FIELD OF THE INVENTION

The present invention relates generally to a method and apparatus for making nanometer-sized particles. More specifically, the method is making a solution of a soluble precursor in a liquid solvent, then continuously flowing the solution through a heated vessel and forming particles within the heated vessel, then recovering the particles by quenching the solution in a cooled region.

BACKGROUND OF THE INVENTION

Particles ranging in size from 1 nm (nanometer, 10~ ⁹ meter) to more than 100 nm exhibit unique and useful surface and interface properties because they contain a high proportion of surface-to-bulk atoms. Uses of these particles include but are not limited to heterogenous catalysts, ceramic materials fabrication, intermetallics, electronics semiconductor fabrication, magnetic recording media, and superconductors.

Production of nanometer-sized particles is currently accomplished in a variety of ways, including gas phase condensation, laser synthesis processes, freeze drying methods, flame or plasma torch reactions, vacuum synthesis methods utilizing sputtering, laser ablation, liquid metal ion sources, reverse micelle solutions, solidification

from the liquid state, and hydrothermal methods. These techniques are typically batch techniques and have limited particle production rate (on the order of gram quantities or less per day) and limited overall production quantity. Additionally, relatively wide ranges of particle sizes are produced by these techniques.

It is a long felt need in the art of nanometer-sized particle production to be able to produce larger quanti¬ ties at faster rates and to be able to control product particle size distribution in order to improve performance and cost of products including but not limited to those enumerated above.

Because the present invention is most related to hydrothermal methods, they are further summarized herein. Hydrothermal methods utilize conditions of elevated temperatures and/or elevated pressures wherein particles are formed by nucleation and grown under these conditions to produce powder products.

Conventional hydrothermal methods begin with making a batch of a solution of a soluble precursor in a liquid solvent. The batch is placed in a vessel. Particles are formed by chemical reactions resulting in nucleation forming precipitates within the vessel. Reactions may be enhanced by heating or pressurization, or both. Heating includes a "ramped" heating stage to bring the solution to a desired temperature. For aqueous solutions, the temperatures are generally in the range from about 90°C to greater than 500°C. The "ramped" heating stage is followed by a holding stage wherein the solution is maintained at the desired temperature, then cooled. Time for ramping and holding typically varies from hours to days depending upon the type of solution and the desired product. Termination of particle growth is achieved by stopping the reaction, generally by cooling the heated solution. Remaining liquid is decanted and the particles dried and recovered.

Particle sizes are a result of concentrations of reactants, the amount of time that the reactants are in contact, and the temperature and pressure of the reactant solution. Using current methods, it is difficult to control the amount of time reactants are in contact at given conditions of temperature and pressure because of large total heat capacity of vessels and equipment.

Particles are made of materials including but not limited to oxides and hydroxides by hydrolysis or oxidation reactions in aqueous solvent systems, and metals in non-aqueous systems; for example, organometallic species, as well as non-oxide ceramic particles formed by reaction of a precursor with a solvent. More speci¬ fically, particle products include but are not limited to iron oxide, titanium oxide, nickel oxide, zirconium oxide, aluminum oxide and silicon oxide. Precursor solutions from which particles are made include but are not limited to aqueous nitrate solutions, sulfate solutions, and oxalate solutions. For example, iron oxide particles may be made from Fe(N0 ₃ ) ₃ or Fe(NH ₄ ) (S0 ₄ ) ₂ .

Further operational details of hydrothermal methods may be found in Hydrothermal Synthesis of Advanced Ceramic Powders, William J. Dawson, Ceram. Bull., 67, 1988 pp. 1673-1677, and in The Role of Hydrothermal Synthesis in Preparative Chemistry. Albert Rabenau, Agnew. Chem. Int. Ed. Engl., 24, 1985, pp. 1026-1040.

Another example of hydrothermal methods is found in the patent 4,734,451 issued on March 29, 1988 to RD Smith, entitled SUPERCRITICAL FLUID MOLECULAR SPRAY THIN FILMS AND FINE POWDERS. Smith teaches the formation of fine powders by dissolving a solid material into a super¬ critical fluid solution and rapidly expanding the solution through a short orifice into a region of low pressure thereby nucleating and forming particles in the region of low pressure. This process differs from the ones described above inasmuch as it is a continuous process and

there is no chemical reaction between the solid material and the supercritical fluid solution. While the Smith process is useful for soluble polymers, organic compounds, and many inorganic compounds, it is not useful for insoluble or substantially insoluble ceramic materials, metal oxides, and other above mentioned sybstantially insoluble materials. In addition to requiring dissolution of the particle forming compound, the Smith process requires carrier solutions which have no liquid droplet formation upon expansion to low pressure, whereas the present invention does not require this limitation.

SUMMARY OF THE INVENTION

The invention is an improvement to hydrothermal methods wherein a solution of precursor and solvent continuously flow through a heated vessel and chemically react to nucleate particle precipitates within the vessel, then flow into a cooled region for recovery of the parti¬ cles. By using continuous flow, the amount of time that the solution is at selected conditions of temperature and pressure are more precisely controllable and shorter (less than one minute and more frequently on the order of seconds) compared to a batch process. Flow of solution into the cooled region results in nearly instantaneous cooling of the solution and terminate particle growth. Use of the continuous process of the present inven¬ tion permits production of materials not producible with existing continuous processes at a much faster rate compared to existing batch methods. Production rates of the present invention are from about tens of grams of particles per hour to about several kilograms of particles per hour. In addition, particle size distributions are narrower compared to batch methods.

The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an embodiment of the present invention.

FIG. 2 is a schematic of a second embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The invention is an improvement to hydrothermal methods wherein a solution of precursor and solvent continuously flow through a heated vessel and the solute chemically reacts to nucleate particle precipitates, then flows into a cooled region for recovery of the particles.

An apparatus for carrying out the present invention is shown in FIG. 1. Solution (1) within reservoir (2) flows into a first tube (3) and is pressurized by a pump (4) . Pressurized solution flows into a reaction vessel (6) that may be any type of closed and pressurizeable continuous flow vessel having an inlet and outlet, but is preferably a tube. The reaction vessel (6) is heated by a heater (8) . Pressure may be maintained within the reaction vessel (6) by any pressure control means, but preferably with a flow restrictor (10) located downstream of both the pump (4) and the heater (8) . The heated solution is cooled by ejection from the end (12) of the

reaction vessel (6) into a chamber (14) having walls (16) that are cooled. The heated solution is rapidly cooled within the chamber (14) . Particles and reacted solution accumulate within the chamber (14) . Particles are recovered by any method including but not limited to settling, filtering, or centrifugation. Remaining liquid is decanted and the particles dried under flowing nitrogen or air.

The heater (8) may be of any type including but not limited to electrical resistance heaters, induction heaters, microwave heaters, fuel fired heaters, and steam coils. It is preferred that the heater be the tube itself resistively heated with electricity.

The flow restrictor (10) may be of any type including but not limited to an adjustable valve, or a non-adjustable orifice such as a nozzle or lengths of small diameter tubing.

The walls (16) may be cooled by any means including but not limited to refrigeration coils, water/ice bath, liquid nitrogen, and dry ice.

A second embodiment of an apparatus according to the present invention is shown in FIG. 2. Instead of an open chamber (14) for cooling and particle collection, a section of the reaction vessel (6) is cooled by a cooling means (21) downstream from the heater (8) . Upon exiting the cooled section of reaction vessel (6) , the solution (1) enters a filter means (22) wherein particles are collected while remaining liquid flows through the flow restrictor (10) to a catch basin (24) . The cooling means (21) may be any means including but not limited to low temperature baths, including water and ice baths, and dry ice, as well as refrigeration cooling coils.

Although it is preferred to use a pump (4) , it is not necessary since the solution (1) is pressurizeable by any means including but not limited to mechanical pistons

with weights on them, overpressure of a gas, and hydraulic head.

The first two embodiments disclose a reservoir (2) holding a solution (1) . Depending upon the desired product and the precursors and solvents necessary to obtain the product, the reaction vessel tube (3) may be provided with multiple ports for staged injection of precursors and solvents. The tube (3) itself may be a concentric tube having an inner and outer tube with an annular space therebetween.

Co-processing of precursors, solvents, or reagents having different reaction temperatures may be accomplished with a multi-port or concentric tube reaction vessel. The heated region of the reaction vessel tube may be controlled to exhibit a temperature variation along its length wherein various compounds may be added into an appropriate temperature zone.

In operation, particle size is determined by many factors including temperature, pressure, type of flow restrictor, and concentration and type of precursor in the solution. Flow rate of solution to achieve a particle production rate depends upon the same factors recited above and may vary over a wide range. For operational convenience, flow rates and tube lengths are selected to provide a residence time of solution (1) within the vessel (3) of less than one minute, and preferably about 2 to 3 seconds. The temperature and pressure of the solution within the vessel may also vary widely depending upon the type of solution and the size of particles desired. Tem- peratures may range from about 25°C (ambient) to greater than 500°C, but are preferably from about 200°C to about 400°C. Pressures are sufficient to prevent substantial vaporization of the solution thereby maintaining the solution substantially in the liquid phase. The terms "substantial" and "substantially" are used because it is recognized that vaporization may not be completely

avoided. Furthermore, some vaporization of solution is not harmful to the process of the present invention.

The process of the present invention is not limited to the type of chemical reaction occurring within the reaction vessel. It is preferred, however, that the reaction take place within the reaction vessel and not within the inlet reservoir or the outlet. The chemical reaction may be an interaction of the precursor with the solvent at elevated temperature conditions, for example oxide formation. The chemical reaction may be a thermal breakdown of the precursor into an insoluble form, for example formation of iron particles from an iron pentacarbonyl/carbon dioxide solution. The chemical reaction may be thermal decomposition of an additional reactant, for example addition of urea decomposing into ammonia in a solution of iron nitrate and forming iron hydroxite particles.

The apparatus and method or process of the present invention can accommodate any combination of precursor and solvent provided that the precursor is soluble in the solute. Solvents may be selected from inorganic and organic liquids. Inorganic liquids include but are not limited to water (aqueous solvent) and ammonia. Organic liquids that may be used as solvents in the present invention include but are not limited to carbon dioxide, hydro=carbons, halogenated hydrocarbons, and alcohols. Precursors that are aqueous soluble include but are not limited to ferric or ferrous salt, for example, ferric halide, ferric sulfate, ferric (periodic chart column 1A element) sulfate; oxalates of potassium, sodium, ammonium, lithium, oxotitanium, zirconium, hafnium, and citrates of zirconium and titanium. Precursors that are soluble in carbon dioxide, especially supercritical carbon dioxide, include but are not limited to Fe(CO) ₅ and Mo(CO) _g .

An additional solute may be oxidizing like urea or reducing like hydrazine, hydrogen gas or sodium borohydride.

EXAMPLE 1 An experiment was performed to produce particles according to the method of the present invention using an apparatus according to FIG. 1. An aqueous solution of iron nitrate (0.1M Fe(N0 ₃ ) ₃ ) was pressurized with a reciprocating pump (4) to a pressure of about 510 Bar (7500 psi) and transported through a reaction vessel (6) . The reaction vessel (6) was 316 stainless steel tubing having an outside diameter of 0.32 cm, a wall thickness of 0.09 cm, and a length of 90 cm. The reaction vessel (6) was heated by resistive electrical heating. The solution had a flow rate of about 50 cc/ in. The tube temperature was held constant for each run and several runs having temperatures ranging from about 225°C to about 400°C were made.

The flow restrictor (10) was constructed of a short length (length less than about 2.5 cm) of capillary tubing having an inside diameter from about 60 micrometers to about 100 micrometers.

The heated solution was ejected into a flask immersed in a water/ice bath. Phase identification and size of the particles was performed using X-ray particle diffraction. Diffraction patterns were obtained using a Philips X-ray diffract- ometer with a copper source operated at 40 kV and 25 mA. Particle size estimates were made by calculations based upon the Scherrer formula as may be found in the book entitled ELEMENTS OF X-RAY DIFFRACTION, 2d edition, by BD Cullity, published by Addison Wesley, Reading, Mass. in 1978. A correction for instrument broadening was made to the Scherrer formula.

Particle size and particle size distribution were obtained using transmission electron microscopy micrographs of particles deposited upon 3 mm carbon coated grids. The particles were micrographed in a Philips EM400T electron microscope operated at 120 keV. Particle size distributions were also obtained using variable temperature Mossbauer spectroscopy as described in the article entitled MOSSBAUER EFFECT STUDIES OF SURFACE IONS OF ULTRAFINE ALPHA-IRON(III) OXIDE PARTICLES, by AM Van der Kraan, published in Phys. Stat. Sol. A, Vol. 18, pp 215, 226 in 1973.

Surface area was obtained by a nitrogen absorption method using a micrometrics ASAP 2000 instrument for further confirmation of the particle size measurements. Results are shown in Table 1. For the identified sample numbers, processing parameters of temperature and pressure are shown. Results of yield, material phase, and particle size are also shown. Particle diameters are reported for three independent measurements along with confirmatory surface area measurements. From Table 1, one can see that particle sizes are larger for higher process¬ ing temperature. One also sees that the particle size range is narrow, showing size range variations of from 8 to 50 nm.

EXAMPLE 2

An experiment was performed to produce particles according to the method and using the apparatus set forth in Example 1. However, in this experiment, an aqueous solution of potassium bis(oxalato)exotitanate (IV) (0.1M K ₂ TiO(C ₂ 0 ₄ ) ₂ ) was used. The solution had a flow rate of about 50 cc/min, and was heated in a reaction vessel tube (6) made of Hastelloy C-276.

Results are shown in Table 2. From Table 2, one can see that the particle size range is narrow, showing range variations of 5 nm.

TABLE 1. Iron-Oxide Powders Produced From FefNO )

TABLE 2. Iron-Oxide Powders Produced From K ₂ TiOfC ₂ 0 ₄ ) ₂

ro

I

EXAMPLE 3

Another experiment was conducted according to the method and using the apparatus of Example 1. In this experiment, a solution of aluminum nitrate (0.1 M) A1(N0 ₃ ) ₃ ) was used. The solution was processed at a temperature of about 400°C and produced very few particles.

Urea (CO(NH ₂ ) ₂ ) was added to the solution and parti¬ cles were made at temperatures from between about 200°C to about 300°C.

EXAMPLE 4

Other experiments conducted according to the method and apparatus of Example 1 but using different combina¬ tions of solutions and precursors. The combinations and resulting particle or powder products are summarized in Table 3.

The results shown in Table 3 are demonstrative of the variety of particles that may be produced with the present invention. While a preferred embodiment of the present inven¬ tion has been shown and described, it will be apparent to those skilled in the art that many changes and modifica¬ tions may be made without departing from the invention in its broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.

TABLE 3. Other Powders Produced