DE102013215498A1 - Iron oxide and silica containing core-shell particles with improved heating rate - Google Patents

Abstract

Core-shell particles whose core contains magnetic, crystalline iron oxide and its shell amorphous silicon dioxide, wherein the shell additionally contains at least one non-magnetic metal oxide or the shell and the core additionally each contain at least one non-magnetic metal oxide.

Description

The invention relates to iron oxide and silica containing core-shell particles with improved heating rate in the magnetic field, their preparation and their use.

In WO03 / 042315 describes the use of iron-silicon oxide particles for inductive heating of adhesive composites. The particles can be obtained either via sol-gel routes or by flame pyrolysis.

In the WO 2010/063557 Iron-silicon oxide particles are disclosed which can be used for inductive heating. The particles have a core-shell structure, with the iron oxide phases hematite, magnetite and maghemite as the core and an amorphous shell of silicon dioxide. The particles are prepared by reacting a mixture of silicon compounds, one of which is a monosilane, and an iron compound in a hydrogen / oxygen flame.

In EP-A-2000439 For example, doped iron-silicon oxide particles having a core-shell structure are disclosed, the choice of doping components being limited to those having magnetic properties. In addition, the particles have a fairly high chloride content. The particles are obtained by flame pyrolysis, whereby reducing gases are introduced into different reaction zones.

In WO2012 / 048985 For example, silica-coated acicular iron oxide particles which may be doped with P, Si, Al, Mg, Co, K or Cr are disclosed. There are no quantities and no information on the compounds that can be used. The doping serves to influence the particle size and shape. It is not known in which chemical form and at which point of the particle, core and / or shell, the doping component is incorporated.

The documents cited in the prior art disclose the use of iron-silicon oxide particles for inductive heating in a magnetic or electromagnetic alternating field. Although the heating times could be significantly improved, the goal remains to further reduce the heating times. It was therefore the object of the present invention to provide a material with which this goal can be achieved.

• a) die Hülle zusätzlich wenigstens ein nichtmagnetisches Metalloxid enthält oder
• b) die Hülle und der Kern zusätzlich jeweils wenigstens ein nichtmagnetisches Metalloxid enthalten.

The invention relates to core-shell particles whose core contains magnetic, crystalline iron oxide and its shell amorphous silica, wherein

• a) the shell additionally contains at least one non-magnetic metal oxide or
• b) the shell and the core additionally each contain at least one non-magnetic metal oxide.

The sum of the proportions of iron oxide, silicon dioxide and non-magnetic metal oxide is generally at least 98% by weight, preferably at least 99% by weight, particularly preferably at least 99.5% by weight, based in each case on the core-shell particles ,

The nonmagnetic metal oxide is preferably selected from the group consisting of alumina, calcia, ceria, chromia, copper oxide, magnesia, silver oxide, titania, tungsten oxide, yttria, zinc oxide, tin oxide and zirconia. Particularly preferred are zinc oxide and alumina.

In the embodiment of the invention wherein both the core and the shell contain nonmagnetic metal oxides, it is understood that the nonmagnetic metal oxide may be the same or different in core and shell. Likewise, in this embodiment, the proportions of the non-magnetic metal oxides may be the same or different.

The best results in terms of the heating rate are obtained when the non-magnetic metal oxide is present in a proportion of 0.5 to 10 wt .-%, based on the core-shell particles. The distribution of this proportion to the core and the shell is not critical. Particularly preferred is an embodiment in which the proportion of the non-magnetic metal oxide is 1.5 to 7.5 wt .-%. Very particular preference is given to the embodiment in which the proportion of non-magnetic metal oxide in the core and shell is in each case from 0.5 to 5% by weight, based on the core-shell particles.

The proportion of silicon dioxide in the core-shell particles according to the invention is preferably 5 to 40 wt .-%, particularly preferably 10 to 20 wt .-%. The proportion of silicon dioxide in the shell of the particles according to the invention is preferably 95 to 99.5% by weight, particularly preferred

The particles according to the invention are largely in the form of isolated individual particles. The individual particles have a largely spherical to bulbous form. The average diameter of the individual particles is generally from 5 to 500 nm, preferably from 50 to 200 nm. Needle-shaped particles are not found. In addition to the isolated individual particles, three-dimensional aggregates of these particles can also be present. In these aggregates, the individual particles are firmly grown together. The proportion of aggregates is less than 50 wt .-%, preferably less than 20 wt .-%, based on the sum of individual particles and aggregates. The determination can be carried out, for example, by image evaluation of TEM images by means of suitable software, as is already known for other magnetic core-shell particles. The BET surface area of the particles according to the invention is generally 5 to 40 m ² / g, preferably 10 to 25 m ² / g.

The proportion of silicon dioxide in the core-shell particles according to the invention is preferably from 5 to 40% by weight of silica, more preferably from 8 to 25% by weight and most preferably from 10 to 20% by weight. The proportion of silicon dioxide in the shell is preferably from 70 to 95% by weight, based on the sum of silicon dioxide and metal oxide.

The case is a tight shell. By dense is meant that at 12 hours contact of the particles at 60 ° C with hydrochloric acid less than 300 ppm iron, hydrogen peroxide less than 10 ppm iron or a NaCl / CaCl ₂ solution less than 50 ppm iron are detectable. The thickness of the shell is preferably 1 to 40 nm, more preferably 5 to 20 nm. The thickness of the shell can be determined, for example, by evaluating HR-TEM images.

The proportion of iron oxide in the core-shell particles according to the invention is preferably 60 to 95% by weight of silicon dioxide, more preferably 75 to 90% by weight and most preferably 70 to 80% by weight. The proportion of iron oxide in the core is preferably 90 to 100 wt .-%, based on the sum of silicon dioxide and metal oxide.

The crystalline iron oxide present in the core of the particles according to the invention may be magnetite, maghemite and hematite as main components. The mentioned interplanar spacings correspond with these iron oxide modifications. Thus, the interplanar spacing of 0.20 nm and 0.29 nm comprises maghemite and magnetite, while the interplanar spacing of 0.25 nm corresponds to maghemite, magnetite and hematite. There are no lattice plane distances detected in the HR-TEM that would have to be assigned to the doping component.

The core of the core-shell particles according to the invention preferably has a ratio (magnetite + maghemite) / hematite of 70:30 to 95: 5, particularly preferably 80:20 to 90:10, and a ratio of magnetite / maghemite of preferably 50: 50 to 90:10, more preferably 60:40 to 70:30. With these conditions, the best heating times are achieved. The composition of the core with respect to maghemite, magnetite and hematite can be determined, this can be done by X-ray diffractometry using Co-K _α radiation in an angular range 2Θ of 10-100 °. Thus, Maghemit is significantly detectable by the reflexes (110) and (211) in the front angle range. The hematite is clearly identifiable because of the freestanding reflexes. The quantitative phase analysis is carried out using the Rietveld method, error about 10% relative.

The core-shell particles according to the invention may contain, in a boundary layer between core and shell, one or more compounds containing iron, silicon and oxygen, which in the HR-TEM have a spacing of the lattice planes of 0.31 +/- 0.01 nm exhibit. This can be determined by XPS-ESCA analysis (XPS = Electron Spectroscopy for Chemical Analysis) and TEM-EDX analysis (TEM) in conjunction with energy-dispersive analysis of characteristic X-rays [EDX] become. These compounds can surround the core in the form of another shell, in addition to silicon dioxide. The thickness of this shell is 0.5 to 2 nm. This shell provides a transition region between amorphous silica shell and crystalline iron oxide core, resulting in an excellent adaptation between core and outer shell. It is currently believed that the phonon transport and thus the heat conduction from core to outer shell is improved by this intensive bond, which can lead to substantially higher heating rates in the application of the particles according to the invention.

The core-shell particles according to the invention also have hydroxyl groups on their surface. These may react with inorganic and organic surface modification agents to form a Van der Waals interaction, ionic or covalent bond. Suitable means for Surface modification may be, for example, alkoxysilanes, carboxylic acids, nucleic acids or polysaccharides.

• a) in einer ersten Zone eines Durchflussreaktors ein Gemisch enthaltend a11) ein oder mehrere Eisenverbindungen oder a12) jeweils ein oder mehrere Eisenverbindungen und Metallverbindungen, a2) ein oder mehrere wasserstoffhaltige Brenngase und a3) ein oder mehrere Sauerstoff enthaltende Gase zündet und abreagieren lässt,
• b) in einer zweiten Zone des Durchflussreaktors zu diesem Reaktionsgemisch jeweils ein oder mehrere Siliciumverbindungen und Metallverbindungen gibt,
• c) nachfolgend das Reaktionsgemisch gegebenenfalls kühlt und den Feststoff von gas- oder dampfförmigen Stoffen abtrennt und
• d) gegebenenfalls den Feststoff anschließend mit einem Mittel zur Oberflächenmodifizierung behandelt, wobei
• e) Eisenverbindung, Siliciumverbindung und Metallverbindung oxidierbar und/oder hydrolysierbar sind.

Another object of the invention is a process for the preparation of the core-shell particles in which

• a) in a first zone of a flow-through reactor, a mixture comprising a11) one or more iron compounds or a12) one or more iron compounds and metal compounds, a2) one or more hydrogen-containing fuel gases and a3) ignites and abreacts one or more oxygen-containing gases,
• b) in a second zone of the flow-through reactor, in each case one or more silicon compounds and metal compounds are present for this reaction mixture,
• c) subsequently, if appropriate, cooling the reaction mixture and separating off the solid from gaseous or vaporous substances, and
• d) optionally subsequently treating the solid with a surface modification agent, wherein
• e) iron compound, silicon compound and metal compound are oxidizable and / or hydrolyzable.

The iron compound and the metal compound are preferably introduced as aerosol in the first zone. The aerosol formation can be carried out in each case from a solution, a dispersion or the substance itself in the form of a liquid in each case using a sputtering gas such as air or nitrogen and a two-fluid or multi-fluid nozzle. It is also possible to generate the aerosol from a solution containing both the iron compound and the metal compound.

The silicon compound and the metal compound may also be introduced into the second zone as an aerosol together or separately. It has also proven to introduce the silicon compound as a vapor in the second zone.

The mean droplet diameter of the aerosols is preferably less than 100 μm, more preferably less than 50 μm.

The iron compound used is preferably iron (II) chloride.

The silicon compound is preferably selected from the group consisting of SiCl ₄ , CH ₃ SiCl ₃ , (CH ₃ ) ₂ SiCl ₂ , (CH ₃ ) ₃ SiCl, HSiCl ₃ , (CH ₃ ) ₂ HSiCl and CH ₃ C ₂ H ₅ SiCl ₂ H ₄ Si, Si (OC ₂ H ₅ ) ₄ and / or Si (OCH ₃ ) ₄ . Particular preference is given to using SiCl ₄ and / or Si (OC ₂ H ₅ ) ₄ .

The metal compound is converted to the non-magnetic metal oxide during the process. Suitable metal compounds are especially salts in the form of nitrates, chlorides and octoates, such as 2-ethylhexanoates. Likewise, organometallic compounds such as alkoxides or acetylacetonates can be used. The metal component is preferably selected from the group consisting of aluminum, calcium, cerium, chromium, copper, magnesium, silver, titanium, tungsten, yttrium, zinc, tin and zirconium. Explicit examples are: aluminum isopropylate, aluminum sec-butoxide, aluminum nitrate, copper nitrate, yttrium nitrate, silver nitrate, zinc nitrate, zirconium nitrate, calcium chloride, magnesium chloride, titanium tetrachloride, titanium isopropylate, zinc octoate and zirconium octoate.

In a particular embodiment of the invention, water or steam can additionally be introduced into the second zone. In this case, the water or the steam is introduced separately from the silicon compound and the metal compound. Preferably, a molar excess of water or water vapor is used. Particularly preferred may be a molar ratio of water / silicon compound of 10 to 100.

As fuel gases, hydrogen, methane, ethane and / or propane can preferably be used. Particularly preferred is hydrogen. The oxygen-containing gas used is mainly air or oxygen-enriched air. As a rule, an excess of oxygen over hydrogen is used. Lambda, the quotient of the amount of fuel to the amount of oxygen, is preferably 1.05-1.50.

The reaction conditions may preferably be chosen so that in the first zone, the average residence time 10 ms to 1 s, more preferably 300 to 600 ms, and the temperature preferably 800 to 1300 ° C, more preferably 950 to 1100 ° C, and in the second zone, the average residence time 0.1 to 10 s, more preferably 1 to 3 s and the temperature is preferably 400 to 900 ° C, particularly preferably 700 to 850 ° C. In the first zone the temperature is measured 50 cm below the ignition point, in the second zone 15 cm above the uppermost point of addition in the second zone.

Suitable surface modification agents are organosilanes, silazanes or polysiloxanes. Usually these agents are sprayed onto the core-shell particles and then treated at temperatures of 120 to 200 ° C, preferably under a protective gas atmosphere, over a period of 1 to 5 hours.

Another object of the invention is a silicone rubber containing the core-shell particles according to the invention. The proportion of these particles is preferably 0.5 to 15 wt .-% and particularly preferably 3 to 6 wt .-%.

Another object of the invention is the use of the core-shell particles according to the invention as a component of rubber mixtures, polymer formulations, adhesives, obtainable by welding in electromagnetic alternating field plastic composite moldings and for the preparation of dispersions.

Examples

analytics

To determine the iron oxide content, the sample was homogenized in the laboratory mill and determined titrimetrically after melt digestion. The Fe (III) content is determined by means of manganometry. The content of Si and the other metal oxide components is determined by means of ICP-OES and then calculated as oxide.

The BET surface area is determined by DIN 66131 ,

The determination of the core components is done by X-ray diffractometry. (Reflection, θ / θ diffractometer, Co-Kα, U = 40 kV, I = 35 mA; scintillation counter, simulated graphite monochromator; angular range (2Θ) / pitch / measuring time: 10-100 ° / 0.04 ° / 6 s ( 4 h)). Using the Rietveld method, a quantitative phase analysis is performed (error about 10% relative). Quantitative phase analysis is performed using set 60 of the ICDD database PDF4 + (2010). The phase analysis and the crystallite done with the Rietveld program SiroQuant ^® version 3.0 (2005).

The thickness of the shell is determined by high-resolution transmission electron microscopy (HR-TEM).

The heating time from 20 ° C to 200 ° C is determined in a silicone composition. The silicone composition is obtained by mixing 33 g ELASTOSIL ^® E50, Fa. Momentive Performance Materials, 13 g of silicone oil type M 1000, Fa. Momentive Performance Materials, 4 g of Aerosil ^® 150, Fa. Evonik and 2.5 g, equivalent to 4, 76 wt .-%, core-shell particles mixed by means of a SpeedMixers 2 × 30 s and 2 × 45 s at 3000 rev / min. Subsequently, the silicone composition is applied in a thickness of about 1 mm on a glass slide. The energy input is made by induction by means of a water-cooled coil with a diameter of 80 mm. The frequency is 510 KHz, the power about 12 KW, Fives Celes GTMC 25 KW, France.

Leaching test: 0.33 g core-shell particles in 20 ml HCl (1 mol / l) or H ₂ O ₂ (0.5 mol / l) or a solution of 8 wt% NaCl and 2 wt% CaCl ₂ in water is stored at 60 ° C over a period of 12 hours. A part of the solution is subsequently analyzed for iron by means of suitable analysis techniques, for example ICP (inductively coupled plasma spectroscopy).

Examples 1 to 10 show the preparation of core-shell particles according to the invention, in which both core and shell contain a nonmagnetic metal oxide. Here, in Examples 1 to 8, the nonmagnetic metal oxide in the core and shell is the same, in Examples 9 and 10 different. Example 11 shows the preparation of core-shell particles according to the invention, in which only the shell contains a nonmagnetic metal oxide.

Example 1: An aerosol obtained by atomizing 4500 g / h of an aqueous solution consisting of 26.1 g of iron (II) chloride, 1.3 g of zinc nitrate and 72.6 g of water, in each case per 100 g of solution, and 3.0 kg / h nitrogen are atomized by means of a two-fluid nozzle. The aerosol thus obtained is reacted with 8.8 Nm ³ / h of hydrogen and 19 Nm ³ / h of air, of which 15 Nm ³ / h of primary air and 4 Nm ³ / h of secondary air, in a first zone. The mean residence time of the reaction mixture in the first zone is about 540 ms. The temperature in the first zone is 979 ° C.

In the stream of the reaction mixture from the first zone 410 g / h of vaporous Si (OC ₂ H ₅ ) ₄ and formed by a two-fluid nozzle aerosols of 4 Nm ³ / h nitrogen and 200 g / h of a 10 weight percent aqueous solution of Zn (NO ₃ ) ₂ through two-fluid nozzle with 200 l / h N ₂ given. Separately, an additional 2.5 kg / h steam are added. The mean residence time of the reaction mixture in the second zone is 1.7 s. The temperature in the second zone is 808 ° C. Subsequently, the reaction mixture is cooled and the resulting solid is deposited on a filter of the gaseous substances.

Examples 2 to 11 are carried out analogously to Example 1. The starting materials are shown in Table 1.

In Examples 1 to 6, aqueous solutions of the metal compounds are used in the first and second zones.

In Examples 7 to 11, an organometallic compound is used as metal compound in the second zone. The octoates are used in the form of 2-ethylhexanoates dissolved in 2-ethylhexanoic acid, the content of octoate in the solution in each case being 50 ± 2% by weight. Titanium (IV) isopropylate and aluminum tri-sec-butylate are used as the substance. The starting materials are also shown in Table 1.

Table 2 shows the average residence times t and the temperature T in the first and second zones.

The physico-chemical properties of the core-shell particles are given in Table 3.

As a comparative example, the powder of Example 6 is made EP-A-2000439 used. This is an iron-silicon mixed oxide powder doped with 1.8% by weight of manganese. The heating time from 20 ° C to 200 ° C is 15 s.

As a further comparative example, the powder of Example 10 is made WO 2012/048985 used. This is an iron-silicon mixed oxide powder doped with 1.8% by weight of phosphorus. The heating time from 20 ° C to 200 ° C is 17 s.

The core-shell particles according to the invention have significantly shorter heating times than powders according to the prior art.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 03/042315 [0002]
• WO 2010/063557 [0003]
• EP 2000439 A [0004, 0048]
• WO 2012/048985 [0005, 0049]

Cited non-patent literature

• DIN 66131 [0035]

Claims (14)

Core-shell particles whose core contains magnetic, crystalline iron oxide and its shell amorphous silica, wherein a) the shell additionally contains at least one non-magnetic metal oxide or b) the shell and the core additionally each contain at least one non-magnetic metal oxide. The core-shell particle according to claim 1, characterized in that the non-magnetic metal oxide is selected from the group consisting of alumina, calcia, ceria, chromia, copper oxide, magnesia, silver oxide, titania, tungsten oxide, yttria, zinc oxide, tin oxide and zirconia. Core-shell particles according to claims 1 to 3, characterized in that the non-magnetic metal oxide is present in a proportion of 0.5 to 10 wt .-%, based on the core-shell particles. Core-shell particles according to claims 1 to 3, characterized in that they contain 5 to 40 wt .-% silica. Core-shell particles according to claims 1 to 4, characterized in that they contain 60 to 95 wt .-% iron oxide. Core-shell particles according to claims 1 to 5, characterized in that the core lattice spacings of 0.20 nm, 0.25 nm and 0.29 nm, respectively +/- 0.02 nm, determined by means of high-resolution transmission electron microscopy , having. Core-shell particles according to claims 1 to 6, characterized in that the determined by X-ray diffractometry ratio of (magnetite + maghemite) to hematite is equal to 70:30 to 95: 5 and from magnetite to maghemite equal to 50:50 to 90:10 is. Core-shell particles according to claims 1 to 7, characterized in that there are between core and shell one or more, the elements containing iron, silicon and oxygen compounds, which in HR-TEM a distance of the lattice planes of 0.31 + / - 0.01 nm. Core-shell particles according to claims 1 to 8, characterized in that they are modified by adsorption, reaction at the surface or complexation of or with inorganic and organic reagents. A process for preparing the core-shell particles according to claims 1 to 9, characterized in that a) in a first zone of a flow reactor, a mixture containing a11) one or more iron compounds or a12) each one or more iron compounds and metal compounds, a2 b) in a second zone of the flow reactor to this reaction mixture in each case one or more silicon compounds and metal compounds, c) subsequently cooling the reaction mixture optionally and the solid d) optionally the solid subsequently treated with a surface modification agent, wherein e) iron compound, silicon compound and metal compound are oxidizable and / or hydrolyzable. A method according to claim 10, characterized in that in the first zone, the average residence time is 10 ms to 1 s and the temperature 800 to 1300 ° C and in the second zone, the average residence time 0.1 to 10 s and the temperature 400 to 900 ° C is. A method according to claims 10 or 11, characterized in that in the second zone additionally water or water vapor is introduced. Silicone rubber containing the core-shell particles according to claims 1 to 9. Use of the core-shell particles according to claims 1 to 9 as a constituent of rubber mixtures, as a constituent of polymer preparations, as a constituent of adhesives, as a constituent of plastic composite moldings obtainable by welding in electromagnetic alternating field, for the preparation of dispersions and for the immobilization of enzymes.