A method for producing nano-titanium carbide particles

Abstract

The present application relates to the technical field of carbide material preparation, and particularly relates to a preparation method of nano titanium carbide particles; a MOF precursor is obtained by low-temperature reaction of a titanium source compound and an organic ligand in a solvent, then fractional temperature rising pyrolysis is carried out, H2 / Ar mixed gas is introduced in the low-temperature stage for slight reduction, small molecule products are removed by intermittent pressure reduction and exhaust in the medium-temperature stage, and extremely low concentration hydrocarbon gas is pulsed for short time exposure and cut back to the protection atmosphere in the high-temperature stage, so that ordered generation of TiC nano grains and particle size inhibition are realized, the TiC particle size is concentrated in 10-50 nm, finally, the selective etching of excess amorphous carbon is carried out by introducing a micro-reducing rare hydrogen atmosphere in the medium-temperature zone, the free carbon content is significantly reduced, and the product purity and crystallinity are improved, the present method realizes fine regulation and control of the precursor structure, the pyrolysis atmosphere and the temperature rising program, and obtains nano TiC powder with uniform distribution and controllable particle size, thereby providing high-quality precursor raw materials for high-performance material manufacturing.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of carbide material preparation, and particularly relates to a preparation method of nano titanium carbide particles. BACKGROUND

[0002] Titanium carbide (TiC) is a typical representative of the transition metal carbide family, which has attracted much attention due to its high melting point, high hardness, high melting temperature, and good strength and chemical stability. In traditional applications, TiC is often used as a wear-resistant coating, a hard alloy additive phase, and a high-temperature structural material. With the rapid development of nanoscience and technology, nano-TiC particles (generally with a particle size in the range of 1-100 nanometers) exhibit more potential applications due to their ultra-high specific surface area, excellent surface activity, and unique size effect. For example, nano-TiC can be used as a reinforcing component of superhard materials to significantly improve the mechanical properties and high-temperature stability of metal-based and ceramic-based composite materials; and can also be applied to the fields of catalysis, energy storage (such as electrode materials), and other frontier fields.

[0003] However, stable preparation of nano-TiC still faces technical difficulties: most of the traditional preparation processes of TiC powder rely on high-temperature carbothermal reduction, chemical vapor deposition (CVD), or self-propagating high-temperature synthesis (SHS). These methods are relatively mature at the macroscopic scale, but it is not easy to stably control the particle size at the nanoscale and at the same time obtain uniformly distributed and high-purity TiC powder. High-temperature processes often lead to particle agglomeration, excessive growth, and the introduction of impurity phases; while wet chemical or sol-gel methods have certain potential in particle size control, but the process is complex, sensitive to the purity of the precursor and operating conditions, and is prone to leave other impurity elements in the product or difficult to scale up production.

[0004] In recent years, some new preparation routes have gradually attracted attention. For example:

[0005] 1. Precursor thermal decomposition method: organic titanium compounds and carbon sources are used to control the reaction conditions at a lower temperature, so that the decomposition products are directly converted into nano-TiC.

[0006] 2. Sol-gel and subsequent heat treatment: through uniform mixing and gelation of titanium salt sol and organic carbon source, the Ti and C components in the gel are in-situ converted into TiC under a controlled atmosphere and temperature program.

[0007] 3. Mechanical alloying and subsequent reaction sintering: titanium powder and carbon black or organic carbon source are mixed and refined to the nanoscale by high-energy ball milling, and TiC nanocrystals are generated at a low temperature under appropriate heat treatment conditions.

[0008] 4. Plasma-assisted synthesis: Utilizing plasma chemical reactions, rapidly decompose and react with precursors, achieve instantaneous high temperature and rapid cooling, conducive to the nucleation and growth control of nano-sized TiC.

[0009] These emerging methods complement the traditional technology route, aiming to achieve the following goals: precise control of TiC nanoparticle size and grain distribution; reduce the preparation temperature and energy consumption, reduce grain growth and agglomeration; effectively control the content of impurity phase and oxygen content in the product, improve purity and performance; balance between production scale and cost, provide feasibility for large-scale industrialization.

[0010] In this context, for the development of nano-titanium carbide particle preparation technology, not only need to choose the appropriate precursor and carbon source, but also need to optimize the mixing and dispersion, atmosphere control, heating program and holding system, in order to obtain nano-TiC products with high purity, uniform particle size distribution and good dispersibility, so as to better meet the growing performance requirements in the fields of hard alloy reinforcement, wear-resistant coating preparation, turbine blade toughening and emerging catalytic materials. SUMMARY

[0011] To solve the above problems, the purpose of the present application is to provide a method for preparing nano-titanium carbide particles, comprising the following steps:

[0012] S1, precursor synthesis step:

[0013] Mixing and reacting titanium source compound and organic ligand in solvent, controlling reaction temperature and reaction time, forming titanium-containing metal organic framework (MOF) precursor;

[0014] S2, precursor pretreatment step:

[0015] The titanium-containing metal organic framework (MOF) precursor obtained in step S1 is centrifuged, washed and dried, the drying temperature is 60-120℃, the drying time is 6-24 hours, and a uniform and fine MOF precursor powder is obtained;

[0016] S3, staged heating pyrolysis step:

[0017] The dried MOF precursor powder is placed in a heating furnace, and a trace of regulating gas is introduced at a specific stage to realize dynamic atmosphere control and complete staged heating pyrolysis, and a fine and uniform nano-TiC grain distribution is obtained;

[0018] S4, selective etching of excess carbon layer step:

[0019] After the furnace temperature is naturally cooled to the predetermined temperature, the treatment furnace is filled with protective gas, and then H2 gas is introduced into the furnace, the volume fraction of H2 is 5-20%, a micro-reducing dilute hydrogen environment is obtained;

[0020] After the temperature is raised to the predetermined temperature and kept, H2 reacts slightly with amorphous carbon on the surface of the solid under heating conditions, thereby selectively etching excess carbon layer and reducing free carbon content;

[0021] S5, a post-processing step:

[0022] After the step of selectively etching excess carbon layer is completed, the residual H2 gas is removed after multiple charging and discharging of the protective gas, and then the furnace temperature is cooled to room temperature, the furnace is opened, and the material is taken out. After sieving, the nano-titanium carbide particles of a predetermined size are obtained.

[0023] Further, the step S1 specifically comprises: the reaction temperature is 25-80℃, and the reaction time is 12-48 hours; the solvent is a mixture of one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methyl pyrrolidone (NMP) or dimethyl sulfoxide (DMSO).

[0024] The titanium source is TiCl4 or Ti(OR)4, wherein R is C1-C4 alkyl, and the organic ligand is selected from a polydentate ligand containing a carboxyl group, a hydroxyl group or a nitrogen coordination group; the crystal grain size of the obtained titanium-containing metal organic framework (MOF) precursor is 100-500 nm.

[0025] Further, the step S3 specifically comprises:

[0026] S31, a low-temperature desorption and atmosphere regulation stage:

[0027] After the temperature is raised to the predetermined temperature and kept, H2 / Ar mixed gas is introduced to reduce the surface of the titanium precursor in the titanium-containing metal organic framework (MOF) and reduce the impurity oxygen content;

[0028] S32, a medium-temperature ordered deconstruction and precursor phase regulation stage:

[0029] After the temperature is raised to the predetermined temperature and kept again, the total pressure of the system is intermittently reduced during the keeping to discharge small molecular gaseous products;

[0030] S33, a high-temperature directional carbonization and particle size inhibition stage:

[0031] After the temperature is raised to the predetermined temperature and kept again, hydrocarbon gas is introduced quantitatively at <0.5vol% in the second half of the keeping period, and the protective atmosphere is cut back after short-time exposure.

[0032] Further, the step S31 specifically comprises:

[0033] The temperature is raised from room temperature to 200-300℃ at a rate of 1-5℃ / min, and kept for 0.5-2 hours to remove the residual pore solvent;

[0034] In the latter half of the heat preservation, 0.1-1 vol% of H2 / Ar mixed gas is pulsed to slightly reduce the surface of the titanium precursor in the titanium-containing metal organic framework (MOF) and reduce the impurity oxygen content, thereby reducing the oxide inclusions in the subsequent carbonization process.

[0035] Further, the step S32 is specifically: continuously heating at 3-10 DEG C / min to 500-700 DEG C, and heat preserving for 1-3 hours, and intermittently reducing the total pressure of the system during the heat preservation to continuously discharge the small molecule gaseous products.

[0036] Further, in the step S32, the intermittently reducing the total pressure of the system refers to: every 30 min, the pressure is extracted to 50-80% of the original pressure, and the air extraction is maintained for 5-10 min, then the air extraction is stopped, and the air extraction is performed again until 30 min.

[0037] Further, the step S33 is specifically: heating at 5-10 DEG C / min to 800-1200 DEG C, and heat preserving for 2-6 hours at the temperature to make Ti react with carbon in situ to form TiC nanocrystalline, and in the latter half of the heat preservation period, a carbon-hydrogen gas is quantitatively introduced at <0.5 vol%, and after a short-time exposure of 5-10 min, the pure protective atmosphere is returned; the carbon-hydrogen gas is CH4, C2H2 or a mixed gas of CH4 and C2H2.

[0038] Further, the step S5 is specifically:

[0039] S51, after the furnace temperature is naturally cooled to 100 DEG C, the treatment furnace is filled with a protective gas, then H2 gas is introduced into the furnace, the volume fraction of H2 is 5-20%, and a micro-reducing dilute hydrogen environment is obtained;

[0040] S52, after starting from 100 DEG C and heating to 300-500 DEG C at a heating rate of 3-10 DEG C / min, heat preserving for 0.5-2 hours, the H2 reacts with the amorphous carbon on the surface of the solid under the heating condition, the excess carbon layer is selectively etched, and the free carbon content is reduced.

[0041] Further, the protective gas is one of pure argon, pure nitrogen, and a mixed gas of nitrogen and argon.

[0042] The application further provides a nano-titanium carbide particle prepared by the method.

[0043] Beneficial effects

[0044] The scheme of the application realizes the fine control of the precursor structure deconstruction and the TiC phase formation process through the organic combination of the staged heating and the dynamic atmosphere regulation, and has the following beneficial effects:

[0045] 1. Precise control of particle size and distribution: Introduce extremely low concentration of hydrocarbon gas during the high-temperature carbonization stage, use short-time pulse exposure and rapid switching to protective atmosphere, effectively inhibit the excessive growth of TiC grains, make the product nanoparticle size concentrate in the range of 10-50 nm, and the distribution is more uniform.

[0046] 2. Reduce oxide inclusions: Introduce trace amounts of H2 / Ar mixed gas and intermittent pressure reduction exhaust during the low-temperature desorption and medium-temperature regulation stages, which not only helps to remove residual solvents and small molecule products from MOF, but also slightly reduces the surface of Ti species, reducing oxygen content, which effectively reduces the oxide impurities in TiC, improves the purity and crystallinity of the product.

[0047] 3. Carbon content optimization and interface purification: In the last etching step, the excess amorphous carbon is selectively removed by a micro-reducing hydrogen environment, which significantly reduces the free carbon content in the final product, thereby obtaining TiC nanopowder with higher purity and cleaner interface, thereby improving its application potential in high-performance materials.

[0048] 4. Process controllability and stability improvement: Use multi-stage heating, dynamic atmosphere and pressure control strategy, process parameters can be flexibly adjusted according to the target particle size and purity requirements. In batch production, it is easier to obtain stable and repeatable product quality and performance.

[0049] In summary, through fine process parameter setting and atmosphere control, the present application realizes high purity, uniform particle size distribution and effective microstructure optimization of TiC nanopowder, providing high-quality raw material basis for the practical application of nano-titanium carbide materials in high-end equipment, functional coating and hard alloy fields. BRIEF DESCRIPTION OF DRAWINGS

[0050] Fig. 1 The flow chart of the method of the present application is shown in the figure;

[0051] Fig. 2 The scanning electron microscope (SEM) photo of the nano-titanium carbide particles prepared by the method of the present application is shown in the figure;

[0052] Fig. 3 The transmission electron microscope (TEM) photo of the nano-titanium carbide particles prepared by the method of the present application is shown in the figure. DETAILED DESCRIPTION

[0053] In order to deepen the understanding of the present application, the present application will be further described in combination with examples, and the present embodiment is only used to explain the present application, and does not constitute a limitation on the protection scope of the present application.

[0054] Example 1

[0055] According to Figs. 1-3As shown, the present embodiment provides a method for preparing nano-titanium carbide particles, comprising the following steps:

[0056] S1, precursor synthesis step:

[0057] The titanium source compound is mixed with the organic ligand in a solvent, the reaction temperature is controlled at 25-80°C, and the reaction time is 12-48 hours, so that a titanium-containing metal organic framework (MOF) precursor is formed; the titanium source is TiCl4 or Ti(OR)4 (R is C1-C4 alkyl), and the organic ligand is selected from a multidentate ligand containing a carboxyl group, a hydroxyl group, or a nitrogen coordination group; the obtained MOF crystal size is 100-500 nm;

[0058] S2, precursor pretreatment step:

[0059] The titanium-containing metal organic framework (MOF) precursor obtained in step S1 is centrifuged, washed, and dried, the drying temperature is 60-120°C, and the drying time is 6-24 hours, to obtain a uniformly refined MOF precursor powder;

[0060] S3, staged temperature rising pyrolysis step:

[0061] The dried MOF powder is placed in an integrated protection environment, an inert atmosphere (such as Ar or N2) is used as the basis, and at the same time, a trace amount of a regulating gas is introduced or a dynamic atmosphere switching is realized at a specific stage, so as to obtain a more fine and controllable nano-TiC grain distribution. The specific process is as follows:

[0062] a. Low-temperature desorption and atmosphere regulation stage:

[0063] The temperature is raised from room temperature to 200-300°C at a rate of 1-5°C / min, and the temperature is kept at this temperature for 0.5-2 hours. Not only the solvent residue in the pores is removed, but also a very low flow (0.1-1 vol%) of H2 / Ar mixed gas is pulsed into the MOF in the latter half of the holding period, so that the surface of the titanium precursor in the MOF is slightly reduced and the impurity oxygen content is reduced, thereby reducing the oxide inclusions for the subsequent carbonization process.

[0064] b. Medium-temperature ordered deconstruction and precursor phase regulation stage:

[0065] Continue to raise the temperature to 500-700°C at a rate of 3-10°C / min, and keep the temperature at this temperature for 1-3 hours. In addition to decomposing the organic ligand into carbon precursor, the total pressure of the system is intermittently reduced (such as reducing the pressure to 50-80% of the original every 30 minutes and maintaining for 5-10 minutes) during the holding period to promote the timely extraction of small molecule gaseous products (such as H2, CO, CO2). In this way, the residual gas partial pressure can be reduced, the tendency of carbon residues to aggregate randomly can be reduced, and a more uniformly distributed nano-carbon precursor structure can be obtained, and Ti can be uniformly distributed in the carbon network at a lower valence.

[0066] c. High temperature directional carbonization and grain size inhibition stage:

[0067] Rise temperature from 5-10 °C / min to 800-1200 °C, and keep the temperature for 2-6 hours to make Ti react with carbon in situ to form TiC nanocrystalline grains. The innovation is that this stage can be achieved by periodically changing the inert gas flow rate or introducing CH4 or C2H2 at a very low concentration (<0.5 vol%) after the second half of the holding period. After a short exposure, immediately switch back to a pure inert atmosphere. This periodic pulse of trace amounts of hydrocarbon gas can promote continuous and stable nucleation of crystal nuclei without significantly increasing the grain size, thereby achieving more precise grain size control and crystal optimization. At the same time, by strictly controlling the atmosphere switching time and gas ratio, excessive grain growth can be inhibited, thereby obtaining a more uniform distribution of TiC nanocrystalline grains.

[0068] S4, selective etching of excess carbon layer step:

[0069] a. Preparation and charging:

[0070] After high-temperature carbonization and furnace cooling, the solid product (TiC product containing excess amorphous carbon) is taken out of the high-temperature furnace and quickly transferred to another treatment furnace (or the same furnace cavity with a change of atmosphere and temperature setting) under the protection of an inert atmosphere. To avoid adsorbing air or moisture, the charging should be carried out in an inert atmosphere glove box or after vacuuming and argon filling.

[0071] b. Atmosphere preparation:

[0072] Fill the treatment furnace with protective gas (such as Ar or N2) to ensure that there is no residual oxygen or water vapor in the system. The pressure and flow rate can be adjusted according to the performance of the equipment, and generally maintained at or slightly below atmospheric pressure.

[0073] If H2 atmosphere treatment is used, the furnace cavity can be repeatedly filled and discharged with Ar or N2 several times to ensure that the residual oxygen content is very low, and then a small amount of H2 is introduced into the furnace. The volume fraction of H2 can be 5-20%, and the rest is protective gas (such as Ar) to obtain a micro-reducing dilute hydrogen environment. The gas flow rate is accurately controlled by a mass flow controller (MFC).

[0074] c. Temperature rise and holding:

[0075] Start from room temperature and heat at a rate of 3-10 °C / min to a temperature range of 300-500 °C (the temperature range can be appropriately selected according to the excess carbon content and particle surface characteristics, such as 350 °C or 450 °C).

[0076] After reaching the target temperature, the temperature is maintained for 0.5-2 hours. During this period, H2 slightly reacts with the amorphous carbon on the surface of the solid under heating conditions (e.g., to generate CH4 or other small volatile molecules), thereby selectively etching, peeling or reducing the excess carbon layer and reducing the free carbon content. During this process, a low flow of H2 / Ar mixed gas should be maintained, and the gas flow rate is usually in the range of 100-500 mL / min (depending on the furnace size and sample amount) to ensure stable reaction gas partial pressure in the furnace chamber.

[0077] d. Process monitoring and safety measures:

[0078] During the etching process, the tail gas composition can be monitored by an online gas analyzer (such as MS or GC). If changes in the concentrations of CH4 or CO, CO2 and other products are detected, it can be determined that the etching reaction is proceeding.

[0079] Strictly prevent mixing of hydrogen and air, and ensure that the furnace body is well sealed during operation. The exhaust port is connected to a safe exhaust system to eliminate the risk of explosion.

[0080] e. Cooling and termination of treatment:

[0081] After the holding period ends, stop heating and naturally or controlledly cool to room temperature under the condition of flowing inert gas. Continue to flow protective gas during the cooling process to avoid oxidation of the product caused by backflow of oxygen.

[0082] When the temperature returns to room temperature, turn off the gas flow and keep the furnace inert. Before the product is removed, a short vacuum or repeated protective gas filling can be performed to ensure a clean environment.

[0083] f. Discharge and inspection:

[0084] Before opening the furnace cover, protective gas can be introduced again to ensure a safe sampling environment and no risk of secondary oxidation. Remove the etched sample and verify the reduction of free carbon phase by X-ray diffraction (XRD) or Raman spectroscopy analysis; transmission electron microscopy (TEM) can further observe the grain size and surface state of TiC particles to confirm that the amorphous carbon has been significantly reduced and the particle size distribution is more uniform.

[0085] S5, post-treatment step:

[0086] After the selective etching of the excess carbon layer step is completed, multiple protective gas filling and discharging are performed to remove residual H2 gas. After the furnace temperature cools to room temperature, the furnace is opened and the material is removed. After sieving, nano-titanium carbide particles of a predetermined size are obtained, and finally a nano-TiC powder with an average particle size of 10-50 nm and uniform particle size distribution is obtained.

[0087] The above shows and describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above-mentioned embodiments, and the above-mentioned embodiments and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A method of producing nano-titanium carbide particles, characterized by, The method comprises the following steps: S1, a precursor synthesis step: a titanium source compound and an organic ligand are mixed and reacted in a solvent, the reaction temperature and reaction time are controlled, and a titanium-containing metal organic framework MOF precursor is formed; the reaction temperature is 25-80 DEG C, the reaction time is 12-48 hours; the solvent is a mixture of one or more of N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl pyrrolidone or dimethyl sulfoxide; the titanium source is TiCl4 or Ti(OR)4, wherein R is C1-C4 alkyl, and the organic ligand is selected from a multidentate ligand containing a carboxyl group, a hydroxyl group or a nitrogen coordination group; the crystal grain size of the obtained titanium-containing metal organic framework MOF precursor is 100-500 nm; S2, a precursor pretreatment step: the MOF precursor obtained in step S1 is centrifuged, washed and dried, the drying temperature is 60-120 DEG C, and the drying time is 6-24 hours, to obtain a uniform and fine MOF precursor powder; S3, a staged temperature rising pyrolysis step: the dried MOF precursor powder is placed in a heating furnace, a protective gas is used as a base gas, a trace amount of a regulating gas is introduced at a specific stage to realize dynamic atmosphere regulation, and the staged temperature rising pyrolysis is completed to obtain a fine and uniform nanometer TiC grain distribution; specifically comprising: S31, a low-temperature desorption and atmosphere regulation stage: the temperature is raised from room temperature to 200-300 DEG C at a temperature rising rate of 1-5 DEG C / min, and the temperature is kept for 0.5-2 hours to remove the residual solvent in the pores; in the latter half of the temperature keeping, 0.1-1 vol% of H2 / Ar mixed gas is pulsed to slightly reduce the titanium precursor on the titanium-containing metal organic framework and reduce the impurity oxygen content, thereby reducing the oxide inclusions in the subsequent carbonization process; S32, a medium-temperature ordered deconstruction and precursor phase regulation stage: the temperature is raised to a predetermined temperature again and kept, and the total pressure of the system is intermittently reduced during the temperature keeping to discharge small molecule gaseous products; S33, a high-temperature directional carbonization and particle size inhibition stage: the temperature is raised to a predetermined temperature again and kept, and a carbon-hydrogen gas is introduced in a constant amount of <0.5 vol% in the latter half of the temperature keeping period, and the protective atmosphere is returned after short-time exposure; S4, a selective etching of excess carbon layer step: after the furnace temperature is naturally cooled to a predetermined temperature, the treatment furnace is filled with a protective gas, then H2 gas is introduced into the furnace, and the volume fraction of H2 is 5-20%, to obtain a micro-reducing dilute hydrogen environment; after the temperature is raised to a predetermined temperature and kept, H2 reacts slightly with amorphous carbon on the surface of the solid under heating conditions, thereby selectively etching the excess carbon layer and reducing the free carbon content; S5, a post-treatment step: after the selective etching of excess carbon layer step is completed, the protective gas is charged and discharged multiple times, the residual H2 gas is removed, the furnace temperature is cooled to room temperature, the furnace is opened, the material is taken out, and the nanometer titanium carbide particles of a predetermined size are obtained after sieving.

2. The method of claim 1, wherein: Step S32 specifically comprises: the temperature is continuously raised to 500-700 DEG C at a temperature rising rate of 3-10 DEG C / min, and the temperature is kept for 1-3 hours, and the total pressure of the system is intermittently reduced during the temperature keeping to continuously discharge small molecule gaseous products.

3. The method of claim 2, wherein the method further comprises: In step S32, intermittently reducing the total pressure of the system means: every 30 minutes, the pressure is drawn to 50-80% of the current pressure, and maintained for 5-10 minutes, then stop pumping, until the full 30 minutes, then pump again.

4. The method of claim 3, wherein the method further comprises: Step S33 is specifically: Rising at 5-10℃ / min to 800-1200℃, and keeping the temperature for 2-6 hours to make Ti react with carbon in situ to form TiC nanocrystalline grains, in the second half of the holding period, introduce hydrocarbon gas at <0.5 vol%, expose for 5-10 minutes, then cut back to pure protective atmosphere; the hydrocarbon gas is CH4, C2H2 or a mixture of CH4 and C2H2.

5. The method of claim 1, wherein: Step S5 is specifically: S51, after the furnace temperature is naturally cooled to 100℃, fill the treatment furnace with protective gas, then introduce H2 gas into the furnace, the volume fraction of H2 is 5-20%, to obtain a micro-reducing dilute hydrogen environment; S52, starting from 100℃, heat to 300-500℃ at a rate of 3-10℃ / min, then keep the temperature for 0.5-2 hours, to make H2 react with amorphous carbon on the surface of the solid under heating conditions, selectively etch excess carbon layer, and reduce the free carbon content.

6. The method of claim 1, wherein: The protective gas is one of pure argon, pure nitrogen, and a mixture of nitrogen and argon.

7. Nanosized titanium carbide particles, characterized in that, Prepared by the method of any one of claims 1-5.

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