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    Synthesis at low temperature of particles of the epsilon phase of iron (III) oxide as a single phase inside an amorphous silica matrix using the sol-gel method. The present invention relates to a sol-gel process for the synthesis at low temperature of a material with high purity in the epsilon phase of iron (III) oxide, ε -Fe2O <sub > 3from low cost raw materials through a one-step process and easy industrial scaling. The material obtained, whether in the form of a powder, monolith or coating, consists of nano-sized panicles embedded in an amorphous silica matrix and having properties that make it suitable for use as a permanent magnet, a catalyst for the dissociation of water, a component of electronic devices or as an absorbent of radio waves. (Machine-translation by Google Translate, not legally binding)
    公开号:ES2666704A1
    申请号:ES201600922
    申请日:2016-11-03
    公开日:2018-05-07
    发明作者:Noemí CARMONA TEJERO；Jesús LÓPEZ SÁNCHEZ；Óscar RODRÍGUEZ DE LA FUENTE
    申请人:Universidad Complutense de Madrid；
    IPC主号:
    专利说明:

     Low temperature synthesis of particles of the epsilon phase of iron (III) oxide as a single phase within an amorphous silica matrix using the sol-gel method. 5 Technical Sector The present invention belongs to the field of particles of magnetic materials. More specifically, the invention relates to the method for obtaining epsilon iron oxide (II) phase particles that can be used in the manufacture of permanent magnets without rare earths, in catalysis for water dissociation or as 10 absorbent materials of radio waves. State of the art The epsilon phase is a polymorph of iron (III) oxide that exists primarily in the form of a nanostructure and whose natural abundance is low. It has an orthorhombic structure that derives from a compact packing of four layers of oxygen. This system consists of three chains whose edges share octahedral positions and another chain whose corners share tetrahedral positions. It is characterized by being composed of six anionic positions (Oj, j = I, 2,3,4,5,6) and 20 four non-equivalent cationic positions (Fe ;, i = 1 2,3,4). The Fe4 position has tetrahedral coordination while the other three have octahedral coordination. It has, therefore, a high structural complexity and its magnetic and electronic behavior has not yet been fully understood. 25 Until now, two magnetic transitions have been reported; one takes place at approximately 500 K and the other at 100 K. At 500 K the epsilon phase transits from a paramagnetic state to a collinear ferrimagnetic state. Below 100 K it is transformed into an immeasurable magnetic structure. From a certain particle size (around 30 nm), the epsilon phase has up to 2 Teslas of 30 coercive field at room temperature. This value is mainly due to its great magnetocrystalline anisotropy (KMC ~ 2-5.105 J / m3), which leads to a monodomain character due to its nanometric size and its non-zero magnetic moment of DESCRIPTION5 Fe3 + ions, explaining a strong spin-orbit coupling. The value of the coercive field is much greater than that observed in BaFel2019 (Bc -0.64 T) or in cobalt ferrites (Bc -0.74 T), thus becoming a great candidate for materials intended for magnetic recording. In the field of magnetic materials, one of the main objectives has always been to find materials with coercive fields as high as possible, to respond to a demand for greater data storage capabilities. On the other hand, the possibility of manufacturing permanent magnets with a high coercive field without the need to incorporate rare earth elements into the material is considered an advantage. In addition, the epsilon phase also has a good magnetoelectric coupling (phenomenon by which a certain magnetic polarization is induced with the application of an electric field 15) and a magnetic frequency of the order of mTHz that can be very useful in a wide range of applications such as the suppression of electromagnetic interference and the stabilization of transmittance (Namai, A. and Ohkoshi, S., Advanced Trends in Wireless Communications, In Tech, 2011, vo1.3). 20 In view of its electronic properties, new possible applications are being discovered since it is a semiconductor (type p) with energy of the gap close to 1.9 eV (Korte, D. et al, Opl. Maler., 2015 (42), 370 -375). In addition, oxidation and reduction potentials prove to be very effective for the production of H2 (as fuel) based on the dissociation of the water molecule (Carraro, G. et al. 25 Adv. Funct. Maler., 2014, (24 ), 372-378). The use of this material, however, has the disadvantage that it is relatively complicated to synthesize in a reproducible manner and to obtain with high purity. Hence, this material does not yet exist commercially. Given its low surface energy and its metastable character, it is energetically more favorable to obtain other phases (hematite and maguernite), both thermally and chemically. Therefore, most studies related to this polymorph arelinked to the synthesis of nanoparticles of samples in powder form by the sol-gel method (Brázda, D. et al. 1. Sol-Gel Sci. Technol., 2009 (51), 78-83). These sol-gel recipes are designed to obtain epsilon nanoparticles embedded in a silica matrix to avoid aggregation between the particles and thus not favor nucleation and the growth of more stable phases (López-Sánchez, J. et al., RSC Adv., 2016 (15), 1039). But the temperatures necessary to obtain this polymorph are somewhat high, between 960 and 1300 oC (Ohkoshi, S.L et al., Sci. Rep., 2015 (5), 14414). 10 US2016 / 01040560 describes a process of synthesis of nanoparticles of size 15 nrn or smaller and containing E-Fe203 as a single phase using the sol -gel method using as starting material nanoparticles of p-FeO (OH) that are coated with silicon oxide and, subsequently, the coated particles are subjected to heat treatment in an oxidizing atmosphere at temperatures between 950 and 1550 oc. It is also described to obtain thin films of nanoparticles of size 15 nrn or smaller containing as single phase E-Fe203 from a dispersion of nanoparticles of p-FeO (OH) coated with silicon oxide, coating the surface of a substrate with said dispersion and, subsequently, subject to thermal treatment in an oxidizing atmosphere and temperatures between 950 and 1250 ° C. If this procedure is performed using Fe (N03) 3'9H20 as the starting material, a pure epsilon phase is not obtained, but a 66% phase E-Fe203 is obtained together with a 34% phase y-Fe: z03. To scale the process of obtaining at the industrial level, reaching these temperatures 25 is quite expensive in time and money. For this reason, apart from the sol-gel process, the synthesis by chemical vapor deposition (Carraro, G. et al., Cryst.Eng.Comm., 2013 (15), 1039) and by pulsed laser deposition have been developed (Gich, M. et al., Adv. Mater., 2014 (3), 4645-4652). 30 Although these techniques have been able to synthesize nanowires and thin films of this material, they have the disadvantage of requiring high and ultra-vacuum. This makes the synthesis of the material even more expensive. Therefore, there is still a need to develop amethod of synthesis of particles of iron (III) oxide, viable and reproducible on an industrial scale and that stable samples are obtained in terms of the concentration of the epsilon phase. The present invention describes a sol-gel recipe that can be used to obtain powder or thin films (coatings) of E-Fe203 at temperatures from 350 oC and without going through intermediate phases, with a homogeneous distribution of epsilon nanoparticles on any substrate of reproducible way. DETAILED DESCRIPTION OF THE INVENTION Synthesis at low temperature of particles of the epsilon phase of iron (III) oxide as a single phase within an amorphous silica matrix using the sol-gel method. The present invention relates to a process for the preparation of a material with high purity in the epsilon phase of iron oxide (1m, E-Fe203, from low cost raw materials, by a single step process and of easy industrial scaling, which has properties that make it suitable for use as a permanent magnet, catalyst for water dissociation, component of electronic devices or as a radio wave absorber.The procedure is based on the sol-gel method for synthesize nanoparticles embedded in a silica matrix Starting from the same sol-gel solution, pure samples of epsilon are obtained both in powder, in monolith, as in thin film at acidic pH 25 and low treatment temperatures, which is a great saving economic and one more step towards production on an industrial scale 30 The process comprises the following steps: a) Prepare a silica sol by alkoxid hydrolysis os or silicon alkyl alkoxides hybrid in hydroalcoholic medium. b) Add an iron oxide (I1I) precursor to the previous sun. c) Add an acid catalyst or organic metal to acidic pH.5 10 d) Add a co-solvent such as a polyalcohol, amma or cyanide. e) Stir until complete hydrolysis and polycondensation. t) Deposit the gel obtained on a substrate to form a thin film of iron oxide (I1I) epsilon phase particles or on a cuvette to form a monolith of iron oxide (III) epsilon phase particles. g) Dry the thin film or the monolith obtained. h) Heat treat the thin film or monolith previously dried in an oxidizing atmosphere at a temperature between 100 oC and 1000 oC with a rise rate between 1 and 5 oC / min Pure samples contain nanoparticles of the iron oxide epsilon phase (III), with diameters between a few nanometers up to 17 nm. The coercive field 15 will increase from O kOe (superparmagnetic case) and will grow as the particle size increases as the treatment temperature increases. Since all organic compounds have evaporated from 350 oC, the epsilon phase is obtained directly in a pure and reproducible manner, both in powder and thin film in a wide temperature range (350-800 oC) with the absence of the rest of the phases The evolution of the Raman spectrum as a function of the temperature up to 600 oC of the monoliths and thin layers obtained confirms that the chemical species formed is epsilon 25 and that there is a correlation in the formation temperature. EMBODIMENT OF THE INVENTION The present invention is further illustrated by the following examples, which are not intended to limit its scope. Example 1This example refers to the preparation of E-Fe203 nanoparticles deposited on Si (10) substrates by dip-coating. Absolute ethanol (CH3Cf-hOH, Panreac> 98%) and 5 the iron oxide precursor are added in a beaker: hydrated iron nitrate (Fe (N03) 3 · 9H20 Sigma-Aldrich> 98%) and nitrate barium (Ba (N03) 2, Sigma-Aldrich> 98%). To ensure the dispersion of the iron oxide nanoparticles, a porous matrix of amorphous silica is used. Tetrarthosilicate (TEOS, SiCsH2004, Sigma-A1drich> 98%) is used as a precursor of this silica matrix. The catalyst used in this solution is nitric acid up to a pH close to 1 to produce hydrolysis and the subsequent polycondensation stage. The sun is homogenized using a magnetic stirrer. Subsequently glycerol (C3Hs03, Sigma-Aldrich> 99.5%) is added. The obtained sun is stirred for 7 days, after which it is applied on Si substrates (100) by dip-coating. The speed chosen to obtain final thicknesses of approximately 200 nm is 2.56 mm / s. Then, the coatings are placed in an oven at 60 oC for 7 days in order to evaporate the excess solvent and water. Finally, a heat treatment is carried out at a temperature between 350 oC and 960 oC with a rising ramp of 1 ° C / min and without a plateau. 20 The results obtained by TEM show that the sizes of the nanoparticles obtained for temperatures close to 960 oC range between 7 and 15 nm. The coatings obtained on the substrate are very homogeneous and have very low RMS roughnesses (approximately 0.5 nm), which is important if they are to be implanted in an electronic device. As can be seen in Figure 1, where the Raman spectra acquired in the flat areas at various temperatures are represented, the epsilon phase samples 30 obtained in a thin layer with a thickness between 200 and 700 nm and a relatively low average surface roughness (approximately RMS roughness of 0.5 nm) are formed by the epsilon phase up to a temperature of 1100 oC and from 960 oCbegins to observe a minor hematite signal located in the fractures of the coating along the sample. From 1200 oC, the only phase present in the sample is hematite. 5 To know the limit where the maximum purity of the epsilon phase in the thin film is guaranteed, this percentage is measured by Mossbauer spectroscopy (Figure 2). For samples obtained at 960 oC the percentage of hematite present is within the limit of resolution of the technique, about 2%. This will be the limit temperature of thin layer treatment at which a purity of epsilon around 10 of 98% can be guaranteed. 15 20 25 Example 2 This example refers to obtaining nanoparticles of E-Fe203 in monoliths of lxlx3 cm3 in size. The process of obtaining followed is the same as that described in example 1, except that the sun obtained after adding the glycerin is introduced in buckets where it is kept for a month in an oven at 60 oC before applying the final heat treatment. The thermal stability range of the monoliths is obtained by thermogravimetric (TG) and differential thermal (A TD) analysis (Figure 1) that determine mass losses (TG) and possible endothermic and exothermic processes (ATD) as a function of temperature . Figure 3 shows the ATD-TG curves obtained on the monoliths. These are heated from room temperature to 1000 oC with a constant ramp of 5 OC / min. The TG curve shows a weight loss of 12% up to 116 oC. The endothermic peak at that temperature of A TD reflects the evaporation of physiabsorbed water 30 and ethanol. This comes mainly from the water of hydrated iron nitrate and the solvent used. From 116 oC to 200 oC, a loss of 10% is observed in the TG curve accompanied by a peak width located around 150 oC in the curveTo TD. This process is attributed to the decomposition of iron nitrate in the amorphous silica matrix. In the next stage, between 200 oC and 300 oC, the slope of the TG curve changes and the fall is more pronounced, with a 33% loss of mass. This is accompanied by a very acute endothermic peak with a maximum of 287 oC in the 5 ATD curve that is associated with the evaporation / decomposition of glycerol. When all the liquid evaporates, the TG curve falls almost abruptly to zero. However, the TG curve represents, between 310 ° C and 390 ° C, a mass loss of 5.6%. In this region the curve A TD exhibits a wide shoulder that ends at 355 oC, indicating another endothermic process. To determine the origin of this last endothermic peak (10 glycerol and iron nitrate are immersed in the silica matrix) an in-situ growth of iron oxide is carried out in a temperature chamber coupled to a CRM and monitored the structural evolution of the coating depending on the temperature up to 600 oC. 15 Figure 4 shows the Raman spectra acquired in that temperature range. The bands that appear in the region between 1100 cm-I and 1700 cm-I are associated with organic groups that have carbon bonds. These bands are present until they reach 300 oC, coinciding with the evaporation of glycerol (see curve A TD of Figure 1). From 350 oC a very wide band around 20 of 700 cm-I was born, which is attributed to iron oxide (III) in the epsilon phase. This band is so wide because the particles are a few nanometers in size. As the temperature increases, the intensity increases and this mode of vibration narrows. This means that the crystallinity and size of the nanoparticles is increasing. Between 450 oC and 600 oC thermal agitation makes Raman analysis difficult. Therefore, it is necessary, at the end of the experiment, to reduce the temperature of the sample to -180 oC to observe the final compound. With this, you get more intensity and resolution of Raman modes. At this temperature it is possible to clearly differentiate, apart from the band in volume at 700 cm-I, the three characteristic modes of epsilon iron oxide in the region between 100-200 cm-l. 30 Curve A TD of Figure 3 shows another change in the slope. An endothermic shoulder begins to grow from 800 oc. Raman measurements made inMonoliths treated at that temperature show the presence of epsilon nanoparticles mostly throughout the sample, with small hematite aggregates distributed in an homogenous manner, so this endothermic rise is attributed to the formation of hematite and supports that only the phase epsilon is present in monoliths from 350 oC to 800 oC. After this temperature, it appears mixed with hematite in a minor way. Example 3 To check if hematite formation begins at the same temperature in the 10 thin-layer and monolith samples, a Raman spectrum is made on samples of epsilon nanoparticles deposited on Si substrates (100) by dip-coating and treated from 400 oC up to 1200 oC. Figure 1 shows the characteristic spectra of these films. As in Figure 4, as the treatment temperature increases, the bands become narrower and their intensity increases. Epsilon iron oxide nanoparticles are increasing in size. A hematite signal is not observed until 960 oC (dashed arrows in the figure). The hematite phase is located in the fractures of the coating and only the epsilon phase is detected in the rest of the sample. 20 To evaluate the purity of the samples synthesized at 960 oC, Mossbauer measurements are made to find the percentage of epsilon present in the sample (Figure 2). The curve of the collected data is adjusted with Lorentzian taking into account a superparamagnetic doublet and three sextets. These are characteristic of the octahedral and tetrahedral positions of the epsilon phase. No signal corresponding to hematite is detected. Therefore, it can be considered that the percentage of this phase is within the limits of resolution of the technique and of the analysis (around 2%). In addition, X-ray diffraction measurements (Figure 5) do not detect any other iron oxide (IlI) polymorph that is not corresponding to the epsilon phase.With these results it is concluded that they are pure samples of epsilon in the form of a thin film, which is stabilized at temperatures below 960 oC. Concerning the magnetic properties, two cycles of 5 hysteresis corresponding to monoliths treated at 700 oC and thin film treated at 960 oC are added by way of example (Figure 6). It can be seen how the coercive field, saturation and remanence magnetization increases in the case of 960 oc. 10 Description of the figures Figure 1 shows the Raman spectra corresponding to samples treated between 400 oC and 1200 oc. At 1200 ° C, epsilon iron oxide has completely transformed into a hematite phase. 15 Figure 2 shows the Méissbauer spectrum of the sample synthesized at 960 oC, with the experimental spectrum (above) and the adjustments to a doublet (curve 1) and three sextetes (curves 2,3 and 4). Figure 3 shows the ATD (solid curve) and TG (dotted curve) of the monoliths as a function of temperature. Each number indicates the temperature at which an endothermic process occurs: 1) evaporation of water and ethanol, 2) evaporation of glycerol, 3) formation of the epsilon phase and 4) formation of hematite, coexisting with the epsilon phase. 25 Figure 4 shows the in-situ growth of epsilon nanoparticles, studied by CRM as a function of temperature (from room temperature to 600 oC). The upper spectrum is acquired at -180 oC to obtain greater intensity and resolution of the Raman bands. 30 Figure 5 shows the X-ray diffractogram of the treated sample at 960 oC (upper diffractogram) and conventional substrate Si (100). Zone 1 corresponds to the contribution of silica an10rfa and zone 2 represents the contribution of the substrateYes (IOO) in flush incidence mode with an angle of 0.70 (upper diffractogram) and oy (inferior diffractogram). Figure 6 shows two cycles of magnetic hysteresis together with a zoom in the 5 origin, corresponding to monoliths treated at 700 oC and thin film treated at 960 oc. The maximum field applied in both cases is 5 Teslas. It can be seen that the samples are not saturated to these applied magnetic fields so high. This is characteristic of the epsilon phase. 
    权利要求:
    Claims (1)
    [1]
    CLAIMS 1. Synthesis at low temperature of particles of the epsilon phase of iron oxide (I1I) as a single phase within an amorphous silica matrix using the sol-5 gel method that comprises the following steps: a) Prepare a sol of silica by hydrolysis of alkoxides or hybrids alkyl-aJcoxide of silicon in a hydroalcoholic medium. b) Add to the previous sol an iron oxide precursor (I1I). c) Add an acid or metal-organic catalyst until acidic pH. d) Add a co-solvent such as, for example, a polyol, amine or cyanide. e) Shake until complete hydrolysis and polycondensation. t) Deposit the gel obtained on a substrate to form a thin film of iron (III) oxide epsilon phase particles or on a tray to form a monolith of iron (III) oxide epsilon phase particles g) Dry the thin film or the monolith obtained. h) Heat treating the thin film or the previously dried monolith in an oxidizing atmosphere at a temperature between 100 ° C and 1000 ° C with a rate of rise between 1 and 5 ° C / min. 2. Synthesis at low temperature of particles of the epsilon phase of iron oxide (IlI), according to claim 1, where the silica precursor is tetra-orthosilicate (TEOS). 3. Synthesis at low temperature of particles of the epsilon phase of iron (III) oxide, according to claim 1, where the precursor of iron oxide (IIT) is hydrated iron nitrate. 4. Synthesis at low temperature of particles of the epsilon iron (III) oxide phase, according to claim 3, where barium substrate is also added.5. Synthesis at low temperature of particles of the epsilon phase of iron oxide (IlI), according to claim 1, where the catalyst is nitric acid. 6. Low temperature synthesis of particles of the epsilon phase of iron oxide 5 (III), according to claim 1, where the pH in the hydrolysis and polycondensation stage is close to 1. 7. Low temperature synthesis of particles of the epsilon iron (III) oxide phase, according to previous claims, where the co-solvent is glycerin. 8. Low temperature synthesis of particles of the iron oxide epsilon phase (IIT), according to previous claims, where the deposition is carried out by dip-coaling. 9. Synthesis at low temperature of particles of the epsilon phase of iron oxide (I1I), according to previous claims, where the deposited sol-gel is dried at a temperature of 60 oC. 10. Low temperature synthesis of particles of the epsilon phase of iron oxide 20 (III), according to claim 8, where the sol-gel dries for a time of 7 days in the case of the thin film. 11. Low-temperature synthesis of particles of the epsilon iron oxide phase (I1I), according to claim 8, where the sol-gel dries for a period of 30 days in the case of the monolith. 12. Low temperature synthesis of particles of the epsilon iron (III) oxide phase, according to previous claims, where the heat treatment is carried out in the presence of air as an oxidizing atmosphere. 13. Synthesis at low temperature of particles of the epsilon phase of iron oxide (I1I), according to claim 12, where the heat treatment is carried out with an upward ramp of 1 oC / min.14. Low temperature synthesis of particles of the epsilon iron (III) oxide phase, according to claims 12 and 13, where the heat treatment is carried out at a temperature between 350 and 960 oC.
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    ES2666704B2|2018-11-14|
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    CN109231974A|2018-10-19|2019-01-18|扬州大学|A kind of method of sol-gel auto-combustion synthesis ε type ferric oxide nano permanent magnet|
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