By Alexander DMITRIYEV, Dr. Sc. (Tech.), Director, Center of High Technologies, Vladimir ANKUDINOV, Cand. Sc. (Tech.), senior researcher of the same Center
Fundamental, or basic research, as a rule, turns with time into applied studies. The final results of these efforts are high technologies with a whole range of practical applications. One such example are what are known as monodispersed systems involving the production and "exploitation" of submillimeter microgranules and microspheres with a number of identical parameters.
This field of research has a history of more than one and a half centuries. Back in 1833 French physicist F. Savard started experiments with jets of water escaping in various directions through fine tubes and orifices or apertures. During his experiments the scientist observed that being "whole", or unbroken in the beginning, these jets later develop necks, or constrictions until finally there appear drops of water of different size which fall in irregular ways and at low angles of scatter.
Later on the scientist established that the length of the unbroken "section" of a jet, the size and scatter of drops depend in the main on the initial disturbances-vibration of the water container (tank), turbulent vortexes appearing on the uneven surface of the nozzles and also on all kinds of external acoustic impacts ("noises").
We do not know what had suggested to the French researcher this idea, but in his subsequent experiments he connected the tank with liquid with the vessel into which it discharges or trickles with a copper wire as a kind of a "backfeed". The results were really impressive. Drops started falling in a regular manner and without scatter, all being of practically one and the same size. It was also established that the rate of scatter of the escaping water can be controlled. This was a method of generation of a regular jet of drops (known as a method of "forced capillary scatter of jets"). All "satellite" drops of different size and direction of motion are eliminated.
As is often the case, the studies of the French researcher could not be put to any practical use there and then. It was remembered some 20 years ago, setting off an interesting and very important trend of research-monodispersion systems
Drawing (left) from a work by Dr. Savard in which he described for the first time the formation of construction "necks" in a jet of water and presented a diagram (right) of the superimposition of regular disturbances upon a jet: a- irregular splitting, b-forced capillary splitting with a positive feedback for producing droplets of similar size.
and technologies for practical applications. Today specialists manipulate not only with water, but with droplets and solid granules from a whole range of materials- solutions, polymers, metals, alloys and ceramics.
The range of potential uses of the new technologies is really large. They can be used to produce identical particles with less than one percent of difference in diameter, form droplet structures "orderly" in space and time and moving in different media with pre-set velocities. The "scatter" of millions of such droplets according to that latter parameter may not exceed 0.1 percent and angular dispersion can be no more than 10 -4 rads.
Now, let us take a look at the basic parameters characterizing monodispersed systems and an approximate range of their applications. The size of the microparticles produced is from 10 to 1,000 mcm, and their relative dispersion is 0.1 - 1.0 percent. Deviations from the spherical does not exceed 0.5 percent. The velocities of microgranules in a flux can be from 3.0 to 70.0 m/s, and with dispersion-10 -5 - 10 -3 percent. The productivity of the new technologies is very high and reaches 10 4 - 10 6 particles/s in a broad range of temperatures-from 14 to 1,500 K.
The "popularity" and potential uses of such systems are also due to the fact that microgranules can be obtained from many media, such as water, metal, bitumen, polymers, copper, paraffin, etc. What is more, the substances thus used can possess relatively high surface tension and low viscosity. They can also contain mechanical inclusions of not more than 3.0 mem. The range includes aqueous and non-aqueous solutions, different alloys and metals, glass, multi-component ceramics, polymers, bioactive substances, vitamins and medicinal preparations, etc.
So, what are the possibilities of such monodispersion systems apart from producing microgranules? So far we do not know them all, but some are already being used: coating the microparticles with surface lay-
Conceptional diagram of an electric droplet-jet printer using droplets of metal.
Photographs ofmouir arches (a) produced by electric droplet-jet technique upon a base with droplets of solder of about 375 mem (area of image-about 1 mm) and two sinusoids, produced by solder droplets upon a copper base (b).
ers with different properties, producing porous and composite droplets and granules, obtaining fine coatings, etc. Speaking about the latter, modern technologies can produce microspheres of 300 to 1,000 mcm in diameter and coatings of 20 - 50 mcm of different materials, such as have been mentioned before. Such "beads" can also be filled with various components (gases, liquids) which makes it possible to use them as "micro-containers'" for the storage and transportation of various substances, including toxic and chemically active ones. What is more- since the thickness of the "shell", or coating, of such microgranules is very low, it can possess hardly any defects which adds up to the strength and durability of such particles. And the final factor deserving of attention is the possibility of applying an electric charge to these beads which makes it possible to "steer up" their movement.
As soon as scientists were able to produce such microgranules they began to be put to practical uses in many branches of industry. In mechanical engineering, instrument making and cryogenic technologies spherical monodispersed droplets are needed made of metals and alloys measuring from 10 to 1,000 mcm. But their production has been a problem, or even two problems. The thing is that speaking of metals, they are mainly rare earths (erbium, holmium, etc.) which have a very high melting point (above 1,000 o C) at which there occur active physico-chemical interaction with other metallic admixtures. We have been able to rule out such reactions by using crucibles and draw plates (or dies) from high-melting metals, like molybdenum.
The second problem was that droplets are rapidly oxidized in the oxygen-rich medium. This being so, we had to design special chambers with vacuum, or filled with some inert gas. In this way we have been able to achieve a stable process of producing monodispersed droplets of metals and alloys. Passing through the chamber, they reach a zone of rapid cooling, such as a container with liquid nitrogen, where they turn into solid granules. Such chambers made of alloys (erbium with nickel, holmium with copper,
Diagram of making items on the basis of 3D-printer monodispersion technology.
etc.) are widely used in what are called regenerative heat exchangers of cryogenic equipment (refrigerators) which can bring down temperature to 2 K. Such equipment finds applications in electronics, medicine (tomographs) and other fields. Working in this direction we have developed units based on what we call cryomonodispersion technologies. The last unit in this range consists of 5 "sections" each performing its special functions. In the first, liquid is prepared by chemical treatment methods. For that the material which we want to turn into mono-dispersed microdrops is dissolved in special components. These are materials with high melting points-various ceramics, chemically active or toxic substances. After that the obtained substance goes into the second section where a capillary jet is produced and liquid droplets generated. These are channeled into a preliminary cooling chamber (3rd section) where their temperature is considerably reduced and a fine hard film, or shell is formed on their surface after which the microspheres get into the 4th section for the final cooling (a container with liquid nitrogen). After that there comes the stage of vacuum sublimation drying (5th section). At this stage solvent components are removed from the microspheres and the final product is ready for use.
On the basis of cryomonodispersion technologies we have obtained some unique powders for multi-component ceramics, including complex piezoceramic and magnetoceramic materials, pure metals and alloys and this is done without heating, at room temperature. And we have been able to develop the manufacturing process and produce monodispersed granulated nuclear fuel on the basis of uranium dioxide.
But coming back to metallic droplets, let us consider some examples of their practical applications. As has been said before, they can carry an electric charge which makes it possible to direct the flux of such microgranules with the help of electric fields- something which is important for many production processes. For example, fluxes of droplets of solder can be used for automatic soldering. Using the same technology, and according to a preset program, it is possible to apply a conducting "relief of solder on the surface of different bases, and bearing in mind that the size of droplets is very small-of 375 mem only-it is easy to produce what are called superminiature "pictures". Technologies of this kind are being successfully developed by specialists of the Moscow Institute of Power Engineering and the Institute of Thermal Physics of the Siberian Branch of the Russian Academy of Sciences.
Another area of utilization of guided fluxes of monodispersed metallic droplets is related to the development of a promising "casting-free" technology of producing items of complex geometry. Here is one such example in which the central role belongs to a movable base connected to a computer. In the process of making a required item a flux of droplets of molten metal is directed vertically down and towards that base, which moves in the required directions, including circular trajectories, in accordance with a computer program. As a result the required item is "formed" upon that base and its configurations can be really fantastic.
Summing up, there is a wide range of applications of monodispersion technologies, including space studies, atomic power production, medicine, farming, chemical and food industries, etc. Take, for example, our latest industrial unit based on a set of technologies, including the monodispersion and cryogenic ones. This is a unique method of cleaning the surface of various objects from dirt, rust, grease and the like. It is based on monodispersed deeply frozen microspherical granules of
ice of the same size (300 mem) accelerated to high velocities by cooled dry jets of air.
The process takes place in the following manner: water is passed through a crude filter and goes into a generator producing monodispersed droplets. These are carried by a stream of cold air (cooled in a special fridge down to -70 - 80C) and are frozen into granules of ice. These are then accelerated to high velocities in a gasodynamic accelerator and chanelled upon the surface to be cleaned.
The result have surpassed all our expectations. Apart from getting an ideally clean surface, the technique provides for an "ecologically friendly" mode of operation (being used is only frozen water which contains no sand, metallic inclusions or different hard powders). Other advantages include a low cost of treatment of a unit of surface and easy handling which makes it possible to make portable cleaners of this kind. The method produces only a "soft" impact on the cleaned surface without causing any defects such as microscratches.
Our method has no analogues anywhere in the world and contains several "know- how". It has passed technical tests in cleaning all kinds of grease and dirt, such as oil and petroleum spots, paints and rust from metallic, plastic, ceramic, concrete, asphalt and other surfaces and has been cleared for use by experts.
On the whole studies and achievements in the field of monodispersion systems and technologies have received a high assessment of the scientific communities in this and other countries. In 1993 we were awarded the State Prize of the Russian Federation, and in 2001 - the Government Prize. Apart from that our innovations won diplomas and silver medals at the 27th International Salon of New Technologies in Geneva in 1999 and at the jubilee 50th World Salon of Innovations, Scientific Developments and Novel Technologies "Brussels-Eureka2001".
Illustrations supplied by A. Dmitriyev.
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