WO2008148588A1

WO2008148588A1 - Silicon-iron mixed oxide powder

Info

Publication number: WO2008148588A1
Application number: PCT/EP2008/053508
Authority: WO
Inventors: Stipan Katusic; Markus PRIDÖHL; Guido Zimmermann; Rainer Hahn; Meike Pelz
Original assignee: Evonik Degussa Gmbh
Priority date: 2007-06-06
Filing date: 2008-03-25
Publication date: 2008-12-11
Also published as: CN101320610A; EP2000439A1; ATE539038T1; EP2000439B1; CN101320610B

Abstract

Silicon-iron mixed oxide powder having magnetic properties in the form of aggregated primary particles, a) for which TEM images show the presence of primary particles composed of spatially separate regions of silicon dioxide and iron oxide and the mean particle diameter of the iron oxide is 2 to 100 nm and b) in which the proportion of silicon, calculated as SiO2, is 5 to 65% by weight iron, calculated as Fe2O3, is 30 to 90% by weight and the proportion of silicon and iron, each calculated as abovementioned oxides, is at least 95% by weight c) in which the proportion of chloride is 0.2 to 3% by weight.

Description

Silicon-iron mixed oxide powder

The invention relates to a silicon-iron mixed oxide powder, and to its preparation and use.

Silicon-iron mixed oxide particles with magnetic properties, especially superparamagnetic properties, are described in the literature. These particles can be prepared, for example, by sol-gel processes or pyrogenic processes. In the latter, generally organic, chloride-free starting materials are used. These processes are uneconomic for the preparation of relatively large amounts.

EP-A-1284485 discloses a process in which chloride- containing starting materials can be used and the resulting silicon-iron mixed oxide particles, in spite of a chloride content of up to 1000 ppm, still have good magnetic properties. The particles disclosed in EP-A-1284485 comprise superparamagnetic iron oxide domains with a diameter of 3 to 20 nm in a silicon- dioxide-containing matrix. Compared to the purely organic processes, the process disclosed in EP-A- 1284485 offers economic advantages, but there is still the desire for inexpensively preparable particles.

It was therefore an object of the present invention to provide particles with good magnetic properties, which are preparable by means of an economically viable process .

The invention provides a silicon-iron mixed oxide powder having magnetic properties in the form of aggregated primary particles, a) for which TEM images show the presence of primary particles composed of spatially separate regions of silicon dioxide and iron oxide and the mean particle diameter of the iron oxide is 2 to 100 nm and b) in which the proportion of - silicon, calculated as Siθ₂, is 5 to 65% by weight iron, calculated as Fe₂U3, is 30 to 90% by weight and the proportion of silicon and iron, each calculated as abovementioned oxides, is at least 95% by weight c) in which the proportion of chloride is 0.2 to 3% by weight.

Magnetic properties are understood to mean ferrimagnetic, ferromagnetic and/or superparamagnetic properties. Preference may be given to inventive powder with superparamagnetic properties.

Superparamagnetic substances do not have permanent (coincident) arrangement of the elementary magnetic dipoles in the absence of external, active magnetic fields. They may have a low residual magnetization.

Preference may be given to inventive powders whose proportion of silicon is 50 ± 10% by weight or 20 ± 10% by weight.

Preference may further be given to inventive powders whose proportion of iron is 50 ± 10% by weight or 80 ± 10% by weight.

The primary particles include those in which the mixed oxide components are present both in and on the surface of a primary particle. In the contact region of silicon dioxide and iron oxide within a primary particle, Si-O- Fe may be present. Furthermore, individual primary particles even composed only of silicon dioxide and/or iron oxide may be present. The primary particles are very substantially pore-free, but have free hydroxyl groups on the surface and may have different degrees of aggregation. The aggregates are three-dimensional aggregates. In general, the aggregate diameter, in one three-dimensional direction in each case, is preferably not more than 250 nm, generally from 30 to 200 nm. Figure 1 shows a schematic of such a three-dimensional structure with an aggregate diameter of 135 nm and 80 nm. Several aggregates may combine to form agglomerates. These agglomerates can be separated again easily. In contrast, the division of the aggregates into the primary particles is generally impossible.

The inventive silicon-iron mixed oxide powder is notable in particular for a high chloride content of 0.2 to 3% by weight, based on the silicon-iron mixed oxide particles. The chloride content originates from the preparation of the particles. The inventive particles are obtained by a pyrogenic process in which chlorine-containing precursors are used. The particles which form contain chlorine generally in the form of hydrochloric acid. This can adhere or be incorporated within the particles which form.

It has, however, been found that chloride contents of 0.2 to 3% by weight have only a negligible influence, if any, on the magnetic properties of the powder.

Preference may be given to a silicon-iron mixed oxide powder having a chloride content of 0.5 to 2.5% by weight. Particular preference may be given to a silicon-iron mixed oxide powder having a chloride content of from 1 to 2% by weight. The total chloride content is determined by Wickbold combustion or by digestion with subsequent titration or ion chromatography.

Moreover, TEM images of the inventive powder show the presence of primary particles composed of spatially separate regions of silicon dioxide and iron oxide. Silicon dioxide may form an envelope around the iron oxide with a thickness of 1 to 15 nm.

The mean diameter of the iron oxide constituents is 2 to 100 nm. The mean particle diameter is preferably less than 70 nm. Particular preference may be given to a range of >20 to 60 nm.

The inventive silicon-iron mixed oxide powder may further comprise at least one or more primary particles which consist of silicon dioxide or iron oxide, i.e. in which silicon dioxide and iron oxide are not present together. The proportions of primary particles which have only silicon dioxide or iron oxide may be evaluated by counting from TEM images; in general, several thousand primary particles are evaluated. The proportions are 0 to a maximum of 5%, generally 0 to <1%, of the counted primary particles.

The silicon dioxide constituent in the inventive mixed oxide powder may be present in either crystalline or amorphous form, preference being given to purely amorphous silicon dioxide.

The iron oxide constituent of the particles of the inventive mixed oxide powder preferably has magnetite and/or maghemite as the main constituent. In addition, it may comprise a total of up to 15%, generally less than 10%, based on the iron oxides, of haematite, beta- Fe203 and iron silicate. Particular preference may be given in particular to a mixed oxide powder in which the proportion of magnetite and/or maghemite, based on the iron oxides, is at least 80%, most preferably at least 90%.

When the intention is to vary the magnetic properties of the powder, it may also be advantageous to provide powders in which the maghemite/magnetite weight ratio is 0.3:1 to 100:1. It is likewise possible that the iron oxide is present only in the form of maghemite.

The proportion of iron oxide, calculated as Fe2U3, in the inventive powder is 30 to 90% by weight. The silicon-iron mixed oxide powder may preferably have a proportion of iron oxide of 50 ± 10% by weight or 80 ± 10% by weight. Particular preference is given to a range of 50 ± 5% by weight or 80 ± 5% by weight.

The sum of silicon dioxide and iron oxide in the inventive powder is at least 95% by weight, preferably at least 98% by weight and more preferably at least 98.5% by weight.

In addition to silicon dioxide, iron oxide and chloride, the inventive mixed oxide powder may comprise at least one doping component. This is preferably selected from the group consisting of the oxides of manganese, cobalt, chromium, europium, yttrium, samarium, nickel and gadolinium.

A particularly preferred doping component is manganese oxide .

The proportion of the doping component is preferably 0.005 to 2% by weight, more preferably 0.5 to 1.8% by weight and most preferably 0.8 to 1.5% by weight, calculated in each case as the oxide and based on the mixed oxide powder.

The doping component is generally distributed homogeneously in the powder. Depending on the type of dopant and the reaction, the doping component may be present in enriched form in regions of silicon dioxide or iron oxide.

The BET surface area of the inventive mixed oxide powder can be varied within wide ranges. A favourable BET surface area has been found to be in the range of 10 to 100 m²/g. Preference may be given to powders having a BET surface area of 40 to 70 m²/g.

The particles of the inventive mixed oxide powder may be enveloped by one or more shells of identical or different polymers or polymer mixtures. Particularly suitable polymers may be polymethyl methacrylates .

The inventive mixed oxide powder features a high saturation magnetization. Preference may be given to mixed oxide powders whose saturation magnetization is 40 to 120 Am²/kg Fe₂O₃ and more preferably 60 to 100 Am²/kg Fe₂O₃.

It has also been found that an advantageous mixed oxide powder is one which has the following features: a) BET surface area 50 ± 5 m²/g b) proportion of

- silicon, calculated as SiO₂, 50 ± 5% by weight

- iron, calculated as Fe₂O₃, 45 ± 5% by weight

- chloride 1.5 ± 0.5% by weight

- manganese, calculated as MnO, 0.5 ± 0.3% by weight, where the sum of the oxides adds up to 100%, c) mean diameter of the iron oxide 10-30 nm d) proportion of (magnetite + maghemite) , based on iron oxide, 90 ± 10% by weight.

It has also been found that an advantageous mixed oxide powder is one which has the following features: a) BET surface area 50 ± 10 m²/g b) proportion of

- silicon, calculated as Siθ2, 10 ± 5% by weight

- iron, calculated as Fe2U3, 85 ± 5% by weight - chloride 1.0 ± 0.2% by weight

- manganese, calculated as MnO, 1.8 ± 0.2% by weight, c) mean diameter of the iron oxide 10-30 nm d) proportion of (magnetite + maghemite) , based on iron oxide, 90 ± 10% by weight.

The invention further provides a process for preparing the inventive silicon-iron mixed oxide powder, in which a) 10 to 60% by weight of one or more vaporous halosilicon compounds, calculated as Siθ2, b) 40 to 90% by weight of iron chloride, calculated as Fe2U3, in the form of a solution and c) optionally 0.005 to 2% by weight of one or more doping compounds, calculated as oxide, d) are fed separately to the high-temperature zone of a reactor, e) are reacted in the high-temperature zone at temperatures of 700 to 2500⁰C with an excess of oxygen or an oxygenous gas, f) and, in a second zone of the reactor downstream of the high-temperature zone, reducing gases are added to the reaction mixture at one or more points in an amount such as to give rise to a reducing atmosphere overall in this second zone, and the temperature is reduced to 500⁰C to 150⁰C, g) the resulting solid is separated from gaseous substances in a further, third zone in which a reducing atmosphere is likewise still present, and h) optionally, sufficient air is added to the gaseous substances that the offgas does not give rise to a reducing atmosphere.

In the context of the invention, a solution is understood to mean one in which the main constituent of the liquid phase is water, water and one or more organic solvents, or a mixture of water with one or more organic solvents. The preferred organic solvents used may be alcohols such as methanol, ethanol, n- propanol, isopropanol, n-butanol or isobutanol or tert- butanol. Particular preference is given to those solutions in which water is the main constituent.

In the context of the invention, a doping component is understood to mean the oxide of an element which is present in the inventive powder. A dopant is understood to mean the compound which is used in the process in order to obtain the doping component. The dopant may be added separately from the halosilicon compound and the iron chloride. This can be done in the form of a vapour or of a solution. The dopant may also be introduced in the form of a vapour together with the halosilicon compound or as a constituent of the iron chloride- containing solution.

In a preferred embodiment, the temperature results from a flame which is generated by igniting a mixture which comprises one or more combustion gases and an oxygen- containing gas and which burns into the reaction chamber .

Suitable combustion gases may be hydrogen, methane, ethane, propane, natural gas, acetylene, carbon monoxide or mixtures of the aforementioned gases. Hydrogen is the most suitable. The oxygen-containing gas used is generally air. As well as the mixed oxides, the reaction mixture also comprises the gaseous reaction products and any unreacted gaseous feedstocks. Gaseous reaction products may, for example, be hydrogen chloride and carbon dioxide .

According to the invention, the reaction mixture is mixed with a reducing gas or a mixture of a reducing gas with air which is added in zone II of the reactor. The reducing gas may, for example, be forming gas, hydrogen, carbon monoxide, ammonia or mixtures of the aforementioned gases, and particular preference may be given to forming gas. Such a reducing gas is, in the process according to the invention, added to the reaction mixture in such an amount as to give rise to a reducing atmosphere.

In the context of the invention, a reducing atmosphere is understood to mean one in which the lambda value is less than 1.

Lambda is calculated from the quotient of the sum of the oxygen content of the oxygenous gas divided by the sum of the iron and silicon compounds to be oxidized and/or to be hydrolysed and of the hydrogen-containing combustion gas, in each case in mol/h.

When, for example, hydrogen and air are used in the high-temperature zone (zone I) and air and forming gas

(80:20 N₂/H₂) are used in zone II, the lambda value is calculated according to the following formula in zone

II and III to be 0.21-excess air from zone

I/O .5 ^• (H₂+0.2 ^• forming gas), based in each case on the amount of gas introduced per unit time. For zone I, the lambda value is greater than 1. When hydrogen and air are used, the lambda value in zone I is determined according to the following formula: 0.21-air/0.5-H₂.

In a preferred embodiment, the residence time in the first zone may be between 0.8 and 1.5 seconds.

In a further preferred embodiment, the sum of the residence times in the second and third zone may be between 15 seconds and 15 minutes.

It is additionally possible for steam to be introduced into the second reactor zone.

Figure 2 shows, by way of example, a schematic setup for the performance of the process according to the invention. I, II and III denote the three reaction zones. In addition: 1 = atomized solution of iron chloride which optionally comprises additional dopants;

2 = oxygen-containing gas, preferably air;

3 = combustion gas, preferably hydrogen;

4a = reducing gas; 4b = steam (optional); 4c = air (optional)

5 = inventive powder deposited on filters;

6 = offgas.

The iron chloride used may preferably be iron (II) chloride (FeC12), iron (III) chloride (FeC13) or a mixture of the two. The iron chloride is introduced as a solution in accordance with the invention. The concentration of the iron chloride may preferably be 1 to 30% by weight and more preferably 10 to 20% by weight, based in each case on the solution. The halosilicon compounds used with preference may be SiCl₄, CH₃SiCl₃, (CH₃)₂SiCl₂, (CH₃)₃SiCl, (CH₃)₄Si, HSiCl₃, (CH₃)₂HSiCl, CH₃C₂H₅SiCl₂, disilanes of the general formula R_nCl₃-_nSiSiR_mCl₃-_m where R=CH₃ and n+m = 2,3,4,5 and 6, and also mixtures of the aforementioned compounds. Particular preference is given to the use of silicon tetrachloride.

It is also possible to use halosilicon compounds from those fractional cuts obtained in the Muller-Rochow synthesis, and the fractional cuts may also comprise proportions of Ci-Ci₂-hydrocarbons . The proportion of these hydrocarbons may be up to 10% by weight, based on one fraction. Usually, these proportions are between 0.01 and 5% by weight, and the proportion of the Ce hydrocarbons, for example cis- and trans-2-hexene, cis- and trans-3-methyl-2-pentene, 2, 3-dimethyl-2-butene, 2- methylpentane, 3-methylpentane generally predominates.

When halosilicon compounds from the Muller-Rochow synthesis are used, this is preferably done in mixtures with silicon tetrachloride.

The invention further comprises a second process for preparing the silicon-iron mixed oxide powder, in which a) 10 to 60% by weight of one or more vaporous halosilicon compounds, calculated as SiO₂, b) 40 to 90% by weight of iron chloride, calculated as Fe₂O₃, in the form of a solution and c) optionally 0.005 to 2% by weight of one or more doping compounds, calculated as oxide, d) are fed separately to the high-temperature zone of a reactor, e) are reacted in the high-temperature zone at temperatures of 700 to 2500⁰C in a flame which is generated by the ignition of a mixture which comprises one or more combustion gases and an oxygen-containing gas and which burns into the reaction chamber, and in which oxygen is used in deficiency, f) in a second zone of the reactor downstream of the high-temperature zone, air or air and steam are added to the reaction mixture at one or more points in an amount such as to give rise to, overall in this second zone a reducing atmosphere or oxidizing atmosphere and the temperature is reduced to 500⁰C to 150⁰C and g) the resulting solid, in a further, third zone, is removed from gaseous substances of the same atmosphere as are present in the second zone, and h) optionally, sufficient air is added to the gaseous substances that the offgas does not give rise to a reducing atmosphere.

In the context of the invention, a reducing atmosphere is understood to mean one in which the lambda value in zone I, II and III is less than 1.

In the context of the invention, an oxidizing atmosphere is understood to mean one in which the lambda value in zone II and III is greater than 1.

With regard to the type of compounds used and the reaction parameters, the statements made for the process already specified apply.

Figure 2 shows, by way of example, a schematic setup for performing this process according to the invention. I, II and III denote the three reaction zones. In addition :

1 = atomized solution of iron chloride which optionally comprises additional dopants;

2 = oxygen-containing gas, preferably air;

3 = combustion gas, preferably hydrogen (excess) ; 4a = air (excess or deficiency) ; 4b = steam (optional) ; 4c = air (optional) ;

5 = inventive powder deposited on filters;

6 = offgas.

The invention further provides for the use of the silicon-iron mixed oxide powder for producing dispersions and pastes.

The invention further provides for the use of the silicon-iron mixed oxide powder as a constituent of rubber mixtures.

The invention further provides for the use of the silicon-iron mixed oxide powder as a constituent of polymer formulations.

The invention further provides for the use of the silicon-iron mixed oxide powder as a constituent of adhesive compositions.

The invention further provides for the use of the silicon-iron mixed oxide powder as a constituent of plastic composite mouldings obtainable by welding in an electromagnetic alternating field.

In a special embodiment of the invention the silicon- iron mixed oxide powder is a component of a HDPE (high density poly ethylene) containing composition. The amount of the silicon-iron mixed oxide powder is preferably in the range of 0.1 to 15 wt . % and most preferably in the range of 0.5 to 5 wt.%, based on the HDPE containing composition. The composition may further comprise additives known to the person skilled in the art, e.g. organic peroxides.

The HDPE containing composition can be used for pipe extrusion. Due to the presence of the silicon-iron mixed oxide powder the curing may be performed in an electromagnetic alternating field.

Examples

Analytical processes

Determination of the BET surface area: The BET surface area of the inventive particles was determined to DIN 66131.

Determination of the content of silicon dioxide and iron oxide: Approx. 0.3 g of the inventive particles are weighed accurately into a platinum crucible and, to determine the ignition loss, calcined at 700⁰C for 2 h in a crucible, cooled in a desiccator and reweighed. After the edges have been rinsed with ultrapure water, the sample material is fumed to dryness on a hotplate with 1 ml of H₂SO₄ p. a. 1:1 and at least 3 ml of HF 40% p. a. The weight loss as a result of the fuming is assumed to be SiO₂ and the remainder to be Fe₂θ3.

Determination of the chloride content: Approx. 0.3 g of the inventive particles are weighed accurately, admixed with 20 ml of 20 per cent sodium hydroxide solution p. a., dissolved and transferred into 15 ml of cooled HNO3 with stirring. The chloride content in the solution is titrated with AgNO₃ solution (0.1 mol/1 or 0.01 mol/1) .

Determination of the adiabatic combustion temperature: It is calculated from the mass and energy balance of the streams entering the reactor. The energy balance takes account both of the reaction enthalpy of the hydrogen combustion and the conversion of the silicon tetrachloride to silicon dioxide and of the iron (II) chloride to iron (II) oxide, and the evaporation of the aqueous solution.

Determination of the residence time: It is calculated from the quotient of the plant volume flowed through and the operating volume flow rate of the process gases at adiabatic combustion temperature.

Determination of the Curie temperature: The Curie temperature is determined by means of thermogravimetry

(TG) . This determination method is based on the behaviour of magnetic substances of losing their magnetizability at a characteristic temperature, the Curie temperature. At this temperature, the alignment of the elementary magnets is prevented owing to increasing thermal motion. When the TG curve of a ferromagnetic curve is measured in an inhomogeneous magnetic field, the magnetic force disappears at the Curie temperature. The inhomogeneous magnetic field is generated by applying two magnets laterally above the oven body. The sudden change in force at the Curie point brings about the end of the apparent weight increase. The Curie temperature corresponds to the extrapolated end of the TG stage. To illustrate the pure magnetic behaviour, the inventive powder is heated up to 1000⁰C 1. in a magnetic field, 2. without a magnetic field, and measurement 2 is subtracted from measurement 1. This difference curve shows the pure magnetic behaviour. At the start of heating, the TG curve shows an increase in the magnetic force, which corresponds to an apparent decrease in the weight. From a particular temperature, the decrease in the magnetic force sets in, which leads to an apparent weight increase. This weight increase ends at the Curie temperature . X-ray diffractograms (XRD) Measurement :

Reflection, θ/θ diffractometer, Co-Ka, U = 4OkV, I = 35mA Linear PSD with Fe filter, sample rotation Slits: 2^χ8 mm, 0.8 mm Angle range (2theta) : 15 - 112.5° Step width: 0.2°

Evaluation: Rietveld program SiroQuant®.

Examples

Example 1 : 0.87 kg/h of SiCl₄ is evaporated and fed into a mixing zone with 7.0 m (STP) /h of hydrogen and 18.00 m³ (STP) /h of air.

In addition, an aerosol which is obtained from a 25 per cent by weight solution of iron (II) chloride, corresponding to 4.60 kg/h of iron (II) chloride, in water by means of a two-substance nozzle is introduced into the mixing zone within the burner by means of a carrier gas (3 m³ (STP) /h of nitrogen) . The homogeneously mixed gas-aerosol mixture burns in zone I of the reactor at an adiabatic combustion temperature of about 1300°C and a residence time of about 40 msec.

Subsequently, 6500 m³ (STP) /h of forming gas (80:20% by vol. of N2/H2) are added to the reaction mixture leaving zone I in a zone II. This cools the entire reaction mixture to 250°C.

In the zone III downstream of the zone II, the solid is deposited out of the gaseous substances on a filter, and 10 m³ (STP) /h of air are added to the offgas stream.

The physico-chemical values of the resulting solid are reproduced in Table 1. Example 2: As Example 1, but with different feedstock amounts for SiCl₄ and FeCl2.

Example 3 : As Example 1, except using a solution of 97 parts of iron (II) chloride and 3 parts of iron (III) chloride instead of a solution of iron (II) chloride.

Example 4 : As Example 1, except using a solution of iron (III) chloride instead of a solution of iron (II) chloride. Furthermore, an additional 6.0 m³ (STP) /h of steam are introduced into zone II.

Example 5-7: As Example 1, except using a solution of 25% by weight of iron (II) chloride and 20% by weight of manganese (II) chloride.

Example 8: 0.28 kg/h of SiCl₄ are evaporated and fed into a mixing zone with 7.0 m³ (STP) /h of hydrogen and 16 m³ (STP) /h of air.

In addition, an aerosol which is obtained from a 25 per cent by weight solution of iron (II) chloride in water by means of a two-substance nozzle is introduced into the mixing zone within the burner by means of a carrier gas (4.0 m³ (STP) /h of nitrogen) . The homogeneously mixed gas-aerosol mixture burns in zone I of the reactor at an adiabatic combustion temperature of about 1230⁰C and a residence time of about 50 msec.

Subsequently, 12 kg/h of quench air are added to the reaction mixture leaving zone I in a zone II. This cools the entire reaction mixture to 280⁰C.

In the zone III downstream of zone II, the solid is deposited out of the gaseous substances on a filter. In the course of deposition, an oxidizing atmosphere is present .

Example 9: As Example 8, except using a solution of 25% by weight of iron (II) chloride and 20% by weight of manganese ( II ) chloride. Furthermore, an additional 8 kg/h of steam are introduced into zone II.

In the zone III downstream of zone II, the solid is deposited out of the gaseous substances on a filter. In the course of deposition, a reducing atmosphere is present .

Feedstocks, use amounts and reaction parameters of Examples 1 to 9 are reproduced in Table 1. The physico- chemical values of the resulting solids are reproduced in Table 2.

Comparative Example 1 (C-I): 0.14 kg/h of SiCl₄ is evaporated at approx. 200⁰C and fed into a mixing zone with 3.5 m³ (STP) /h of hydrogen and 15 m³ (STP) /h of air. In addition, an aerosol which is obtained from a

10% by weight aqueous iron (III) chloride solution, corresponding to 1.02 kg/h of iron (II) chloride, by means of a two-substance nozzle, is introduced into the mixing zone within the burner by means of a carrier gas

(3 m³ (STP) /h of nitrogen).

The homogeneously mixed gas-aerosol mixture burns there at an adiabatic combustion temperature of about 1200⁰C and a residence time of about 50 msec.

After the flame hydrolysis, the reaction gases and the powder formed are cooled, and the solid is separated from the offgas stream by means of a filter. In a further step, treatment with steam-containing nitrogen removes hydrochloric acid residues which still adhere on the powder. The powder has an iron oxide content of 50% by weight, a BET surface area of 146 m₂/g, a chloride content of 368 ppm and a saturation magnetization of 17 Am₂/kg.

Comparative Example 2 (C-2): As C-I, except using 0.23 kg/h of SiCl₄ and 0.41 kg/h of FeCl₃. The powder has an iron oxide content of 50% by weight, a BET surface area of 174 m²/g, a chloride content of 220 ppm and a saturation magnetization of 6.5 Am₂/kg.

Comparative Example 3 (C-3) : As C-I, except using 0.21 kg/h of SiCl₄ and 0.40 kg/h of FeCl₂. The powder has an iron oxide content of 25% by weight, a BET surface area of 143 m²/g, a chloride content of 102 ppm and a saturation magnetization of 10.4 Am²/kg.

X-ray diffractometry (XRD) : Haematite is identifiable unambiguously owing to the unobscured reflections. The reflections of magnetite and of maghemite overlap one another to a very high degree. Maghemite is detectable significantly on the basis of the (110) and (211) reflections in the acute angle range. With the aid of the Rietveld method, quantitative phase analysis is performed (error approx. 10% relative) . Figure 3 shows the X-ray diffractogram of the powder from Example 5.

Magnetization : The saturation magnetization is the maximum achievable magnetic moment per unit volume. The saturation magnetization is attained in infinitely large magnetic fields. The magnetization which is established at an external field of B = 5 T corresponds approximately to the saturation magnetization and is employed as a measure of the magnetizability . The saturation magnetization of the inventive examples from Examples 1 to 9 is significantly higher than that of Comparative Examples 1 to 3. High-resolution transmission electron microscopy

(HR-TEM) : For the manganese-containing inventive powders from Examples 5-7, the lattice spacings were determined by means of HR-TEM images. The powders exhibit lattice spacings of 0.25 nm, 0.26 nm and

0.27 nm. These values agree very well with the reference values for maghemite 0.25 nm, magnetite

0.252 nm and haematite 0.269 nm. Values which would suggest the presence of a manganese oxide are not found. It can also be concluded from this that manganese is incorporated into the lattice of the iron oxide .

The inventive silicon-iron mixed oxide powder features excellent magnetic properties. Contrary to the adverse influences, described in the literature, of chloride on the magnetic properties, the present invention shows that up to 3% by weight of chloride in the powder has no effect on the magnetic properties. Instead, the novel inventive processes allow the preparation of powders with a high chloride content, whose saturation magnetizations are significantly above those from the prior art. The processes additionally allow large amounts of powder to be prepared inexpensively.

The inventive powders can be used in various ways.

Examples 11 A-C: Preparation of dispersions

Example HA: 12.0 g of the powder from Example 5 are added to 108 g of distilled water and then sufficient IM NaOH is added that the pH is between 9.1 and 9.2. By means of a dissolver, dispersion is effected at 2000 rpm for 5 min. The pH of the dispersion is 9.1, the content of inventive powder is 10% by weight and the mean particle diameter d₅o is 320 nm. Example HB: As Example HA, except, after dispersion by means of a dissolver, dispersion by means of an Ultraturrax over a period of 5 min at 10 000 rpm. The pH of the dispersion is 9.1, the content of inventive powder is 10% by weight and the mean particle diameter d₅o is 180 nm.

Example HC: As Example HB, except, after dispersion by means of a dissolver and Ultraturrax, dispersion by means of ultrasound over a period of 4 min at an amplitude of 80%. The pH of the dispersion is 9.1, the content of inventive powder is 10% by weight and the mean particle diameter d₅o is 140 nm.

Example 12: Rubber mixtures

The inventive silicon-iron mixed oxide powder can be incorporated into rubber mixtures. This can be done, for example, with the feedstocks shown in Table 3A and the method shown in Table 3B.

The samples can be heated rapidly in a high-frequency field, in which case the amount of inventive powder is simultaneously low.

Example 13: Adhesive compositions

25 g of the powder from Example 1 are suspended in 100 ml of ethanol and 20 g of oxybis- (benzosulphohydrazide) are added as a blowing agent. The mixture is heated to 60⁰C with stirring for 5 hours and then the solvent is drawn off on a rotary evaporator. The dry formulation is ground in a ball mill for 3 minutes and then screened. The fraction with a particle size of nominally less than 63 μm is used for the further experiments. 10 g of this powder are mixed with 300 g of the moisture-curing one-component polyurethane adhesive dinitrol PUR 501 FC (Dinol GmbH) in the Planimax mixer (Molteni) provided with kneading hooks. The mixture is kneaded at level 1 (150 rpm) for 15 minutes.

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23

Table 1 : Reaction parameters of inventive Examples 1 to 9

* 4:1 parts by volume of H₂/N₂; after cooling; *** red. (ox.) = deposition of the powder in reducing (oxidizing) atmosphere

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24 Table 2: Physico-chemical values of the powders from Examples 1 to 9

= mean particle diameter of iron oxide; b) as Fe2θ3,- c) as Mnθ2.

Table 3A: Feedstocks for rubber mixture

Keltan®: DSM; Corax®: Degussa; Edenor: Caldic Deutschland; Sunpar: Sun Oil Company; Rhenogran®: Rheinchemie; (§)ρhr: parts per hundred rubber; (*) based on the overall, accelerated rubber mixture Table 3B: Preparation of the rubber mixtures

The adhesive thus modified is used to produce a thick- layer adhesive bond from a sand-blasted and degreased aluminium sheet and a 3 mm-thick float glass pane. The overlap length is 25 mm and the adhesive layer thickness 3 mm. After a curing time of 1 week at 25°C and 50% relative air humidity, the bond is separated again by inductive excitation.

The bond is separated by excitation with an STS M230 semiconductor generator. The excitation frequency of this generator is 300 kHz. For inductive excitation of the adhesive in the adhesive bond, a coil with three turns and an internal diameter of 3 cm is used. The adhesive surface is oriented at right angles to the coil axis in the middle of the coil. At a power of 3000 W and an action time of 2 min, the adhesive is destroyed by the foaming of the blowing agent. The two joined parts can be separated in a simple manner.

Example 14: Plastic composite mouldings obtainable by welding in an electromagnetic alternating field

2 kg of the inventive mixed oxide particles from Example 5 are mixed in the melt with 8 kg of polyamide granules (Vestamid® L1901; designation according to ISO 1874-1: PA12, XN, 18-010; Degussa AG) in a Berstorff ZE25-33D twin-screw extruder at 250⁰C and a throughput of 10 kg/h, extruded and granulated. The granules are subsequently extruded to sheets of thickness 1 mm.

One such sheet is placed between one sheet each of the same or different plastic base material (without inventive powder) , and adhesive tape is wound firmly around this multilayer structure. Subsequently, the multilayer structure was placed into an electromagnetic alternating field for given times at 100% power. The inductor coil has the following data: dimensions: 200 x 45 x 40 mm (L x W x H) , material: rectangular copper tube 10 x 6 x 1 mm, cross-sectional power area: 28 mm², coil supply length: 120 mm, number of coil turns: 3, coil winding length (eff .) : 35 mm, coil diameter (internal) : 20 mm to 40 mm, inner coil area: 720 mm², inductance (at 100 kHz) : approx. 270 nH, working frequency: 323 kHz

The high-frequency semiconductor generator used has the following data: manufacturer: STS - Systemtechnik Skorna GmbH, model designation: STS M260S, clamp power: 6 kW, inductance range: 250 - 1200 nH, working frequency: 150 - 400 kHz (323 kHz with the inductor coil used) .

After the sample has been removed from the alternating field, the adhesive strength is assessed according to the following marks:

In a multilayer structure a) Vestamid® L1901 - Example 14A - Vestamid® L1901 or b) Vestamid® L1901 - Example 14A - Trogamid® X7323 or c) Vestamid® L1901 - Example 14A - Trogamid® X7323 and a welding time of 30 s, an adhesion mark of 4 in each case is found. The marks mean: 0 = no adhesion, 1 = slight adhesion, 2 = some adhesion; can be separated with a little effort, 3 = good adhesion; can be separated only with great effort and possibly with the use of tools, 4 = inseparable adhesive bond; separation only by breakage of cohesion.

Claims

Silicon-iron mixed oxide powder having magnetic properties in the form of aggregated primary particles, a) for which TEM images show the presence of primary particles composed of spatially separate regions of silicon dioxide and iron oxide and the mean particle diameter of the iron oxide is 2 to 100 nm and b) in which the proportion of

- silicon, calculated as Siθ₂, is 5 to 65% by weight

- iron, calculated as Fe₂U3, is 30 to 90% by weight

- and the proportion of silicon and iron, each calculated as abovementioned oxides, is at least 95% by weight c) in which the proportion of chloride is 0.2 to 3% by weight.

Silicon-iron mixed oxide powder according to Claim 1, characterized in that the proportion of silicon, calculated as Siθ₂, is

50 ± 10% by weight or 20 ± 10% by weight.

Silicon-iron mixed oxide powder according to Claims 1 and 2, characterized in that the proportion of iron, calculated as Fe₂U3, is 50 ± 10% by weight or 80 ± 10% by weight.

Silicon-iron mixed oxide powder according to Claims 1 to 3, characterized in that the proportion of chloride is 0.5 to 2.5% by weight .

5. Silicon-iron mixed oxide powder according to Claims 1 to 4, characterized in that the primary particles have an envelope of silicon dioxide having a thickness of 1 to 15 nm.

6. Silicon-iron mixed oxide powder according to Claims 1 to 5, characterized in that the iron oxide comprises, as the main constituent, magnetite and/or maghemite.

7. Silicon-iron mixed oxide powder according to Claim 6, characterized in that the proportion of magnetite and/or maghemite, based on the iron oxides, is at least 80%.

8. Silicon-iron mixed oxide powder according to Claims 6 and 7, characterized in that the maghemite/magnetite weight ratio is 0.3:1 to 100:1.

9. Silicon-iron mixed oxide powder according to Claims 1 to 8, characterized in that it comprises at least one doping component.

10. Silicon-iron mixed oxide powder according to Claim 9, characterized in that the proportion of doping component is 0.005 to 2% by weight.

11. Silicon-iron mixed oxide powder according to Claims 1 to 10, characterized in that its BET surface area is 40 to 70 m²/g.

12. Silicon-iron mixed oxide powder according to Claims 1 to 11, characterized in that the particles are enveloped with poly (meth) acrylates .

13. Silicon-iron mixed oxide powder according to Claims 1 to 12, characterized in that it has a saturation magnetization of 40 to 120 Am²/kg of Fe₂O₃.

14. Silicon-iron mixed oxide powder according to Claim 1, characterized in that it has the following features: a) BET surface area 50 ± 5 m²/g b) proportion of

- silicon, calculated as SiO₂, 50 ± 5% by weight

- iron, calculated as Fe₂O₃, 45 ± 5% by weight

- chloride 1.5 ± 0.5% by weight

15. Silicon-iron mixed oxide powder according to Claim 1, characterized in that it has the following features: a) BET surface area 50 ± 10 m²/g b) proportion of - silicon, calculated as SiO₂, 10±5% by weight

- iron, calculated as Fe₂O₃, 85 ± 5% by weight

- chloride 1.0 ± 0.2% by weight - manganese, calculated as MnO, 1.8 ± 0.2% by weight, c) mean diameter of the iron oxide 10-30 nm d) proportion of (magnetite + maghemite) , based on iron oxide, 90 ± 10% by weight.

16. Process for preparing the silicon-iron mixed oxide powder according to Claims 1 to 15, characterized in that a) 5 to 65% by weight of one or more vaporous halosilicon compounds, calculated as Siθ2, b) 30 to 90% by weight of iron chloride, calculated as Fe2U3, in the form of a solution and c) optionally 0.005 to 2% by weight of one or more doping compounds, calculated as oxide, d) are fed separately to the high-temperature zone of a reactor, e) are reacted in the high-temperature zone at temperatures of 700 to 2500⁰C with an excess of oxygen or an oxygenous gas, f) and, in a second zone of the reactor downstream of the high-temperature zone, reducing gases are added to the reaction mixture at one or more points in an amount such as to give rise to a reducing atmosphere overall in this second zone, and the temperature of the reaction mixture is reduced to 500⁰C to 150°C, g) the resulting solid is separated from gaseous substances in a further, third zone in which a reducing atmosphere is likewise still present, and h) optionally, sufficient air is added to the gaseous substances that the offgas does not give rise to a reducing atmosphere.

17. Process according to Claim 16, characterized in that the temperature results from a flame which is generated by igniting a mixture which comprises one or more combustion gases and an oxygen- containing gas and which burns into the reaction chamber .

18. Process according to Claims 16 or 17, characterized in that the reducing gases used are forming gas, carbon monoxide, hydrogen, ammonia or mixtures of these gases .

19. Process according to Claims 16 to 18, characterized in that the residence time in the first zone is between 0.8 and 1.5 seconds .

20. Process according to Claims 16 to 19, characterized in that the sum of the residence time in the second and third zone is between 15 seconds and 15 minutes.

21. Process according to Claims 16 to 20, characterized in that steam is additionally introduced into the second reactor zone.

22. Process according to Claims 16 to 21, characterized in that the iron chloride used is iron (II) chloride.

23. Process according to Claims 16 to 22, characterized in that the halosilicon compound used is silicon tetrachloride.

24. Process for preparing the silicon-iron mixed oxide powder according to Claims 1 to 15, characterized in that a) 5 to 65% by weight of one or more vaporous halosilicon compounds, calculated as Siθ2, b) 30 to 90% by weight of iron chloride, calculated as Fe2U3, in the form of a solution and c) optionally 0.005 to 2% by weight of one or more doping compounds, calculated as oxide, d) are fed separately to the high-temperature zone of a reactor, e) are reacted in the high-temperature zone at temperatures of 700 to 2500⁰C in a flame which is generated by the ignition of a mixture which comprises one or more combustion gases and an oxygen-containing gas and which burns into the reaction chamber, and in which oxygen is used in deficiency, f) in a second zone of the reactor downstream of the high-temperature zone, air or air and steam are added to the reaction mixture at one or more points in an amount such as to give rise to, overall in this second zone a

- reducing atmosphere or

- oxidizing atmosphere and the temperature is reduced to 500⁰C to 150⁰C and g) the resulting solid, in a further, third zone, is removed from gaseous substances of the same atmosphere as are present in the second zone, h) and optionally, sufficient air is added to the gaseous substances that the offgas does not give rise to a reducing atmosphere.

25. Use of the silicon-iron mixed oxide powder according to Claims 1 to 15 for producing dispersions.

26. Use of the silicon-iron mixed oxide powder according to Claims 1 to 15 as a constituent of rubber mixtures.

27. Use of the silicon-iron mixed oxide powder according to Claims 1 to 15 as a constituent of polymer formulations.

28. Use of the silicon-iron mixed oxide powder according to Claims 1 to 15 as a constituent of adhesive compositions.

29. Use of the silicon-iron mixed oxide powder according to Claims 1 to 15 as a constituent of plastic composite mouldings obtainable by welding in an electromagnetic alternating field.

30. HDPE composition containing the silicon-iron mixed oxide powder according to Claims 1 to 15.