THEREOF
Field of the invention
The present invention relates to methods for magnetic conversion of microporous metal materials from paramagnetic to ferromagnetic. In particular, the present invention relates to a conversion method for changing (converting) magnetic behavior of microporous austenitic stainless-steel materials from paramagnetic to ferromagnetic.
The invention also relates to a ferromagnetic microporous stainless-steel filter obtainable thereby as well as to their uses, particularly in applications where an efficient heater/heat generator or heat exchanger for fluids, gas or liquid, is required.
Background of the invention
Most of the heating systems for fluids are based on electrical resistances that are placed inside the fluid to heat it via Joule effect. Also, there is a possibility to directly heat metal pipes via Joule heating. This is used in heaters that can be applied to heat tap or shower water.
However, the heating of the fluids in both cases is not efficient due to a poor heat transfer from the Joule heated conductor to the fluid. To increase the heating efficiency and the heat transfer capability porous materials can be used. For example, a Ni foam heated by Joule effect has been employed to heat and disinfect air. [L Yu et al: Catching and killing of airborne SARS- CoV-2 to control spread of COVID-19 by a heated air disinfection system, Materials Today Physics 15, 100249 (2020)].
Moreover, these heating systems require electrical contacts to the heating resistance or to the metal pipe, which complicates the design and increases the costs. To reduce the complexity, wireless heating of a porous structure inside a fluid pipeline could be achieved by magnetic induction or optical heating. In this context, porous stainless-steel structures can provide a cost-effective alternative to combine wireless magnetic heating and/or optical heating, and to efficiently transfer the heat to the fluid going through the porous structure. However, the majority of the porous stainless-steel structures are composed of non-ferromagnetic austenitic microstructures, whose magnetic induction heating efficiency is limited. The conversion of the austenitic to a martensitic phase stainless-steel can modify the magnetic behavior to exhibit ferromagnetism at room temperature. A widespread method to obtain magnetic (martensitic) steel from non-magnetic (austenitic) steel is by a quenching process of the later from high temperatures. [M. Wendler, C. Ullrich, M. Hauser, L. Kruger, O. Volkova, A. WeiB, J. Mola, Quenching and partitioning (Q&P) processing of fully austenitic stainless steels. Acta Mater. 133 (2017) 346-355.] The process is usually based on heating the austenitic steel to high temperature, typically around 1000 °C, and then rapidly cooling the material (i.e., quenching) to room temperature, or below, usually using a liquid, like water, oil or liquid nitrogen. The transformation of austenitic stainless steel into the martensitic phase is typically used to achieve an increase in its hardness and strength. Austenitic stainless steel is known for its excellent corrosion resistance and formability, but it has relatively low strength compared to other steel types. By subjecting the austenitic stainless steel to a specific heat treatment known as quenching, it can be transformed into a harder and stronger phase called martensite. However, this transformation has not been applied to enhance the magnetic heating efficiency. On the other hand, the large size of the microstructures in porous stainless makes that the optical heating efficiency is also low.
Nur Acar et al. in “Effect of pressure on the magnetic and structural properties ofX2CrNiMo17- 12-2 austenitic stainless steel prepared by powder metallurgy method’, Journal of Molecular Structure, discloses the effect of pressure in a stainless-steel powder which is treated combining a mechanical compression of 400-600 MPa with a thermal treatment in inert Argon atmosphere. No final porous structure is specifically described. Additionally, the complete annealing process takes about 10 hours, in which process a maximum temperature of 1200°C is reached. Figure 1 reveals that the samples were subjected to several thermal treatments, first at 600°C for 30 minutes and second at 900°C for 30 minutes before sintered at 1200 °C for 1hour. The inert Argon atmosphere in such conditions (combined mechanical compression and high annealing temperature) is needed to prevent oxidation. As it is described therein, this cycle of annealing is responsible of enabling to delubricate/debind, reduce of oxides, remove impurity in the sample and provide the uniform temperature distribution throughout heating process. Mechanical features of treated stainless-steel samples are examined.
On the other hand, US11478761 B2 discloses an annealing of long stainless-steel fibers, “long fibers” is emphasized, of diameter 25-30 microns in a mesh that are woven (as in a textile) to leave very small gaps between fibers, through which the fluids can penetrate. The magnetic transformation is not studied. The purpose of these meshes is not for magnetic or optical heating. The surface mesh is superhydrophilic and used to separate oil and water in an oil and water mixture or for photocatalytic degradation of methylene blue and other organic contaminants. A wavelength or range of wavelengths of light is selected based on the absorbance of the specific contaminant to be degraded. No thermal treatment for this purpose is mentioned therein.
CN107164704 discloses again an annealing at controllable atmosphere. This patent describes porous material, mechanical properties (elastic young modulus) close to bone. The purpose of CN’s patent is a microstructured stainless steel with mechanical properties close to those of bone. The stainless-steel powder and a porogen agent are sintered at high temperatures and under a reductive H2 atmosphere to prevent the oxidation of the sintered grains and keep metallic structure. CN107164704 does not describe wetting capabilities or optical or magnetic heating capabilities of the filter by self-heating for applications of the present invention.
Therefore, there is still the need to provide an alternative to the prior art that resolves the shortcomings thereof.
There is still the need to find more efficient systems to heat flowing fluids in several applications, including air disinfection, thermo-catalytic processes, or water heating.
Therefore, the present invention is also concerned with providing improved heat generators that are capable of improving heat exchange between fluids.
Description of the invention
The present invention was made in view of the prior art described above, and a first aspect of the invention is to provide a method for magnetic conversion of a microporous metal material from paramagnetic to ferromagnetic.
The method is characterized in that the microporous metal material comprises a microporous austenitic stainless-steel material as raw material, and the method comprises the steps of: i) thermal annealing the microporous austenitic stainless-steel material as raw material, wherein thermal annealing comprises heating the microporous austenitic stainless- steel material to an annealing temperature comprised of between 500°C and 1 ,200°C, and maintaining the microporous stainless-steel material at the annealing temperature for a period, and then, ii) cooling down the annealed microporous stainless-steel material to room temperature, wherein the cooling is carried out by allowing the annealed microporous stainless-steel material to cool under room temperature, so the annealed microporous stainless-steel material is slowly cooled, or alternatively wherein the cooling down is carried out by immersing the annealed microporous stainless-steel material in an organic solvent, preferable isopropanol bath, that is at room temperature, so the annealed microporous stainless-steel material cools quickly as defined in claim 1.
The annealing process of step i) is carried out under atmospheric conditions. This means that in this step no gas treatment is carried out, i.e. , no inert gases or other components are used during annealing. Compared to Nur Acar et al., annealing in accordance with the method of the present invention does not require mechanical compression nor inert atmosphere. Additionally, the annealing is performed in a single step, no cycles are carried out, only heating and holding over a period of time. In addition, lower annealing temperatures and a shorter total time can be used without the characteristics of the filter obtained deviating from its purpose.
As will be described below, the microporous austenitic stainless-steel filter converted to ferromagnetic of the invention has the ability to be self-heated by magnetic induction, which is not possible with the material described by Nur Acar et al.
Compared to CN 107164704, the annealing of the invention does not require controllable atmospheres, it is performed under atmospheric conditions. Additionally, the austenitic stainless-steel microporous filter converted to ferromagnetic of the invention is capable of functioning as a heat generator (self-heating) by magnetic induction or optical absorption and/or at the same time as a heat exchanger for fluids, gases or liquids, which pass through the heated filter.
According to the present invention, preferable microporous metal material as raw material consists of a microporous austenitic stainless-steel filter. Still more preferable, it is a microporous austenitic stainless-steel filter, for example, an Amespore® disc filter that is commercially available in the state of the art. More data about the Amespore® disc filters can be found at the web page https://amespore.com/en/find-out-your-amespore-filter/stainless- steel-disc-filters/, whose content is briefly included in Appendix of this invention.
Surprisingly, the method of the first aspect is capable of: changing the magnetic behavior from paramagnetic to ferromagnetic of a microporous austenitic stainless-steel material; increasing the surface area due to the nanostructuring of the surface during the thermal annealing (step i) and the cooling (quick and slow) down the annealed material (step ii); enhancing the magnetic specific absorption rate (SAR) and/or optical absorption efficiency in a wide light spectral range.
Preferable annealing temperature (step i) is comprised of between 800°C and 1 ,100°C, more preferable between 940°C and 960°C.
Usually, the annealing is performed in a conventional furnace, such as a tube or tubular furnace.
Advantageously, the thermal annealing (step i) of the invention can be also carried out in an inductive furnace. The annealing is much faster heating and much lower energy consumption than in a conventional tubular furnace.
Therefore, preferable thermal annealing step i) is carried out in an inductive furnace.
In this annealing embodiment by an inductive furnace, the filter or metal powder must be sintered. Metal powder or filter not been sintered have very low efficiency of inductive heating and not suitable for inductive furnace.
Preferable organic solvent (step ii) consists of isopropanol. Other pure organic solvents can be used as well.
Preferably, the microporous austenitic stainless-steel material as raw material includes an average pore diameter equal to or lower than 80 pm, preferably an average pore diameter equal to or lower than 25 pm, more preferable equal to or lower than 10 pm.
The microporosity can be alternatively measured by the average particle size of the particles delimiting the pores in the microporous material. Therefore, the microporous austenitic stainless-steel material as raw material includes an average particle size lower than 100 pm, preferably an average particle size lower than 50 pm.
The microporous austenitic stainless-steel material as raw material can be initially cleaned by immersing the microporous austenitic stainless-steel material in an organic solvent bath under ultra-sonication and then drying with a nitrogen flow. Preferable organic solvent is isopropanol.
Optionally, the microporous austenitic stainless-steel material as raw material can be further initially chemically treated to remove metals and oxides that can be present at their surface, whereby the availability of the iron at the surface of the raw material be assured.
Optionally, the chemical treatment is carried out before the thermal annealing of step i), and comprises chemically treating the microporous austenitic stainless-steel material as raw material in an acidic mixture comprising hydrochloride acid and nitric acid in a volume ratio from 3:1 to 1 :3. Preferable acidic mixture is in an equimolar ratio 1 :1.
In a second aspect, the present invention also provides a ferromagnetic microporous stainless- steel material obtainable by the method defined in the first aspect of the invention.
Surprisingly, the obtainable microporous stainless-steel material is ferromagnetic, and the surface nanostructured. The microporous filters obtained according to the first aspect of the present invention are composed of a 3D matrix of stainless-steel microparticles.
The authors of the present invention have found that, in addition to converting from paramagnetic to ferromagnetic, the obtainable microporous stainless-steel material is superhydrophilic or superhydrophobic in accordance with the way of cooling (slow or quick). In fact, when the cooling step is carried out by allowing to slow cool down under room temperature, then the obtainable ferromagnetic microporous stainless-steel material is also hydrophilic.
On the other hand, when the cooling step is carried out at by immersing in a pure organic solvent bath that is at room temperature, then the obtainable ferromagnetic microporous stainless-steel material is also hydrophobic.
Compositionally, the surface of the obtainable ferromagnetic microporous stainless-steel material is also varied in accordance with the way to cool down, slow or quick. In the first way (slow), the surface of the ferromagnetic microporous stainless-steel material is all of iron oxides, and in the second way (quick) the surface of the ferromagnetic microporous stainless- steel material is of carbon.
In a preferable embodiment, the material is a filter.
Thus, the ferromagnetic microporous stainless-steel filter of the second aspect, compositionally, the filter’s surface is either all of iron oxides, or alternatively, is of carbon.
In a further aspect, the present invention is directed to the use of the ferromagnetic microporous stainless-steel material obtainable by the method defined in the first aspect as a heat generator.
In addition, the filter working as a heat generator is also capable of exchanging the heat generated to the fluids that pass through it with improved efficiency. Therefore, its use is also as a fluid heat exchanger.
Stainless steel microporous ferromagnetic material of the invention as a heat generator and as a heat exchanger is capable of saving energy. Using a magnetic field, the temperature reached is around 45% higher, and close to 75% higher using optical heating. Heating can be done wirelessly, that is, without direct contact between the ferromagnetic material and the energy source, either magnetic or optical.
Advantageously, the heat generator is able to be heated by magnetic induction and/or by optical absorption to act as a very efficient heat exchanger in fluids flowing therethrough.
The method in accordance with the first aspect of the present invention can be also an alternative to fabricate ferromagnetic microporous stainless-steel materials.
Thus, the method of the first aspect of the present invention encompasses a method to fabricate ferromagnetic microporous stainless-steel filters, in which iron powder compacted as raw material is subjected to the annealing treatment described in step i), and then cooling down as described in step ii) to obtain a ferromagnetic microporous stainless-steel filter. The annealing temperatures of step i) may also allow compacted iron powder to be sintered as a raw material, so that the obtainable ferromagnetic microporous stainless-steel material may also be sintered.
The ferromagnetic microporous stainless-steels material can be used in applications where an efficient heat exchanger for fluids is required. Applications such as air disinfection, thermo- catalytic processes, or water heating among others are susceptible to be performed using a ferromagnetic microporous stainless-steel material or filter obtainable in accordance with the first aspect of the present invention as an efficient heat exchanger.
Definitions
According to the present invention, the term “annealing” has the common meaning in the art that is a heat treatment process which alters the microstructure of a material to change its mechanical or electronic properties. Typically, in the art, in steels, annealing is used to reduce hardness, increase ductility, and help eliminate internal stresses. A typical annealing process includes three main stages. During the annealing process, the metal is heated to a specific temperature where recrystallization can occur. At this stage, any defects caused by deformation of the metal are repaired. The metal is held at that temperature for a fixed period, then cooled down to room temperature.
According to the present invention, the expression “austenitic stainless steel” means a specific type of stainless-steel alloy. Stainless steels can be classified by their crystalline structure into four main types: austenitic, ferritic, martensitic, and duplex. Austenitic stainless steels possess austenite as their primary crystal structure (face-centered cubic). This austenite crystal structure is achieved by sufficient additions of the stabilizing elements of austenite: nickel, manganese, and nitrogen. Their crystalline structure makes the austenitic steels essentially non-magnetic.
In accordance with the present invention, the term “material” in the expression “microporous austenitic stainless-steel material” is a filter, sintered or not, or metal powder.
The expression "an average size of less than 50 pm ", where "50" is a variable for the number of micrometers, is defined herein as the average particle size, as measured by any conventional means such as dynamic light scattering or microscopy, of a sampling of particles wherein the average is less than about 50 micrometers in diameter, assuming for purposes of the calculation that the irregular particles have an approximate diameter, that is, that they are approximately spherical. This assumption is purely for the calculation of average particle size, due to the particles often being non-spherical in shape. Methods used to measure particle size include dynamic light scattering, scanning electron microscopy or transmission electron microscopy (see also Appendix)
In the cooling down step, the expression “atmospheric conditions” means the atmospheric pressure and the ambient or room temperature, usually 1 atmosphere of pressure and 23°C of temperature. The ambient or room temperature includes values from 15°C to 35°C.
Brief Description of the Drawings
To better understand the description made, a set of drawings has been provided which, schematically and solely by way of non-limiting example, represent practical cases of various embodiments.
Figure 1 depicts the morphology of microporous austenitic stainless-steel filters (Amespore®) commercially available in the art.
Figure 2 depicts the morphology of a ferromagnetic microporous stainless-steel filter obtained in Example 4 using slow cooling (blue surface filter).
Figure 3 depicts the morphology of a ferromagnetic microporous stainless-steel filter obtained in Example 4 using rapid cooling, in two different magnification scales, (a) 8,000 and (b) 60,000 (black surface filter).
Figure 4 depicts a bar graph of the effect on porosity of magnetic induction heating (132 kHz, 500 Oe) of 20 L/min constant airflow through treated filters (striped columns), treated with an annealing temperature of 950°C and with rapid or slow cooling, versus untreated filters, porosity varies for average pore diameters of 5 pm, 10 pm, 25 pm, 40 pm, 60 pm and 80 pm.
Figure 5 depicts a bar graph of the effect of the annealing temperature, between 650°C and 1 ,000°C, on the magnetic induction heating (132 kHz, 500 Oe) after 2 minutes (100 W) in a constant airflow of 20 L/min flowing through filters of average pore diameter of 10 pm. The filters were treated with annealing at different temperatures during 1h and thereafter rapid or slow cooled.
Figure 6 depicts the obtained data of the effect of the period maintaining to the annealing temperature on the magnetic induction heating (132 kHz, 500 Oe) after 2 minutes (100 W) in filters of average pore diameter of 10 pm. The filters were treated with an annealing temperature of 950°C at different periods of time and thereafter rapid or slow cooled.
Figure 7 depicts four graphs of the magnetic induction heating effect for four treated filters of different average pore diameters (5 pm, 60 pm), subjected to different magnetic frequencies (78 kHz, 404 kHz) and magnetic field amplitude of 125 Oe. The squares show the data obtained with thermally treated filters and with rapid or slow cooling, and the circles show the data obtained with untreated filters. Figure 8 depicts four graphs of the magnetization reversal loop effect for four treated filters of different average pore diameters (5 pm, 60 pm), subjected to different magnetic frequencies (78 kHz, 404 kHz). Black cycles show the data obtained with thermally treated filters using an annealing temperature of 950°C and with rapid or slow cooling, and grey circles show the data obtained with untreated filters.
Figure 9 depicts a graph of magnetic induction heating efficiency versus different magnetic field amplitude (Oe), H= 100 Oe, 125 Oe, 150 Oe, 175 Oe, in filters of average pore diameter of 10 pm, treated with an annealing temperature of 950°C.
Figure 10 depicts a graph of the effect of optical heating (917 nm wavelength and 15 W power) on treated filters (black line) versus untreated filters (grey line).
Figure 11 depicts a graph that compares the magnetic heating efficiency for untreated filters versus treated filters with a thermal annealing in a tubular furnace and then slow or quick cooling down.
Figure 12 depicts several pictures of disc filters Amespore® to comparatively show the untreated filters versus treated filters of Example 6.
Figure 13 depicts a graph of the comparative results obtained by thermal annealing (step i) using tubular furnace versus inductive furnace.
Figure 14 depicts a graph of the effect of optical heating that compares filters obtained by slow cooling versus quick cooling and also versus untreated filters.
Figure 15 depicts SEM images by SEM QUANTA 650 FEG, with a voltage of 5kV of untreated and treated filters with slow and quick cooling: untreated filters (first column); with slow cooling (second column) and fast cooling (third column).
Figure 16 depicts SEM images by SEM QUANTA 650 FEG, with a voltage of 5kV, of treated filters using an inductive furnace at the thermal annealing and, then, slow and quick cooling down.
Figure 17 depicts the XPS spectra (X-ray photoemission spectroscopy) obtained with SPECS PHOIBOS 150 hemispherical energy analysers, under ultra-high vacuum conditions. It shows the comparative of the C1s, O1s y Fe 2p spectra for the quick and slow cooled filters versus untreated filters.
Figure 18 depicts the Contact angles determined with the Drop Shape Analyzer DSA25S (KRUSS) of untreated filter that is hydrophilic (final contact angle es 0°), first image, slowly cooled filter that is superhydrophilic, the water droplet is absorbed very fast by the filter network (final contact angle es 0°), second image, and the fast cooled filter (I PA) that shows superhydrophobic behavior (144.1°), third image.
Detailed description of the invention
As described above, the present invention provides a method for magnetic conversion of microporous metal materials from paramagnetic to ferromagnetic.
In particular, the present invention relates to a conversion method for changing magnetic behavior of microporous austenitic stainless-steel materials from paramagnetic to ferromagnetic.
The conversion method is suitable for changing the austenitic stainless steels which is paramagnetic to a ferromagnetic behavior.
As described above, preferable thermal annealing step i) is carried out in an inductive furnace.
The magnetic conversion method of the first aspect transforms their magnetic properties from paramagnetic to ferromagnetic and generates micro/nano-structuration of the pore surface (Figures 2 and 3 (a), (b)).
In addition, the cooling step ii) (quick cooling) in an organic solvent (preferably, isopropanol) results in the deposition of a carbon layer on the material (filter) surface.
Advantageously, the ferromagnetic behavior of the converted filter enables a high improvement of the magnetic induction heating efficiency. Furthermore, the nanostructured surface and the deposited carbon layer enable boosting the optical heating efficiency.
The conversion method also provides a powerful protection for corrosion in both air and water conditions, even at very high temperatures.
The potential applications include air disinfection, methane pyrolysis for generation of H2 without CO2 emission, other thermo-catalytic processes in gas o liquid phase, and even efficient decentralized water heating.
The obtainable ferromagnetic microporous stainless-steel reveals an efficient heating of fluids by magnetic induction and/or optical absorption.
The filters obtainable using the method of the first aspect of the present invention were surprisingly improved compared to the untreated filters, as it will be discussed below.
Effect of the Porosity Filter on the Magnetic Induction Heating-
As can be observed in Figure 4, the effect of the porosity on the magnetic induction heating was surprisingly improved using an average pore diameter equal to or lower than 25 pm. Moreover, the filters with smaller average pore diameters, that is of about 5 pm, reached a still higher temperature in the filter, even above of 400°C, compared to filters having a higher average pore diameter, particularly higher than the threshold of 25 pm.
Therefore, heating of the porous material by magnetic induction is further improved with an average pore diameter equal to or lower than 25 pm.
Effect of the Annealing Temperature on the Magnetic Induction Heating-
As shown in Figure 5, the maximum temperature, expressed in Celsius degrees, achieved in filters heated by magnetic induction is obtained by using an annealing temperature range between 900°C and 960°C.
Therefore, heating of materials by magnetic induction is further improved when materials are thermally treated (step i) using annealing temperatures from 900°C to 960°C, independent of the benefits derived from cooling down (step ii) by a slow or quick way.
Effect of the period maintaining the Annealing Temperature on the Magnetic Induction Heating-
As shown in Figure 6, the effect of the period maintaining the annealing temperature on the magnetic induction heating of the annealed filter does not change too much after the first 15 minutes, and does not affect or even goes down after 1 hour maintaining the annealing temperature.
The filters obtained by thermal annealing at 950°C and either slow or quick colling were subjected to magnetic inductive heating (power 100W). The maximum temperature reached at the filters was collected after 2 minutes heating by magnetic induction at these conditions.
The results in Figure 6 show that the optimal annealing temperature and time holding at such temperature that maximize inductive magnetic heating are a thermal annealing at 950°C and holding for 1 hour at said temperature, respectively, without major differences in the type of cooling performed.
Therefore, the preferable period maintaining the annealed material to the annealing temperature (step i) is equal to or lower than 1 hour, preferably equal to or lower than 30 min.
-Effect of the porosity of the filters on the Magnetic Induction Heating -
Several assays of magnetic heating for different average pore diameters and magnetic frequencies, at fixed magnetic field amplitude, were performed.
As shown in Figure 7, the effect of the porosity of treated filters of average pore diameter of 5 pm versus 60 pm, subjected to a magnetic frequency of 78 kHz versus a magnetic frequency of 404 kHz with a fixed magnetic field amplitude reveals that the lower is the average pore diameter (5 pm) and the magnetic frequency (78 kHz) the higher is the heating (°C) in the treated filters by magnetic induction.
Therefore, the optimal magnetic field frequency for magnetic induction heating depends on the pore size and, therefore on the size of the particles that form the porous structure. The smaller is the pore size, the smaller is the dimension of the particles that form the porous structure.
This effect is because for small pores, that is < 25 pm, preferable 5 pm, and low frequencies, that is < 200 kHz, 78 kHz, the ferromagnetic behavior dominates in the particles and the heating due to hysteresis losses is in the same range as the heating due to Eddy currents.
In contrast, for large frequencies (> 404 kHz), the heating due to Eddy currents dominates and the hysteresis losses contribution given by the ferromagnetic behavior is weak.
For large pore filters (>60 pm), the effect of the thermal treatment is small and the hysteresis losses in the larger particles that form the porous structure have a negligible contribution.
Therefore, the use of ferromagnetic microporous stainless-steel material obtainable by the method defined in the first aspect of the invention as a heat generator as well as in use as a heat exchanger in fluids is preferable having a small average pore diameter, preferably equal to or lower than 25 pm, still more preferable below 10 pm, and combined with low magnetic frequencies, preferably from 70 kHz to 200 kHz.
The authors of the present invention have found that the magnetic heating efficiency after the thermal treatment is similar for treatments with slow and quick cooling (step ii).
For a fixed magnetic field amplitude, in general, the heating efficiency increases with the frequency. However, it is more convenient from the practical point of view to work at lower frequencies due to the possibility to use cheaper electronic circuits. Moreover, higher magnetic fields can be obtained at lower frequencies using similar electric energy consumption, and the magnetic heating efficiency drastically increases with the amplitude of the magnetic field.
Ferromagnetism in the converted microporous stainless-steel filters -
As shown in Figure 8, the magnetization reversal loops of the four filters confirmed the conversion from paramagnetic to ferromagnetic of the austenitic microporous stainless-steel filters.
For small pores (5 pm) and low frequencies (78 kHz) (first graph, left), the ellipsoidal magnetization reversal loops with positive slope reflect the effect of the ferromagnetism in the treated filters (black circle), compared to the ellipse with negative slope of the untreated samples (grey circle), which is sign of the Eddy currents and diamagnetic-like behavior under alternating magnetic fields. The difference in amplitude of the loops is correlated with the higher heating efficiency in the treated samples.
For high frequency (404 kHz), the difference in amplitude and internal area of the loops is smaller, thus confirming that for large frequencies the magnetic heating process is dominated by Eddy currents.
In contrast, for large pores (60 pm), the magnetization reversal loops barely change, which explains the minimal enhancement of the magnetic heating efficiency in this case.
Effect of Magnetic induction heating efficiency on magnetic field amplitude -
As shown in Figure 9, the magnetic induction heating efficiency increases as long as the magnetic field amplitude is increased.
Effect of Optical Heating
As shown in Figure 10, the induced temperature change in the treated filter subjected to an optical heating is increased compared with the untreated filter using a laser light beam with 917 nm wavelength and 15 W power.
Examples
Hereinafter, the present invention is described in more detail and specifically with reference to the Examples and Figures, which however are not intended to limit the present invention.
Example 1 Cleaning of microporous austenitic stainless-steel filters
6 virgin (untreated) filters, commercially available (Amespore®) (Figure 1) of different average pore diameters of 5 pm, 10 pm, 25 pm, 40 pm, 60 pm and 80 pm were used for the assays.
The initial step was cleaning the filters in isopropanol for 5 minutes under ultra-sonication and dried with a nitrogen flow.
Example 2 Chemical treatment of microporous austenitic stainless-steel filter
The cleaned filters of Example 1 were submitted to a chemical treatment by immersing the filters in a mixture of HCkHNCh (1 :1), then rinsed with isopropanol for 5 minutes and dried with a nitrogen flow.
Depending on the average pore diameter of each one of the filters (5 pm, 10 pm, 25 pm, 40 pm, 60 pm and 80 pm), the time (seconds) in the acid immersion was varied. The time in the acid mixture increases as the average pore diameter is reduced. The filters were immersed for 45 seconds for the average pore diameter of 5 pm, 10 seconds for the average pore diameter of 10 pm, and 5 seconds for the average pore diameters of 25 pm, 40 pm, 60 pm and 80 pm. The chemical treatment removes the chromium and nickel oxide layers usually present in the stainless-steel surface of the filters.
Example 3 Annealing treatment (step i) of microporous austenitic stainless-steel filter
A tube furnace (Lindberg Blue Mini-Mite™TF55030A-1) was used for the high temperature treatment. The filters were introduced in a quartz tube.
The thermal annealing step i) was performed by heating the filters of the above average pore diameters (with the ends of the tube uncapped) at a rate of 1 °C/s until an annealing temperature of 950°C was reached. The filters were then kept at this annealing temperature for 1 hour.
Example 4 Cooling treatment (step ii) of microporous annealed stainless-steel filter
The filters of Example 3 were cooled down to room temperature.
The cooling of step ii) was carried out in two different ways.
-Slow cooling-
The first way of cooling down was done by removing the quartz tube from the furnace including the filters inside it and allowing to cool it to room temperature, yielding dark blue filters of morphology shown in Figure 2.
-Quick cooling -
In the second way, the cooling down was carried out by removing the filters directly from the furnace and immediately immersing them into an isopropanol bath at 25°C. This cooling showed that the filters turned a black color with morphology shown in Figure 3 (a), (b).
Example 5 Surface composition of ferromagnetic microporous stainless-steel filter
The surface composition of the filters obtained in Example 4 (slow and quick cooling) were compared by SEM-EDX. The results showed that the surface composition of the slowly cooled down filters became all iron oxides as shown in Table 1 , second column, below. The results also showed that the surface composition of the rapidly cooled down filters in the isopropanol bath caused carbon incorporation into the surface composition as shown in Table 1 , third column, below.
Table 1 - Comparison of surface composition using SEM-EDX
Figure imgf000015_0001
Figure imgf000016_0001
The blue filters obtained by slow cooling were highly hydrophilic, whereas the black carbon coated filters obtained by quick cooling were highly hydrophobic.
Example 6 Method for magnetic conversion from paramagnetic to ferromagnetic using a tubular furnace in the thermal annealing
- Raw material- Microporous austenitic stainless-steel sintered filters with average pore size of 10pm.
-Thermal annealing- The filters were heated with a heating ramp of 60°C/min until reaching 950°C, then holding at such temperature for 1 hour. The thermal annealing was performed under atmospheric conditions, air at standard conditions.
Cooling down: Two different ways of cooling down were performed using the half of the filters annealed in each way of cooling down.
-Slow cooling-
The first way of cooling down was done by removing the quartz tube from the furnace including the filters inside it and slowly cooled with a ramp of -60°C/min, allowing to cool they to room temperature, yielding dark blue filters of morphology shown in Figure 15 (second column).
-Quick cooling -
In the second way, the cooling down was carried out by removing filters directly from the furnace and immediately immersing them into an isopropanol bath at 25°C. This cooling showed that the filters turned a black color with morphology shown in Figure 15 (third column).
Assays performed based on the filters obtained in Example 6
Particular conditions are, if needed, described in each assay o Comparison of magnetic heating efficiency for untreated and treated filters with slow and guick cooling (Figure 11)
Comparative assays to evaluate the efficiency of the magnetic induction heating in converted ferromagnetic filters of the second aspect of the invention (Example 6) versus untreated filters were performed.
The untreated filters, “Virgin”, were microporous austenitic stainless steel sintered filters having an average pore size of 10 pm. The treated filters are the filters obtained in Example 6.
To test the magnetic induction heating efficiency the filters were subjected to an alternating magnetic field of 137 kHz (power 100W), under a constant air flow of 15 L/min. The measurements represented in Figure 11 show that the magnetic heating efficiency is enhanced for the treated filters with respect to the untreated. The heating efficiency is practically identical for treated filters with thermal annealing and quick or slow cooling. The temperature values were measured at the filter by optical thermometer. o Quick cooling step (ii) in different solvents (Figure 12)
In accordance with Example 6, the quick cooled by immersion was assayed using different organic pure solvents: pure IPA, MetOH, EtOH, or acetone, all at room temperature and standard conditions.
Form Figure 12, a very similar nanostructured carbon surface was observed in all the treated filters using different organic solvents versus the untreated filter.
The pure acetone, pure methanol and pure ethanol can be used as well for quickly cooling. In all the filters a very homogeneous coverage was formed with similar nanostructured carbon surface. o Optical heating (Figure 14)
Comparative assays were performed to compare the effect of optical heating when the filters were obtained by cooling down using slow cooling versus quick cooling and also the optical heating of untreated filters.
The untreated filters “Virgin” and treated filters were illuminated with a collimated laser beam at a wavelength 808 nm and power of 15W for 2 min to induce the optical heating under atmospheric conditions (static conditions, no air flow).
The results show that the treated filter with quick cooling has highest optical heating efficiency thanks to the enhanced optical absorption of the carbon layer. The treated filter with slow cooling also exhibits higher optical heating efficiency compared to the untreated filter “Virgin” due to the enhanced optical absorption caused by the corrugated surface. o SEM pictures - Conventional tubular oven (Figure 15)
The SEM images show that the surface of the untreated filters (first column) have stainless steels grains with smooth surface. Contrarily, the annealing treatment in conventional furnace and slow cooling (second column) increases the surface roughness yielding submicron oxidized structures with sharp angles and rather flat sides. And, in contrast, the annealing and fast cooling (third column) in organic solvent provides a carbon coating formed of submicron (100-500 nm) carbon nanoparticles homogeneously distributed on the surface.
Compared to US11478761 B2, the microporous filters according to the present invention are composed of a 3D matrix of stainless-steel microparticles, which are sintered to achieve a mechanically robust and electrically connected microporous 3D structure, having tunable porosity by means of the size of the microparticles. o XPS, X-ray photoemission spectroscopy (Figure 17)
From Figure 17 it can be seen the effect of the different treatments. The comparative of the C1s, O1s y Fe 2p spectra shows the increase of the O content in the slowly cooled filter, and the increase of the C in the fast cooled filter. The C layer in the fast cooled filter masks the iron signal, showing the efficient C coverage.
These results show that the treated filters using slow cooling have the highest content of oxidized Fe and oxygen on the surface. In contrast, in the fast cooled, the iron signal is negligible because it is homogeneously covered by the carbon layer, whose signal is the highest of all the samples. The filters submitted to XPS were microporous austenitic stainless steel sintered filters with average pore size of 10 pm, either untreated “Virgin” or thermal annealing in tubular furnace and then either slowly cooled in atmosphere or fast cooled by immersion in IPA. o Contact Angle (Figure 18)
The measurements show that the untreated filter is hydrophilic and it is able to absorb the water droplets. In the case of the treated filters with slow cooling, the surface is superhydrophilic and the water droplets are absorbed much faster by the filter. In contrast the treated filters with fast cooling in I PA are superhydrophobic and the water droplets are repelled, forming a contact angle larger than 140°.
The authors of the present invention have guantified the hydrophilicity (superhydrophilic behavior of the filters). In the “virgin” filter, it takes 0.5 seconds for the water droplet to be absorbed (hydrophilic), while after slow cooling treatment (superhydrophilic), it takes 0.25 seconds.
The filters were microporous austenitic stainless steel sintered filters with average pore size of 10 pm, either untreated “Virgin” or treated in tubular furnace and then either slowly cooled in atmosphere or fast cooled by immersion in IPA).
Example 7 Method for magnetic conversion from paramagnetic to ferromagnetic using a inductive furnace in the thermal annealing - Raw material- Microporous austenitic stainless-steel sintered filters with average pore size of 10pm.
-Thermal annealing- In the induction furnace, the filters were fast heated by induction at a magnetic frequency of 137 kHz, using coil diameter of 25 mm, 9 turns, and electric power consumption of 100W. The heating ramp measured was of 1900°C/min. The maximum temperature of 950°C was kept during 1 hour. The thermal annealing was performed under atmospheric conditions, air at standard conditions. The induction furnace was an Equilab EQH- 3.0.
The filters were heated to reach a temperature of 950°C, then holding at such temperature for 1 hour.
Cooling down: Two different ways of cooling down were performed using the half of the filters annealed in each way of cooling down.
-Slow cooling-
The first way of cooling down was done by removing the filters from the inductive furnace and allowing to cool they to room temperature in atmospheric conditions with a rate of -90°C/min, yielding filters of morphology shown in Figure 16 (first column).
-Quick cooling -
In the second way, the cooling down was carried out by removing filters directly from the induction furnace and immediately immersing them into an isopropanol bath at 25°C, yielding filters of morphology shown in Figure 16 (second column).
Assays performed based on the filters obtained in Example 7
Particular conditions are, if needed, described in each assay o Magnetic Inductive treatment (Figure 13)
Comparative assays of annealing (step i) using tubular furnace versus inductive furnace were performed and the results shown in Figure 13.
The thermal annealing of ferromagnetic sintered filters of Example 6 was compared with the thermal annealing of ferromagnetic sintered filters of Example 7.
From figure 13, it can be seen that the thermal annealing (step i) in an inductive furnace has advantages with respect to the thermal annealing in a tubular furnace. It is much faster heating and much lower energy consumption than in a conventional tubular furnace.
In addition to these advantages, both thermal annealing, induction and tubular furnaces, allow similar transformation from paramagnetic to ferromagnetic, and hydrophilicity or hydrophobicity depending on the quick or slow cooling performed. Figure 13 also shows that the filters obtained by thermal annealing using inductive furnace provide similar enhanced inductive heating. o SEM pictures - Magnetic Induction furnace (Figure 16)
SEM pictures were obtained at the filters treated using a magnetic Induction furnace for thermal annealing to evaluate its effect in the surface of the filters obtained. Figure 16 shows that when the thermal annealing treatment is performed in a magnetic induction furnace and slow cooling, the surface of stainless-steel grains that form the filters is modified yielding a 3D nanoporous structure of oxidized metal (pores size 100-600 nm). Using the same thermal annealing treatment condition but with quick cooling down in IPA, yields also a porous surface but with higher porosity (ca. 1 micron), that is covered by pyrolytic carbon.
It was concluded that the filters were fast heated by induction using an inductive furnace.
Again, compared to US11478761 B2, the microporous filters according to the present invention are composed of a 3D matrix of stainless-steel microparticles, which are sintered to achieve a mechanically robust and electrically connected microporous 3D structure, having tunable porosity by means of the size of the microparticles.
-APPENDIX-
AmesPore® SSU stainless steel disc filters
(https://amespore.com/en/find-out-your-amespore-filter/stainless-steel-disc-filters/)
The material used to produce these disc filters is AISI 316L or AISI 316 stainless steel. They are made (SSU) by uniaxial compaction of powder in a rigid tool with the negative shape of the part, and then sintered. The pore size is adjusted by changing the compaction pressure and/or the particle size of the powder.
The diameter or width of these filters is determined by the tooling, while their height is adjustable from approximately 1 ,5mm to the maximum length indicated in the tables.
We have a variety of stainless-steel filter discs in permanent stock that can be shipped within 24 hours. Please click the links to view our permanent stock or to view the different material grades that are available.
AISI 316L or AISI 316 stainless steel filters made by uniaxial compaction
The material used to produce these filters is AISI 316L or AISI 316 stainless steel. They are made by uniaxial compaction of powder in a rigid tool with the negative shape of the part, and then sintered. The pore size is adjusted by changing the compaction pressure and/or the particle size of the powder.
Figure imgf000021_0001
Porous AISI 316L stainless steel: Characteristics in uniaxial compaction
Figure imgf000022_0002
Figure imgf000022_0001
1 . Intermediate grades can be manufactured on demand.
2. Measured with a porometer.
3. Equivalent to the bubble point, determined in-out according to the ISO 4003:1977 standard at a continous flow of 5ml/min.
4. Determined by a single pass at 0.01 m/s with a 2mm thick sample.
5. Measured according to the ISO 4022:2018 standard under the following conditions: pressure slope 250 s/bar, maximum pressure difference 1 .5 bar, maximum flow 3.3 l/s.
Figure imgf000023_0001
AmesPore® SSU stainless steel disc filters
Figure imgf000023_0002
(The letters SSU indicate uniaxially pressed stainless steel and the letter D indicates disc)
C HARACTE RIZATIO N O F AM ES PO RE® FI LTE RS
The characteristics and properties of AmesPore® filters are determined in the laboratory by the following tests:
Porometry curve determined according to ISO 4003: 1977, from which the pore size distribution curve is derived, and from which the average, minimum and maximum pore size is obtained.
Bubble point according to ISO 4003: 1977, determined as the appearance of the first bubble or continuous flow.
Filtering efficiency according to ASTM F795-88 (1993).
Permeability coefficient according to ISO 4022:2018 in both water and air.
Density and total porosity according to ISO 2738:1999. Degree of cleanliness.
Chemical composition, including carbon, oxygen and nitrogen content. Corrosion resistance.
Mechanical properties, such as radial tensile strength and shear strength. Metallography and fractography, including an electron scanning microscope. Advanced measurement equipment.
Figure imgf000024_0001
Sintered metallic filters
AmesPore® porous metallic filters and components are sintered stainless steel or bronze parts with high porosity (between 25% and 60% by volume).
They are produced by the powder metallurgy technology, which consists of shaping a mixture of metal powders in a mould or tool that has the negative shape of the part and then sintering the resulting preform. The preform may be created by gravity filling, by uniaxial or isostatic pressing or by extrusion, depending on the material to be shaped, the desired porosity and the component geometry.
Sintering consists in heating the preform to a temperature lower than the melting point of the base metal (between 700°C and 1300°C) under carefully controlled conditions of atmosphere and time. The temperature causes the powder particles to weld together.
The result of this process is a structurally functional metallic part with a controlled microporosity level.

Claims

1 . A method for magnetic conversion from paramagnetic to ferromagnetic of a microporous metal material, characterized in that the microporous metal material comprises a microporous austenitic stainless-steel material, and the method comprises the steps of: i) thermal annealing the microporous austenitic stainless-steel material as raw material, wherein thermal annealing comprises heating the microporous austenitic stainless- steel material to an annealing temperature comprised of between 500°C and 1 ,200°C, and maintaining the annealed microporous stainless-steel material at the annealing temperature for a period, and then, ii) cooling down the annealed microporous stainless-steel material to room temperature, wherein the cooling is carried out by allowing the annealed microporous stainless-steel material to cool under room temperature, so the annealed microporous stainless-steel material is slowly cooled, or alternatively cooling by immersing the annealed microporous stainless-steel material in an organic solvent bath that is at room temperature, so the annealed microporous stainless-steel material is quickly cooled, wherein the thermal annealing of step i) is carried out under atmospheric conditions, so no gas treatment is carried out.
2. Method of claim 1 , wherein the microporous austenitic stainless-steel material as raw material is a filter, sintered or not.
3. Method of claim 1 , wherein the microporous austenitic stainless-steel material as raw material is a metal powder, sintered or not.
4. Method of any one of previous claims, wherein the thermal annealing of step i) is carried out in a tubular furnace.
5. Method of any one of previous claims, wherein the thermal annealing of step i) is carried out in an inductive furnace, provided that the microporous austenitic stainless-steel material as raw material is sintered.
6. Method of any one of previous claims, wherein the thermal annealing of step i) is carried out at a temperature between 850°C and 1 ,000°C, preferable between 940 °C and 960°C.
7. Method of any one of previous claims, wherein the thermal annealing of step i) is maintained at the annealed temperature for a period of about 1 hour.
8. Method of any one of previous claims, wherein in the cooling down of step ii), the organic solvent is selected from the group consisting of isopropanol, acetone, methanol and ethanol.
9. Method of claim 8, wherein in the cooling down of step ii), the organic solvent consists of isopropanol.
10. Method of any one of previous claims, wherein before the thermal annealing of step i), the microporous austenitic stainless-steel material as raw material is chemically treated in an acidic mixture comprising hydrochloride acid and nitric acid in a volume ratio from 3:1 to 1 :3.
11. Method of claim 10, wherein the acidic mixture is in an equimolar ratio 1 :1.
12. Method of any one of previous claims, wherein the method further comprises an initial step of cleaning the microporous austenitic stainless-steel material, wherein the cleaning includes immersing the microporous austenitic stainless-steel material in an isopropanol bath under ultra-sonication and then drying with a nitrogen flow.
13. Method of any one of previous claims, wherein the microporous austenitic stainless-steel material as raw material is a filter that includes an average pore diameter equal to or lower than 80 pm.
14. Method of claim 13, wherein the average pore diameter is equal to or lower than 25 pm, preferably equal to or lower than 10 pm.
15. A ferromagnetic microporous stainless-steel material obtainable by the method defined in claims 1 to 14.
16. Ferromagnetic microporous stainless-steel material of claim 15 which is a sintered filter.
17. Ferromagnetic microporous stainless-steel material of claim 15 which is a non-sintered filter.
18. Ferromagnetic microporous stainless-steel material of any one of claims 15 to 17, wherein the material is a filter, sintered or not, and has an average pore diameter equal to or lower than 25 pm, preferably equal to or lower than 10 pm.
19. Ferromagnetic microporous stainless-steel material of claim 15 which is a sintered metal powder.
20. Ferromagnetic microporous stainless-steel material of claim 15 which is a non-sintered metal powder.
21 . Ferromagnetic microporous stainless-steel material of any one of claims 15 to 18, wherein the ferromagnetic microporous stainless-steel material is a hydrophilic filter.
22. Ferromagnetic microporous stainless-steel material of claim 21 , wherein the filter surface is nanostructured and composed of iron oxides.
23. Ferromagnetic microporous stainless-steel material of any one of claims 15 to 18, wherein the ferromagnetic microporous stainless-steel material is a hydrophobic filter.
24. Ferromagnetic microporous stainless-steel of claim 23, wherein the filter surface is nanostructured and composed of carbon.
25. Use of a ferromagnetic microporous stainless-steel material defined in claims 15 to 24 as a heat generator.
26. Use of a ferromagnetic microporous stainless-steel material defined in claims 15 to 24 as a heat exchanger.
27. Use of any one of claims 25-26, wherein the ferromagnetic microporous stainless-steel material as a heat generator is self-heated by magnetic induction and/or optical absorbance.
28. Use of any one of claims 25-26, wherein the ferromagnetic microporous stainless-steel material as a heat exchanger exchanges its reached temperature by self-heating to treat fluids flowing therethrough.
PCT/EP2024/070425 2023-07-18 2024-07-18 A method for magnetic conversion from paramagnetic to ferromagnetic of microporous metal materials, converted ferromagnetic microporous metal material and uses thereof Pending WO2025017138A1 (en)

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Citations (2)

* Cited by examiner, † Cited by third party
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CN107164704A (en) 2017-04-17 2017-09-15 华南理工大学 A kind of porous low mould austenitic stainless steel and preparation method thereof
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