Monodispersion technology, its applications in instmment making, medicine, biotechnology, power engineering, electronics, space studies and other areas have been the subject discussed by two leading Russian experts in the field-Rector of the Moscow Institute of Power Engineering, Research Director of the Center of High Technologies, RAS Corresponding Member, Yevgeny Ametistov, and Director of the same Center, Alexander Dmitriev, Dr. Sc. (Tech.).

Although research in this field was launched some 20 years ago, the main achievements have been scored but recently. Having said that, what is this all about and why the results of these studies are in such high demand today?

This technology is based on the application of spherical particles (granules) from 10 to 1,000 mkm in size with dispersion of 0.1-1.0 percent; they are made of different materials: from water solutions and cryogenic liquids to molten metals. For their production a team of researchers, headed by Prof. Ye. Ametistov and Dr. A. Dmitriev, developed generators of monodispersion particles and sets of units with a capacity of 10 ⁴ -10 ⁶ granules/s which make it possible to form complex droplet structures. Deviations of the latter from sphericity are only 0.5 percent, their flux velocity 3- 70 m/s, operating temperatures range - 14-1,500 К. Droplets of this kind, which simply cannot be produced by any other known techniques, are now in high demand for research purposes.

If we apply one and the same electric charge to each of the newly formed spherical particles, they can be considered a kind of an analog of electrons, and their flux can be controlled the way it is done in a cathode-ray tube. And that means that it should be possible, by altering the excitation signal, to regulate the movement of droplets after disintegration: launching them in a steady flux, produce "packs or sections" with a desired number of particles in each of them, produce fluxes or jets of granules of two different sizes (what are called satellite regimes), etc. Systems of this kind can consist of microgranules of different structure (uniform, hollow, multilayer, porous), in different phasic states (liquid, solid), and made up of different materials (glass, metals, alloys, polymers, composites, etc.).

One of the most promising trends in these new technologies are what we call electrodroplet markers for graphic-digital labeling of various articles. Their performance is based on the formation of a flux of monodispersed droplets of dies, or inks, which, upon their formation, gain the same electric charge. After that they fly past electrodes emitting electric pulses according to a special program depending on the purpose of labeling. As a result, droplets deviate at different angles and label the product with a required inscription.

Another example: monodispersion technology making use of precision dosage generation of charged droplets. This provides the basis for the operation of automated dosing apparatuses, or batchers, of harmful, toxic or radioactive substances with an unprecedented degree of high accuracy. Apparatuses of this kind are in high demand in medicine, pharmacology, electronics and other fields. Their principle of operation is similar to the

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Coherent fluxes of monodispersion microspheres (geometry and structure): a - unidimensional flux; b - flat flux; c - two- dimensional flat droplet shroud; d - cylindrical coaxial droplet shroud; e - coherent structure of microspheres of two dimensions (basic droplets and satellites).

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Operational diagram о fan electric jet marker for labelling goods and products.

electro-droplet jet pumps, but the flux of droplets is controlled in a much more sophisticated way and requires a built-in processor. Thanks to all that the substance is channelled directly into containers, located on a platform or a moving conveyor belt. If need be, one can place into each of them different doses of preparations ejected in every concrete case from what are called monodispersion ejector heads. The rate of this process is very high and reaches 10 ⁵ microdoses/s.

The same principle is used for devices for "sorting out" of cells and other bioactive products. Weak laser irradiation makes it possible to "diagnose" in a contactless way their composition and likely breaks of structure. In this manner one can carry out proximate tests of blood and other biological fluids, which is very important in view of the growing number of both natural and technogenic disasters in the world.

Monodispersion technologies are very effective in medicine-for the making of drugs. The potential of precision dosing, or batching, makes it possible to produce multicomponent preparations with a preset menu of the necessary substances, including polyvitamins and nutritional additives.

And last, but not least - there is one area in which the above technologies are very fruitful and sometimes simply indispensable. This area is space research. Take, for example, one of what seems to be exotic, but time-tested technologies - the development of what are called droplet radiation heat exchangers for the cooling of satellites and orbital stations. In this process, essentially, the heat from the built-in heat exchanger of a space vehicle is supplied into a droplet generator where droplets are turned into what is called a shroud. This is then discharged into space in the form of low-pressure vapor. After flying for tens and hundreds of meters there, the droplets are precipitated and gathered up in a special collector from where they are channelled into the onboard heat exchanger, and the cycle has a rerun.

By the rate of heat removal and the ratio of the effective payload of the system to the power level emitted by the heat exchangers, units of this kind surpass all of the known analogues. And they become simply indispensable if the level of heat removal from a space vehicle exceeds 100-500 kW. So far they are unrivaled by their mass and size, by prompt readiness for operation and by their "invulnerability" to meteorite bombardment. The authors point out that they are keeping up studies and experiments in conjunction with experts from the Keldysh Research Center (former Research Institute of Thermal Processes). Their testing stand, which is a model of a droplet space radiator with systems of droplets generation, collection and diagnostics, has passed flight tests with much success on board the MIR space station.

The two authors of the present review cite examples of the likely uses of their technology in a not too distant future. These include what they call a modeling of micrometeorite phenomena with the help of accelerated monodispersed particles; systems of contactless refueling of space vehicles based on the dispersion of the fuel and oxidant into droplets, their cooling and freezing, acceleration of their jet in a special unit for their delivery from a low orbit to a space vehicle.

And more of the same. At all thermal power stations now fuel is atomized by injectors before burning. But in the currently available industrial units of this kind the spray particles produced are very different in size, which causes their incomplete combustion

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Diagram of a precision monodispersion batcher.

and high levels of toxic emissions. Injectors based on monodispersion technologies produce droplets of oxidizer and fuel of quite definite dimensions, which ensures their almost complete combustion, boosts the efficiency of the power station, and reduces harmful discharges.

And atomic power stations also fit into this picture. It turns out that their safety levels can be sharply increased if the nuclear fuel is used in the form of monodispersed granules - something that should rule out in principle accidents like the Chemobyl disaster. The explanation for that is simple: a granulated fuel rod contains fuel wrapped up in protective layers of pyrolytic carbon and silicon carbide which, transparent to neutrons, retain radioactive decay products inside the particle. It is not destroyed even in a worst emergency because its shell is very strong, so that a reactor of this kind could sus-

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Diagram of droplet radiation heat exchanger for space probes.

tain a loss of its coolant without a melting of its core.

Another major problem for atomic power stations is to provide conditions in which a nuclear reaction comes to a halt by itself when a preset temperature is exceeded. Since the high (8 MeV/nucleon) energy of nucleon bonding in the nucleus makes the reaction insensitive to temperature, the only way to achieve the desired effect is to build a core in the form of a complex heterogeneous system of monogranules of fuel and graphite filler. Their production from dioxine of uranium or plutonium will make it possible to cope not only with the safety problem, but resolve a number of associated tasks of energetics. For example, the monodispersion technology can help produce plutonium spheres for new generations of nuclear reactors - something that will help convert plutonium to peaceful uses. This new technology, developed by Prof. Ye. Ametistov and Dr. A. Dmitriev in collaboration with experts from the All-Russia Research Centers of chemical technology and inorganic materials, is now practically ready for use.

Monodispersion systems are now in demand in research centers equipped with cyclotrons, proton synchrotrons and the like. In most of these accelerators of elementary particles corpuscular targets are used in the form of threads or plates at cryogenic temperatures. But, as proved by experience, what are called spherical analogs are more preferable because they provide for a higher degree of detection and are practically ecologically safe (in practical terms ruling out scatter of fragments of heavy ions beyond the walls of the accerating chamber in experiments with colliding beams). And it is monodispersion technologies which make it possible to produce corpuscular targets, such as hydrogen ones with cryogenic temperatures.

Experts of the Moscow Institute of Power Engineering, working together with their counterparts from the Moscow Institute of Theoretical and Experimental Physics and the Julich Accelerator Research Center, Germany, have developed a hydrogen cryostat containing a special system of regeneration and control of droplets with their subsequent freezing. With this aim in view liquid hydrogen is supplied into a generator wherein a jet of homogeneous droplets of 10-30 mkm in diameter is produced, propagating at 20-30 m/s through a system of vacuum locks in which they are frozen into granules. Passing through a superconducting magnet, the granules get into the accelerator channel where some of them interact with a high-intensity proton beam and evaporate, disintegrating into ions which are registered by a detector. The remaining granules, not captured in the jet, get into a hydrogen collector. This method can be used in thermonuclear synthesis for making spherical targets of heavy hydrogen where granules feed nuclear fuel both to thermonuclear units with magnetic containment of plasma (tokomaks) and to targets used in

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pulsed thermonuclear synthesis (laser, beam). Research is now under way in many countries into effective methods of generation of spherical targets- something that can provide the basis of giant- scale energetics of the future.

Gaining momentum recently has been a new trend in power engineering in which hydrogen is used as fuel. Its application, however, is far from being safe since accumulations of large volumes of this gas pose an explosion hazard. One of the proposed solutions consists in producing strong (such as plastic) spherical hollow capsules, filled with hydrogen at a high (and even supercritical) pressure. The fuel "packed up" in this way is strictly batched and absolutely safe. The identity of the thus obtained particles by mass makes it possible to apply to them equal electric charges and manage them with the help of respective fields, thus ensuring the required supply of fuel and oxidizer into the combustion chamber.

Since the walls ofmicrogranules are made of high-strength but easily oxidized polymers, which bum up together with hydrogen, this opens up prospects for developing basically new structures, such as internal combustion engines for different applications operating on hydrogen fuel. This fuel will be absolutely safe, environment-friendly and convenient for transportation and storage.

In many technological processes a central role belongs to all kinds of heat exchangers. Some of them, say, regenerative ones designed for cryogenic gas machines, have to be made of rare- earth alloys. We at our Institute have developed a method of producing corresponding monodispersed particles on the basis of erbium, holmium, neodymium, ytterbium, etc. These products are in high demand on the US market and in China. And we are also looking into the possibilities of producing multicomponent composite alloys for aircraft and rocket technology, bullet-proof jackets and ammunition stores on the basis of hollow metal granules, densely packed together and "bound" by a special matrix. Such materials possess unique mechanical and thermophysical properties, are very light and can sustain high pulse strains, or loads.

And more. It turns out that the necessary granules can be obtained not only by melting, but also by heating up the respective material up to the melting point in an acid medium. From a solution thus formed we can obtain by the known methods jets or heterogeneous droplets which are frozen up in liquid nitrogen and are channelled into a special unit for vacuum sublimation drying, where they turn into solid granules. This technique makes it possible to produce high-quality ceramic powders (piezoelectic ceramics, magnetoceramics, etc.), including all sorts of complex or multicomponent materials.

In their final assessment of the achievements of Russian scientists in the field ofmonodispersion technologies, the authors of the review come to the conclusion that this will be one of the main areas which will determine scientific and technological progress in this new century.

Ye. Ametistov, A. Dmitriev, "New Branch of Science and Practice - Monodispersion Technologies", - VESTNIK RAN, Vol. 71, No. 9, 2001

Prepared by Arkady MALTSEV

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