APPARATUS AND METHOD FOR PREPARATION OF FREE NANOPARTICLES

Technical Field

[0001] The present disclosure relates to an apparatus and method for preparing free-standing nanoparticles.

Background of the invention

[0002] Considering the importance of preparation of materials in smaller sizes (up to few hundred nanometers but primarily below hundred nanometers) is increasing, processes such as laser ablation, hydrothermal, chemical vapor deposition etc., are used for the preparation of such materials with desired particle sizes.

[0003] However, the industrial application of all the above processes often suffer from high cost, poor yields, agglomeration of nanoparticles, limitation of particle size and are often less versatile. Milling is one such process that is normally used for the preparation of finely-grained structural materials. Ball mills are commonly used in laboratories and in industries to reduce the size of materials (powders) rapidly. Generally, milling apparatuses of the type viz., Planetary, Spex and Vibratory are used for the preparation of finely-grained structural materials.

[0004] Ball milling is a technique where the size of particle decreases due to collision between vial, ball, and powder. So, the final powder size depends on the geometry and design of vial, ball, and powder. Also, the grinding and collision method depends on the motion of the container and its arrangement. Depending on the interaction between ball, powder, container and motion, the ball mill is divided into two categories. The first arrangement is an agitator ball mill, which comprises a grinding container having cylindrical grinding chamber wall. An agitator is provided with projecting agitation tools, which is arranged inside a grinding chamber. The agitator and the grinding chamber can be rotated around their respective axes, which are parallel to each other. In the past, there are several modifications in the motion of the agitator ball mill have led to the development of new mills with better efficiency and productivity. These modifications include the dynamic motion generator i.e. change from mechanical to an electromagnetic motion generator. However, reducing the size of powder below a certain limit continues to remain as a challenge using the agitator ball mill. Furthermore, the efficiency of the mill is reported to be low due to ineffective collision between grinding parts.

[0005] Planetary ball mill includes a revolution turning, which consists of turns around its own axis and across a central axis. In this type of mill, the powder is filled in a vial with balls and the rotation of vial is like a planetary motion in space. The rotation of vials gives a collision between powder, ball and container which finally reduces the size of the powder. These types of mills are directly driven with a mechanical motor; hence the power consumption is high, although it has an easy scalability.

[0006] Vibrating ball mills are more efficient than the rotary ball mills and the vibratory ball mills can be used for variety of materials. It produces more uniform sized particle than the rotary ball mill and handles larger volumes of materials per horse power input more efficiently than other ball mills. In other words, it has been found that a drive mechanism, which is capable of imparting predetermined amplitude of vibration at a predetermined optimum frequency, produces a more efficient and uniform grinding. In most of the cases the electromagnetic vibration is used as a source of vibration. Apart from these individual mills, there are several designs of mills where combination of grinding part has been used and tried to improve the efficiency.

[0007] It has also been observed that the temperature in ball mill plays a critical role in size reduction of the powder. The temperature decides the state of material during deformation. It has been found that at the low temperature, polymers and bio materials are very easy to deform. Metals, ceramics and its related class of materials also show a high size reduction at low temperature. There are two general ways of cooling the powder. Firstly, liquid Nitrogen is used as a medium, where the liquid Nitrogen is continuously poured on the powder inside ball container. It cools the system at liquid Nitrogen temperature and reduced particle size of the material is obtained. However, such a method leads to formation of nitride and contamination from the system. The second method of cooling is the cooling of container externally with the help of liquid Nitrogen circulation. This method is limited due to its hindrance in motion of the container.

[0008] The agitator mills can be cooled in both the possible ways but still it has low efficiency, which is not desirable. The planetary mill can be cooled in the above method.

However, continuous cooling continuous to remain as a challenge. The recently developed mills have shown external cooling. However, the scaling and the efficiency of such mills are limited. The vibrating mill is found to be very effective for low temperature deformation process. But the use of electromagnet and method of liquid nitrogen cooling acts as a challenge for designing an efficient mill.

[0009] In a room temperature milling, the selected material undergoes a high degree of welding and sintering due to the generation of heat. In order to obviate these heat-related welding and sintering problems, milling apparatuses, such as stirrer milling by Leverinia, functioning under low temperatures have been used. However, some of peculiar limitations associated with such apparatuses include low yield and contamination of the material. The apparatuses of this nature are also faced with the problems of continuous cooling during the course of milling. Vibratory mills do render more instrumental freedom as well as high deformation rate for the milling material. However, the direct usage of cooling agents such as liquid nitrogen in conjunction with a selected milling material (wet milling),not only leads to the formation of nitride or compounds of nitrogen but also contributes to an uncontrolled and non-uniform cooling of the selected milling material. It is also significant to note that contamination due to the operating environment as well as milling in liquid nitrogen may lead particles with thin layer of oxide or nitride which deteriorate the properties. However, such wet milling operations often result in lower deformation rate of the selected milling material. Accordingly, the challenges that are faced in such known milling systems include limitations in achieving a reduced particle size, due to the presence of multiple grains in the milling material, which are nanometric size and the particles are much coarser (often in the order of micrometers). The ball milling reduces grains to a nanosize but the particles containing these grains are still big submicron particles. Additionally the particles often agglomerate and get sintered producing boundaries. These sintered crystals affect properties both due to its bigger particle size and also the non-uniform grain size distribution. Additionally, the distribution of grain sizes of such milling material is often broad. In order to obtain better properties and fine tuning of the applications, size of the nanoparticles must be in a narrow range. Further, many of the recent applications demand the nanoparticles to be uniform and in colloidal form. Therefore, it is generally perceived that such milling processes do not result in providing free nanoparticles. Another issue is contamination during ball milling either from container or from environment. Thus ball milled powder is generally processed further through powder metallurgy routes for possible structural applications. In order to broaden the process of free standing nanoparticle without any contamination an inert atmosphere is needed.

[0010] Further, the demand for nanoparticles, which are uniform, free and in colloidal form has been on the rise. Nanoparticles below 20nm size show extremely exciting properties but all the existing processes of preparing such a fine particle has in general very low yield. Although yields can be enhanced in ball milling process, the minimum particle size presents a limitation in this class of processes. Therefore, it is preferable to have an apparatus and a method, which can produce ultra-fine nanoparticles (5nm-l um) at low temperature without compromising on purity.

[0011] Due to the above, the applications of ball mill products are limited mostly to structural applications. However, it is observed that selection of appropriate apparatus can reduce contamination.

[0012] In addition, it is also preferred to provide an apparatus and method to prepare nanoparticles that are free from nitride formation, loosely-bonded agglomerated nanoparticles and free standing nanoparticles.

Objects of the present invention

[0013] An object of the present invention is to provide an apparatus for preparing free-standing nanoparticles from various materials, under different thermal conditions, particularly under cryogenic conditions.

[0014] Another object of the present invention is to provide an apparatus having with a plurality of milling chambers arranged in different planar positions, for milling and coating objects/material of different shapes and configurations.

[0015] It is an object of the present invention to provide an apparatus in which a plurality of milling elements is used for milling.

[0016] Still another object of the present invention is to provide an apparatus to mill and provide coating to the desired material.

[0017] Further object of the present invention is to provide a method to milling a given material for obtaining free-standing nanoparticles and to provide a coating to the selected material.

Brief description of the drawings

[0018] FIG.1 is a broad schematic block diagram of the system in which the apparatus for the preparation of nanoparticles is used.

[0019] FIG.2 is schematic arrangement of compression connected to the system of the present invention.

[0020] FIG.3 is a perspective view of the apparatus of the present invention.

[0021] FIG.4 is a cross-sectional view of the apparatus of the present invention.

[0022] FIG.5 (a) and (b) are the cross-sectional views of the apparatus of the present invention depicting exemplary shapes of the inner milling chamber.

[0023] FIG.6 is a cross-sectional view of an exemplary aspect of the apparatus of the present invention with a plurality of vibrators connected to the milling chamber and arranged vertically to the floor axis of the apparatus.

[0024] FIG.7 is a cross-sectional view of an exemplary aspect of the apparatus of the present invention depicting a plurality of milling elements arranged in the milling chamber.

[0025] FIG.8 is a cross-sectional view of an exemplary aspect of the apparatus of the present invention depicting separate divisions of a base member, arranged at an angle to each other.

[0026] FIG.9 is a cross-sectional view of an exemplary aspect of the apparatus of the present invention depicting the horizontal arrangement of the vibrators.

[0027] FIG.10 is a cross-sectional view of an exemplary aspect of the apparatus of the present invention depicting the arrangement of plurality of milling chambers.

[0028] FIG. 11 (a), (b) and (c) are cross-sectional top views of the base member of the apparatus of the present invention, shown in a triangular, square and circular shapes, having means for arranging a plurality of vibrators.

[0029] FIG. 12 (a) depicts images of Fe material captured at various stages (i.e., stages 1 to 8 at a time interval of 15 minutes) of milling.

[0030] FIG.12 (b) depicts images of ZnO material captured at various stages (i.e., stages 1 to 8 at a time interval of 15 minutes) of milling.

[0031] FIG. 12 (c) depicts nanoparticles of Cu material that is subject to milling.

[0032] FIG. 13 depicts digital images of processed colloids covering three examples of Cu, Zn and Fe and their particle size analysis.

[0033] FIG. 14(a) is X-Ray Powder Diffraction (XRPD) of Cu material taken after milling in the apparatus of the present invention.

[0034] FIG. 14(b) is an SEM image captured at a relatively low magnification of nanoparticles of Cu by transferring drops of solution on a carbon film.

[0035] FIG.15 (a) is a transmission electron microscopy (TEM) image of the milled sample of Cu is prepared on a graphite coated TEM grid.

[0036] FIG. 15(b) is histogram of milled Cu material depicting particle sizes.

[0037] FIG. 16(a) is X-Ray Powder Diffraction (XRPD) of Zn material taken after milling in the apparatus of the present invention.

[0038] FIG. 16(b) is an SEM image captured at a relatively low magnification of nanoparticles of Zn.

[0039] FIG. 17 (a) is a transmission electron microscopy (TEM) image of the milled sample of Zn is prepared on a graphite coated TEM grid.

[0040] FIG. 17(b) is histogram of milled Zn material depicting particle sizes.

[0041] FIG. 18(a) is X-Ray Powder Diffraction (XRPD) of Fe material taken after milling in the apparatus of the present invention.

[0042] FIG. 18(b) is an SEM image captured at a relatively low magnification of nanoparticles of Fe.

[0043] FIG. 19 is a TEM image of Cu and histogram depicting the particle size of the milled material and the absence of impurities in the milled material.

[0044] FIG.20 is a TEM image of the milled sample of ZnO depicting its particle size.

Summary of the present invention

[0045] The present invention provides an apparatus for preparing free-standing nanoparticles, comprising, a first base member connected to a movable second base member supported with load bearing members. A vibrator is connected to the second base member to impart vibratory motions. A movable external chamber connected to the first and second base members. The movable external chamber is provided with jacket or an insulator to circulate temperature-controlling liquid and maintain thermal conditions of the apparatus. A movable and sealed inner milling chamber disposed in the external chamber and connected to the first and second base members to store milling material. Milling element is disposed in the inner milling chamber for performing milling of the material. Conduits are provided to control environment/medium of the apparatus. The present invention also provides a process for the preparation of free-standing nanoparticles under cryo/high temperature conditions using the apparatus of the present invention.

Detailed Description of the invention

[0046] The ball mill apparatus of the present invention, for the preparation of free- standing nanoparticles or a fine powder, advantageously under cryomilling conditions, is now described, initially referring to FIG.1 of the accompanied drawings. FIG.1 is an exemplary high level schematic architecture depicting a system using the ball mill apparatus of the present invention. The ball mill system comprises an air compressor 1, a vibration controller unit 2, vibrator 3, milling apparatus 4, gas circulation unit 5, and temperature medium/fluid unit 6. The gas circulation unit 5 is connected to the milling chamber 4 to pressurize the milling apparatus 4 and to enable circulation of liquid nitrogen circulate through the apparatus. The temperature medium/fluid unit 6 is also connected to the ball mill apparatus to control the temperature of the apparatus.

[0047] A heating arrangement is also connected to milling apparatus 4 to operate the apparatus at higher temperatures in the range of 30-600°C. Accordingly, the milling apparatus 4 of the present invention can also be used for high-energy milling of milling material. The heating arrangement can be connected to the apparatus, in which the heating arrangement can be based on induction or conduction. The heating arrangement can be used for milling the milling material that has already undergone cryogenic milling for further treatment.

[0048] The air compressor 1 is connected to the milling apparatus 4, as shown in FIG.l, to supply compressed dry air and to impart a vibratory motion to the vibrator 3, which is also connected to the milling apparatus 4. Any suitable air compressor can be used along with the milling apparatus 4 of the present invention, preferably a compressor that can generate a pressure in the range of 1-10 bar and to operate the vibrator 3 at a pre¬determined frequency. The capacity of the compressor 1 as shown here is only indicative in nature and can be suitably modified as per the milling needs of the apparatus.

[0049] The compressor 1 as shown in FIG.2 is connected to an air flow controller unit 7 and is regulated by a timer 8. The timer 6 helps in running the vibrator 3 at desired frequency cycles. The vibrator 3 receives compressed air from the compressor 1 for causing vibration and the vibration is controlled by the air flow control unit 5. The vibrator 3 is arranged to vibrate a frequency in the range of 30-120Hz. However, the indicated frequency range is variable and is determined in accordance with the required capacity of the ball milling apparatus 4. The compressed air passes through a lubrication unit and a pressure controller valve as arranged in the vibration controller unit 2. The role of the lubrication unit is to minimize friction and it is decided by capacity of the vibrator 3. The rate of air flow is controlled by the pressure control valve, which determines the amplitude of the vibration.

[0050] In an aspect of the present invention, constructional features of the milling apparatus 4 that is used for preparing nanoparticles from selected starting or milling materials, are now described in detail, by referring to FIG.3, which is perspective view of the apparatus 4 and FIG.4, which is a sectional view of the ball milling apparatus 4. The ball milling apparatus is particularly advantageous to prepare free-standing nanoparticles from the selected starting or milling materials.

[0051] The ball milling apparatus 4 comprises a first base member 9. The first base member 9 provides a mounting support to the milling apparatus 4. The first base member 4 is constructed with a combination of horizontal and vertical planks, legs or bars that are connected to form a frame as shown in FIG.3. The frame can be built by using any suitable materials such as metals, alloys, polymers or any other materials that can withstand the working load of the milling apparatus 4, especially those materials that can withstand the vibratory movements that are caused during the operation of the milling apparatus 4. The first base member 9 can also be constructed as a solid block or box or pedestal can be suitably used as a platform for mounting the milling apparatus 9. The first base member 9 is equipped to be fixed to a surface in a stationary condition or can be provided with elements such as rollers, castors or sliders, with suitable locking arrangements for rendering easy mobility to the milling apparatus 4. The height, load-bearing capacity, width and other dimensions of the first base member 9 can be suitably altered depending upon size of the milling apparatus 4 and its capacity. The upper portion of the first base member 9 is provided with a first pass-through opening 10, along with a bracket.

[0052] Load-bearing members 11 are mounted on the first base member 9 as shown in FIG. 3. In an exemplary aspect, the load bearing members 9 are shown mounted on the corner locations of the upper portion of the first base member 9. However, these load bearing members 11 can also be arranged at other different locations of the first base member 9, depending upon the size, weight, configuration, direction of movements etc., of the milling apparatus 4. The load bearing members 11 can be advantageously selected from a set of load-bearing springs, electromagnets, hydraulic pistons or any other devices that can absorb shock and provide biasing effect on the application of load factor.

[0053] A second base member 12 is mounted on the load bearing members 11, as shown in FIG. 1. The planar axis of the second base member 12 is same as that of the first base member 9. The mounting of the second base member 12 to the load bearing members 11 is done by welding the bottom surface of the second base member 12 to the upper surfaces of the load bearing members 11. The second base member 12 can also fitted to the load bearing members 11 detachably, by other fitting arrangements such as thread, press-fit or other suitable arrangements. The second base member 12 acts like a platform on which the milling apparatus 4 is mounted.

[0054] The second base member 12 is mounted on the load bearing members 11 and is movable along a vertical and horizontal axis of the milling apparatus. The second base member 12 is made of same material as that of the first base member 9. The second base member 12 can be a sheet or a platform. The second base member 12 is provided with a second pass-through opening 13 with a bracket.

[0055] An intervening space 14 is provided between the first and second base members 9 and 12, to facilitate the vibratory movements of the ball milling apparatus 4 along with the second base member 12.

[0056] A vibrator 15 is permitted to pass through the openings 10 and 13 of the first and second base members 09 and 12 and fitted to the base members through brackets. The vibrator 15 in this disclosure, as an exemplary embodiment, is shown as a pneumatic vibrator, which is connected to a compressor 1, as shown in FIG. 1. Other types of vibrators such as rotary vibrators, piston vibrators etc., can also be suitably adapted for use. The vibrator 15 provides the desired movements, rotation or vibration to the milling chambers of the apparatus.

[0057] In another aspect of the present invention, at least a movable external chamber 16 is connected to the second base member 12 and the vibrator 15. The external chamber 16 is attached to the second base member 12 in a manner that the movements of the external chamber 16 correspond and reciprocate with the movements of the second base member 13 that are caused by the vibrator 15. In other words, the both second base member 12 and the external chamber 16 vibrate together. The external chamber 16 is a hollow container of suitable configuration such as square, oval, cylindrical etc., and which acts as housing for an inner milling chamber, as hereinafter described and is provided with a lid or cover 17 to seal the external chamber 16.

[0058] In further aspect of the present invention a channel 18 is arranged inside the external chamber 16. The channel 18 is filled a temperature-controlling liquid with temperature of -100°C such as inorganic temperature-controlling liquids liquid nitrogen helium, or a mixture thereof, to maintain cryogenic temperature conditions within the external chamber 16. In order to maintain the cryogenic temperature levels a mixture of temperature-controlling liquids such as organic liquids such as methanol, ethanol and benzyl alcohol etc. Conduits 19 and 20 are connected to the channel 18 for supplying the temperature-controlling liquid into the chamber 16 at a desired flow rate through the temperature medium/fluid unit 6. A thermal jacket or insulator 21 is arranged on the external chamber 16 to thermally insulate the channel 18 and to maintain cryogenic conditions within the chamber 16. Alternately, temperature-controlling liquid can also filled inside external chamber 16 to maintain cryogenic conditions inside the chamber 16.

[0059] A movable inner milling chamber 23 is arranged inside the external chamber 16. The inner milling chamber 23 is advantageously formed of two portions, an upper portion 23a and a lower portion 23b. The material 24 that is required to be milled is loaded into the lower portion 23b and is covered by the upper portion 23a, which acts a lid or cover. A suitable locking arrangement is provided in the upper portion 23a, to lock the upper portion 23a to the bottom portion 23b. The lock arrangement can be a thread or press-fit that locks the upper portion 23a with the bottom portion 23b with a sealing member such as an O-ring, bellow seal, thermo seal or any suitable sealing materials that can withstand the temperature, pressure and abrasive nature of the milling material 24. It is to be noted here that the aforementioned two portions 23a and 23b of the inner milling chamber 23 are provided to enable an easy fabrication. The thermal jacket 21 provides an insulating support, preferably to the bottom portion 23b, to maintain the desired thermal conditions for the milling material 24. It is also within the purview of the invention to use a unitary inner milling chamber 23, without any partitions or portions.

[0060] In yet another aspect of the present invention, the milling apparatus 4 is equipped to process the milling material 24 in vacuum, inert or controlled atmosphere. Accordingly, gas conduits 25 and outlet conduit 20 are in flow communication with the movable inner milling chamber 23, to facilitate the circulation of gas that is supplied through a gas circulation unit 5 (FIG.l) and to control vacuum conditions inside the chamber through a vacuum pump. The conduits 25 and 26 are provided with suitable valve arrangement to switch between the vacuum pump and the gas circulation unit 5. In this disclosure, Argon (Ar) gas is supplied to the milling apparatus 4 to maintain a constant inert atmosphere. However, other gases such as oxygen, C02, methane, ammonia, nitrogen etc., can be suitably used, for controlling the inner atmosphere of the milling apparatus 4. For instance, if the selected milling material is titanium powder, which forms titanium nitride in the presence of nitrogen. Therefore, it is necessary to control the effect of nitrogen on titanium by controlling the inner atmosphere.

[0061] In this disclosure movable inner milling chamber 23 is made of stainless steel and is coated with suitable anti-corrosive material such as Teflon. The other suitable materials such as alloys of metals can be also suitably used.

[0062] In further aspect of the present invention, at least a grinding element 22 is arranged in the movable inner milling chamber 23, for the grinding of the milling material 24. The material for making the grinding element 22 can be advantageously chosen from tungsten carbide (WC), steel and other materials such as zirconium and alumina etc.

[0063] In yet another aspect of the present invention, the inert atmosphere is generated for the movable inner milling chamber 23, without actually exposing the milling material 24 to the effects of liquid nitrogen, such as formation of corresponding nitrides, since the movable inner milling chamber 23 is hermetically sealed from the temperature-controlling liquid.

[0064] In further aspect of the present invention, as shown in FIGs.5(a) and FIG.5(b), the movable inner milling chamber 23 can be formed in different shapes, for instance conical (a), cylindrical (b) or any other suitable shapes for milling materials of different types.

[0065] In yet another aspect of the present invention, the apparatus of the present invention the ball mill apparatus 4 can be also suitably adapted as shown in FIG. 6, by using to a plurality of vibrators 15. The vibrators 15 impart predetermined amplitude to the vibratory movements at a predetermined optimum frequency, in order to produce a more efficient and uniform grinding.

[0066] In another aspect as shown in FIG.7, the apparatus of the present invention the ball mill apparatus 4 can be also suitably adapted to hold a plurality of grinding elements 22. In this arrangement, the movable inner milling chamber 23 is provided with a plurality of grinding elements 22, which are in the combination of balls 22 and rollers 22a. The balls 22 and rollers 22a can be of variable sizes and their numbers inside the movable inner milling chamber 23 can be suitably varied, depending upon the density and quantity of the milling material 24 that is required to be milled. The combination of balls 22 and rollers 22a facilitate the milling of the material in inner milling chamber 23 of different configurations as described above. More particularly, these milling elements 22 and 22a facilitate the enhanced milling.

[0067] In yet another aspect of the present invention, the apparatus of the present invention the ball mill apparatus 4 can be also suitably adapted, as shown in FIG. 8, in which the second base member 12 is provided with two divisions or partitions and are independently connected to the separate vibrators 15. In this arrangement the two divisions of the second base member 12 are inclined and arranged at an angle to each other. The inclined angle is provided to enhance the grinding capacity of the inner milling chamber 23. In other words, in an inclined angle the spatial movement of the grinding elements 22, 22a is constricted thereby increasing compact intensity of the grinding elements 22, 22a vis-a-vis the milling material 24. In this arrangement the movements/vibrations of the divisions can be individually controlled and can be operated individually, alternately and in combination.

[0068] In yet another aspect of the present invention, the apparatus of the present invention the ball mill apparatus 4 can be also suitably adapted, as shown in FIG.9 the two divisions 12a and 12b of the second base members 12 are arranged in horizontal directions and they are positioned opposing each other. In this aspect, the vibrators 15 are also arranged in opposing directions. Such an opposing arrangement of the vibrators 15 exert vibrations to the movable inner milling chamber 23, from opposing directions and from sideways, which oscillate the grinding elements 22 and 22a in predominantly longitudinal directions, to render grinding of the milling material 24. It is also understood here that the vibrators 15 are co-axial to each other and can also be arranged at variable angles to each other.

[0069] It is also an aspect of the present invention, as shown in FIG.10, a pair of movable inner milling chambers 23 along with the outer chambers 16 are arranged on the respective second base members 12 to provide simultaneous milling of the material 24 in more than one milling chamber. The movable inner milling chambers 23 are connected to plurality of vibrators 15 as shown in FIG.10.

[0070] In yet another aspect of the present invention, as shown in FIG. 11, various exemplary configurations of the second base members 12 are shown. The second base members 12 are in the shape of a triangle with vibrators 15 arranged randomly. To these randomly arranged vibrators, a plurality of external chambers 16 and the corresponding inner milling chambers 23 are connected to perform milling operations. In this arrangement, the vibrators 15 are disposed to operate continually and sequentially to perform the milling operations. By adopting this arrangement, the compacting strength of the milling elements can be enhanced, since the compacting forces are acting from opposing or parallel directions.

[0071] In aspect of the present invention the milling apparatus of the present invention enables milling of the material both in controlled gaseous or liquid medium.

[0072] The present invention also provides a process for the preparation of free- standing nanoparticles and colloids of materials and in particular metals and alloys by high energy cryomilling, under an inert atmosphere and at low temperature (below -100° C) followed by separation of these loosely attached particle to obtain free standing nanoparticles/colloids, by using the milling apparatus of the present invention.

[0073] The process for the preparation of the free-standing nanoparticles, the process comprising the steps of selecting a material for milling. In this process, milling materials such as Fe, Cu, Zn and ZnO are used to demonstrate, in exemplary manner, the preparation of corresponding free-standing nanoparticles. However, the apparatus and process steps of the present disclosure support milling of other milling materials such as metallic, non-metallic, alloys, organic and inorganic material etc., or any other material where nanoparticles particles are required to be made.

[0074] In this process, a selected raw material is disposed in at least a movable and sealed inner milling chamber that is arranged in a movable outer chamber. The inner milling chamber is cooled or the temperature is controlled through a channel covered with a jacket that is supplied with a reduced amount of a temperature-controlling liquid and maintaining the inner milling chamber temperature in the range of 77-300 K. The temperature-controlling liquid is an inorganic or organic liquid selected from nitrogen helium, methanol, ethanol and benzyl alcohol or combination thereof. The milling is carried out under an inert atmosphere and the inert atmosphere is generated by employing inert gases selected from argon, ammonia and carbon monoxide or a combination thereof, preferably ammonia. The inner temperature of the milling apparatus is continuously monitored using a probe and the temperature is maintained in the range of about at about 77-300 K. The milling of the material is performed in the presence of at least a movable milling member, without physically exposing the material to the temperature-controlling liquid, to obtain free-standing nanoparticles.

[0075] After the process of milling the milling material (powder) is transferred to a container containing an alcohol, preferably methyl alcohol and mildly ultrasonicated with an ultrasonicator for about 15 minutes. The process of ultrasonication results in a colloid of nanoparticles suspended in alcohol medium. The particle size of the suspended nanoparticles are estimated using a light scattering system such as Zeta Sizer (Model ZS90), which employs a laser based noninvasive back scattering technique. This instrument can measure sizes from lOu to 0.3nm. The nanoparticles are separated by any other suitable methods of separation such as ultrasonication or agitation.

[0076] In yet another aspect of the present invention the process wherein the selected raw material can be in the form of pieces, chips or shots (Fe).

[0077] In further aspect of the present invention, the process wherein the raw material is milled under variable temperature conditions such as low, room and high temperatures to obtain the corresponding free-standing nanoparticles.

[0078] In yet another aspect of the present invention, the process wherein the nanoparticles are in colloidal form.

[0079] In further aspect of the present invention, the nanoparticles as obtained by employing the apparatus and method of the present invention are subjected to X-ray diffraction (XRD) studies. Evolution of microstructure of the sample materials is studied using a Field Emission Scanning Electron Microscope and a Field Emission Transmission Electron Microscope. In order to address, the issue of contamination of the cryo-milled nanopowders, a composition analysis using a Field Emission Electron Micro Probe Analyzer (EPMA). The Energy-dispersive X-ray Spectroscopy (EDS) measurement is followed by the Wavelength dispersive X-ray spectroscopy (WDS) measurements with a finely converged beam using two different crystals (PJET & LFED), covering the energy range of all possible contaminations (B to W). Within the accuracy of the selected microprobe (sigma of 0.01), the absence of any contaminant is determined.

[0080] The following are non-limiting examples that are provided to illustrate the subject matter of the present invention. In this process, as an exemplary embodiment, three different kinds of metals Cu (FCC), Zn (HCP), Fe (BCC) and ZnO other alloys are subjected to the milling process and the corresponding results are observed.

Example 1

[0081] 5 grams of Copper chips (Cu) are loaded into the inner milling chamber of the apparatus the inner milling chamber is hermetically sealed. Liquid nitrogen is circulated through the jacket arranged on the external chamber and chamber temperature is maintained at about 150° K. Argon gas is circulated into the external chamber and the pressure is maintained at lbar. The cryomilling is performed for about 3 hours to obtain nanoparticles of Cu. lOmg of the nanoparticles (powder) is mixed with 50 ml of water and the solution is ultrasonicated for 15 minutes to obtain a colloid of nanoparticles suspended in an alcohol medium.

[0082] The X-ray diffraction patterns of the nanoparticles reveal the increase in broadening of the diffraction peaks in three hours of milling process. The process of cryomilling reduced the crystallite size down to 30nm after 3hrs. SEM images of the cryomilled Cu powders reveal that cryomilling for 3hrs has enabled the average particle size reduction of the Cu powder from about 10 urn to 30 nm. With an aim to observe the size and morphology of the Cu powders with better clarity, the same is subjected to extensive ultrasonication followed by TEM study. The particle size and morphology of the as received Cu powder, which is 3-hrs ball-milled product, also confirm the particle size of 30nm. Moreover, the nanoparticles are evidently isolated from each other and having close to spherical shape. After cryomilling, the size distribution becomes narrower (between 10-50nm) and lying between 20-30nm. The microstructural features of the cryomilled Cu particles evidenced in the SEM and TEM images are in contrast to the established mechanism of grain size reduction of the ductile metals, which involve repetitive welding and fracture events during ball milling leading to the formation of nanoscale microstructure of the aggregates.

Example 2

[0083] 5 grams of Zinc chips (Zn) are loaded into the inner milling chamber of the apparatus the inner milling chamber is hermetically sealed. Liquid nitrogen is circulated through the jacket arranged on the external chamber and chamber temperature is maintained at about 150° K. Argon gas is circulated into the external chamber and the pressure is maintained at 1 bar. The cryomilling is performed for about 3 hours to obtain nanoparticles of Zn. lOmg of the nanoparticles (powder) is mixed with 50 ml of water and the solution is ultrasonicated for 15 minutes to obtain a colloid of nanoparticles suspended in an alcohol medium.

[0084] Considering the fact that the minimum size reduction is related to melting point of the materials, it is established that the materials with lower melting point cannot reduced to very small size. In process steps of the present invention Zn and commercially pure Zn powder (99.95% Zn) is used for the preparation of free-standing nanoparticles. The XRD patterns obtained from Zn milled shows absence of any contaminants in the milled samples. The crystallite size has reduced to 9 ±2 nm after about 3 hours of cryomilling. The SEM images of the cryomilled material shows the particle size to be approximately 7 nm. Comparing the results with crystallite size measurement from XRD analysis, it is clear that the average crystallite is almost same in both cases. Therefore, SEM micrographs reveal the formation of free standing Zn nanocrystallites, after cryomilling. With an objective to observe the size and morphology of the Zn powders with better clarity, a detailed TEM study of all the samples is conducted to elucidate the aspect of grain refinement due to combined milling at cryogenic temperatures. The bright field micrograph depicts the particle size and morphology of Zn powder cryomilled for about 3 hours. The histogram generated from the TEM bright field images showing crystallite size distribution. Therefore, it stands established that cryomilling has resulted in the reduction of individual crystal size below 6 nm. Moreover, the ball-milled crystallites are evidently isolated from each other and having a shape close to hexagonal. The particle size analysis of these particles gives a size of lOnm, which matches with the above observations.

Example 3

[0085] 5 grams of Ferrous shots (Fe) are loaded into the inner milling chamber of the apparatus the inner milling chamber is hermetically sealed. Liquid nitrogen is circulated through the jacket arranged on the external chamber and chamber temperature is maintained at about 150° K. Argon gas is circulated into the external chamber and the pressure is maintained at lbar. The cryomilling is performed for about 3 hours to obtain nanoparticles of Fe. lOmg of the nanoparticles (powder) is mixed with 50 ml of water and the solution is ultrasonicated for 15 minutes to obtain a colloid of nanoparticles suspended in an alcohol medium.

[0086] Contamination during milling is one of the key challenges. The contaminations can be from container or environment. In the same line, the heating during milling leads to high temperature oxidation in the material. The process of the present invention helps preparing free- standing nanoparticles that are free from the oxidation effects. In current example pure Fe (99.95%) is cryomilled in similar conditions, as described above. EPMA for bulk composition has shown the content of oxygen <2%. The purity of Fe can be further verified by VSM magnetic measurement which shows magnetic saturation very close to pure Fe.

[0087] FIG.12(a) depicts various stages (stages 1-8) of images of Fe material captured at various time intervals of 15 minutes during the course of milling operations using the apparatus of the present invention. Stage 1 depicts an image of Fe material prior to milling and whereas the image of stage 8 depicts the milled Fe nanoparticles. Various intermediate stages i.e. stages 2-9 are shown depicting the progressive milling of Fe material. The reduction of size of the initial chip is clear and in long duration change in metallic color reveals formation of small particles. The color of oxide particle confirms its reduction of size.

Example 4

[0088] 10 grams of Zinc Oxide (ZnO) are loaded into the inner milling chamber of the apparatus the inner milling chamber is hermetically sealed. Liquid nitrogen is circulated through the jacket arranged on the external chamber and chamber temperature is maintained at about 150° K. Argon gas is circulated into the external chamber and the pressure is maintained at 1 bar. Low temperature milling is performed for about 3 hours followed by room-temperature milling for about 10 hours. 20mg of the nanoparticles (powder) is mixed with 50 ml of water and the solution is ultrasonicated for 15 minutes to obtain a colloid of nanoparticles suspended in an alcohol medium. The particle size analysis shows formation of small size particles. The TEM image clearly reveals formation of ZnO nanoparticles.

[0089] FIG. 12(b) depicts various stages (stages 1-8) of images of ZnO material captured at various time intervals of 15 minutes during the course of milling operations using the apparatus of the present invention. Stage 1 depicts an image of ZnO material prior to milling and whereas the image of stage 8 depicts the milled ZnO nanoparticles. Various intermediate stages i.e. stages 2-9 are shown depicting the progressive milling of ZnO material.

[0090] FIG. 12 (c) depicts powder form of nanoparticles of Cu material that is subjected to milling.

[0091] In an aspect of the present invention the particle size of the suspended nanoparticles as obtained from the apparatus and process of the present invention are estimated using a Zeta Sizer (Model ZS90), which employs a laser based non-invasive back scattering technique, which can measure sizes of particles from lOjo, to 0.3nm. The corresponding digital images are as shown in FIG.13, which confirm the formation of colloids of Cu, Zn and Fe along with their particle size analysis. The size distribution of exemplary materials such as Cu, Fe, Zn and ZnO are plotted as histograms. It is observed from the histograms that the mean particle size for Zn, Cu, Fe and ZnO nanoparticles is viz., 30, 8, 50 and 6 nm respectively.

[0092] In another aspect of the present invention, the milled samples corresponding to Cu, Zn and Fe (in powder form) are subjected to XRD at 153K, to study the reduction in grain size to and the corresponding data are obtained in reflection mode using Cu Ka doublet radiation in the 20 range of 10-120° with a step size of 0.008°. The XRD patterns of Cu as shown in FIG.14 (a) reveal no major impurity and the broadening of the diffraction peaks shows reduction of size. Further, cryomilling reduced the crystallite size down to 30nm for Cu after 3 hours. In another aspect of the present invention evolution of microstructure of the milled sample Cu is studied using a field emission scanning electron microscope and a field emission electron probe micro analyzer is used for measuring bulk composition. FIG.14(b) shows SEM image at a relatively low magnification of nano particles of Cu prepared by transferring drops of solution on a carbon film.

[0093] In yet another aspect of present invention transmission electron microscopy (TEM) image of the milled sample of Cu is prepared on a graphite coated TEM grid. The lattice resolved image as shown in FIG.15a and 15b depict relatively larger particles and in general having single crystalline form.

[0094] In another aspect of the present invention, the milled sample Zn (in powder form) is subjected to XRD at 153K, to study the reduction in grain size to and the corresponding data are obtained in reflection mode using Cu Ka doublet radiation in the 20 range of 10-120° with a step size of 0.008°. The XRD patterns as shown in FIG.16(a) reveal no major impurity and the broadening of the diffraction peaks shows reduction of size. Further, cryomilling reduced the crystallite size down to 7nm for Zn after 3 hours. Evolution of microstructure of the milled sample Zn is studied using a field emission scanning electron microscope and a field emission electron probe micro analyzer is used for measuring bulk composition. FIG.16(b) shows SEM image at relatively low magnification of nanoparticles of Zn prepared by transferring drops of solution on a carbon film.

[0095] In yet another aspect of present invention transmission electron microscopy (TEM) image of the milled sample of Zn is prepared on a graphite coated TEM grid. The lattice resolved image as shown in FIG.17a and 17b depict relatively larger particles and in general having single crystalline form.

[0096] In another aspect of the present invention, the milled sample corresponding Fe (in powder form) is subjected to XRD at 153K, to study the reduction in grain size and the corresponding data are obtained in reflection mode using Cu Ka doublet radiation in the 28 range of 10-120° with a step size of 0.008°. The XRD patterns as shown in FIG.18 (a) reveal no major impurity and the broadening of the diffraction peaks shows reduction of size. Further, cryomilling reduced the crystallite size down to 50nm for Fe after 3 hours.

The evolution of microstructure of the milled samples of Fe is studied using a field emission scanning electron microscope and a field emission electron probe micro analyzer is used for measuring bulk composition. FIG. 18(b) shows SEM image at a relatively low magnification of nano particles of Fe prepared by transferring drops of solution on a carbon film.

[0097] In another aspect of the present invention, the milled sample of Cu is subjected to transmission electron microscopy and the particle size of the milled material is found be about 20 nm in size as shown in FIG.19. The TEM has also not shown any impurities as can be seen from the inset of FIG. 19.

[0098] In yet another aspect of the present invention the milled sample of ZnO is subjected to transmission electron microscopy and the particle size of the milled material is found be about 7 nm in size as shown in FIG.20.

Advantages

[0099] The ball mill apparatus of the present invention is highly efficient and consumes less power. The power consumption of the ball mill apparatus is about 50 % less than known mills.

[00100] The ball mill apparatus of the present invention consumes less liquid Nitrogen, which is about 60 % less than the known mills.

[00101] The ball mill apparatus of the present invention provides a better temperature control, which can vary from liquid nitrogen temperature to 500°C.

[00102] The size reduction of the milling material is done is done in inert atmosphere (e.g. Ar) and vacuum.

[00103] The milling process of the present invention is performed in conjunction with different types of vials depending on milling material.

[00104] The milling apparatus of the present invention can also be used to provide coating to various objects.

[00105] The reduction in particle size of the materials of the present invention enhances the properties of the materials.

[00106] It will thus be seen that the embodiments as set forth above, are efficiently attained and since certain changes may be made in carrying out the present invention without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense.

[00107] It is also understood that the following claims are intended to cover all the generic and specific features of the invention herein described and all statements of the scope of the invention, which as a matter of language might be said to fall therebetween.

We Claim:

1. An apparatus for preparing free-standing nanoparticles, comprising:

(a) a first base member;

(b) a movable second base member arranged co-axial to the first base member, with an intervening space formed between the first and second base members;

(c) at least a vibrator connected to the first and second base members and;

(d) at least a movable external chamber, connected to the first and second base members and the vibrator;

(e) at least a movable and sealed inner milling chamber, disposed in the external chamber and connected to the first and second base members and to store milling material; and

(f) at least a movable milling member, disposed in the inner milling chamber.

2. The apparatus as claimed in claim 1, wherein an insulating jacket with a coolant disposed to envelope the external chamber and the inner milling chamber.

3. The apparatus as claimed in claim 1, wherein a plurality of load bearing members disposed between the first and second base members.

4. The apparatus as claimed in claim 1, wherein the second base member is unitary or with divisions.

5. The apparatus as claimed in claim 1, wherein the divisions of the second base member are arranged in plane parallel or at an angle to each division.

6. The apparatus as claimed in claim 1, wherein the inner milling chamber is cylindrical, oval or circular.

7. The apparatus as claimed in claim 1, wherein the movable milling member is a ball, pestle, roller, pellet or a combination thereof.

8. The apparatus as claimed in claim 1, wherein a single or plurality of movable milling members arranged in the inner milling chamber.

9. The apparatus as claimed in claim 1, wherein a plurality of movable external chambers and inner milling chambers connected to the base members.

10. The apparatus as claimed in claim 1, wherein a plurality of vibrators connected to the movable external chambers and positioned vertically, horizontally or at any intermediate positions to the floor axis.

11. A process for the preparation of free-standing nanoparticles, the process comprising the steps of:

(a) disposing milling material in at least movable and sealed inner milling chamber that is arranged in a movable outer chamber;

(b) controlling the temperature of the inner milling chamber through a channel covered with a jacket circulating a reduced amount of a temperature-controlling liquid;

(c) milling the material in the presence of at least a movable milling member, in an inert atmosphere, and under vacuum, without physically exposing the milling material to the temperature-controlling liquid; and

(d) obtaining free-standing nanoparticles of the milling material.

12. The process as claimed in claim 11, wherein the milling material is selected from metallic, non-metallic, alloy, organic, inorganic or ceramic material.

13. The process as claimed in claim 11, wherein the milling is performed at a temperature in the range of about 77-300° K.

14. The process as claimed in claim 11, wherein the temperature-controlling liquid is an inorganic or an organic liquid, selected from nitrogen helium, methanol, ethanol and benzyl alcohol or combination thereof.

15. The process as claimed in claim 11, wherein the inert atmosphere is generated by employing inert gases selected from argon, ammonia and carbon monoxide or a combination thereof.

16. The process as claimed in claim 11, wherein the material is metallic, non-metallic, alloy, organic, inorganic or a ceramic material.

17. The process as claimed in claim 11, wherein the movable milling member is ball, pestle, roller or a combination thereof.

18. The process as claimed in claim 11, wherein the nanoparticles are in colloidal form.