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Author: Nauka v Sibiri (Science m Siberia), 1999 Prepared by Arkady MALTSEV
One of this country's leading centers of research in the field of new materials and their properties is the Institute of Catalysis (named after Academician Sergei Boreskov) which belongs to the Siberian branch of the Russian Academy of Sciences. Its researchers are making a study of sophisticated problems, e.g. thermodynamic properties and volumetric effects in electrolyte solutions placed into the pores of various substances of definite geometry. The importance and promising nature of this research have been confirmed by a special grant offered to an Institute team for its study: "Investigation of Thermodynamic Properties and Structure of Solutions and Crystallohydrates of Inorganic Salts Dispersed in Monodispersive Nanopores: Dimensional Effects". Dr. Mikhail Tokarev, a staff member of the Laboratory of Energy-Accumulating Processes and Materials of the Institute, takes a closer look.
One major problem facing researchers specializing in this field is what accounts for changes in the properties of substances during their transition from a solid, or integral state into an ultra-dispersive condition. Studied in sufficient detail thus far are the properties of minor metallic clusters (fragments consisting of a few scores of atoms) and fine films, and also the lowering of the freezing temperature of water, or organic solvents, in fine capillaries. But much less is known about what is called the multicomponent ultradispersive systems, such as electrolyte solutions.
Studying the dimensional effects in such solutions, scientists rely on what they call selective sorbents of water (SSW) which have been developed at the Laboratory of Energy-Accumulating Processes and Materials of the Institute. In outline these materials can be described as a matrix into the pores of which a highly hydroscopic, or moisture-absorbing substance is introduced-usually some inorganic salt. New materials produced in this way possess high sorption and energy-accumulating capacity, a low regeneration temperature and other unique properties.
Beforehand, scientists had investigated in detail SSW systems of two types which contained salt in the micro- and mesopores of silica gel. And they discovered many interesting facts. For example, the sorption capacity of crystallohydrates in a mesoporous system is considerably increased and no crystallohydrates are formed in a microporous structure. Calori-metric methods revealed a 15-20C drop of the melting temperature of solutions and crystallohydrates in mesoporous systems.
But did such changes in silica gel pores belong only to this salt or were common for all ultradispersive systems in general? What kind of a model could be designed in order to describe and explain this effect?
Proceeding from the available experience and methods, Mikhail Tokarev points out, the research team set itself to synthesize and study the largest possible range of systems incorporating porous matrixes with different, but well-defined structures into which different salts were introduced.
The subsequent studies proved: it was the porous structure of the matrix, and not its chemical nature that determines to a large extent the properties of the salts introduced therein.
Since in the scientists' view the different "modes of behaviour" of solutions and crystallohydrates in nanopores are associated with
changes in their structure and molecular mobility, the subsequent and more detailed investigations involved a set of mutually complementary physico-chemical methods, including nuclear-magnetic resonance, infrared radiation, dielectric spectroscopy and what is known as dynamic thermomechanical analysis.
The experiments proved that marked changes of properties of the materials under investigation occurred when researchers introduced them into micropores, which can be explained by a non-uniform distribution of salt ions and water molecules incorporated into the electrolyte. And quantitative description of such systems is impossible without resorting to the theory of non-homogeneous liquids.
With that in mind, the researchers built models of behaviour of electrolyte solution in the pores by means of mathematical modelling. And it was proved that the "salt- water" system in nanopores was not homogeneous. What is more, the specific internal energy density of a model system grows at smaller pores radii within certain limits-a conclusion borne out by experimental data.
On the basis of thermodynamic analysis data the researchers built what they called temperature-independent curves of water sorption by salt solutions in nanopores with the use of but one independent parameter-the free energy of sorption.
In this discussion of the project Mikhail Tokarev also dwelled on their plans for the future. He said the researchers were confident that the already established regularities of the process under investigation and alterations of the physical-chemical properties of such solutions and of salt hydrates in matrixes of different chemical nature and porosity would lead to qualitative assessments of the dimensional effects in such systems. Apart from their theoretical value, these studies will have a range of practical applications, some of which were highlighted by Mikhail Tokarev in his interview.
He said that one of the oldest, and so far unresolved problems in the world was the problem of heat accumulation on a daily and seasonal scale. The underlying idea is fairly simple-it should be possible to accumulate heat in the summer and use it for heating in the winter. Residential heating projects of this kind are already under serious consideration in some Western countries, Germany for one. In pilot units of this kind heat is "stored" in large water tanks of more than 70 m3. Preliminary calculations conducted by our scientists together with their German counterparts indicate that giant structures of this kind could well be replaced with relatively inexpensive SSW units of 3-4 m3 only.
Another possible application of sorbents is building on their basis of what specialists call absorption thermal pumps and freon-free refrigerators. Theoretical studies with the participation of Italian specialists have revealed some obvious advantages of the Russian-made materials over the traditional systems using zeolite, silica gels, etc. In the first place, they operate at lower temperatures: while a temperature of 230C is required to attain a high capacity of a thermal unit based on the "zeolite-water" tandem, the use of the materials suggested by the Institute's scientists brings down this temperature to 120C and broadens the range of applications to a considerable extent. And since the same SSW help boost the unit capacity of such units, their size can be appreciably reduced for use in small premises and even automobiles.
But the range of potential applications of sorbents does not end there. They can be used, for example, for what is called deep gas drying, with the process being accelerated severalfold, and the temperature of sorbent regeneration 50 to 150C lower than usual-a fact promising tangible energy savings. On the basis of the Omsk Branch of the Institute of Catalysis, commercial production has been started of such drying agents on the scale of up to 500 tons a year.
In yet another application, SSW materials have been used for heat shields operating on the principle of water evaporation. Experiments have proved that their lifetime is an order of magnitude longer than that of the commonly accepted materials, and the heat conductivity factor can be below 5 mW (mK). The Institute's scientists, working in cooperation with experts from the Novosibirsk Institute of Light Industry, have developed flexible "heat-shield" fabrics containing one of the modifications of the new sorbent. What is also important is that all of these materials are for multiple use because they automatically "restore" their original condition by absorbing water from the ambient air.
This very characteristics prompted what looked like a fantastic idea: extract water from the ambient air and then use it. This idea proved not too far-fetched: because at high levels of air humidity at night-time the sorbent will accumulate water and later the heat of the day will have this water evaporated and collected in a special condenser. The first pilot units of this kind reached a capacity of up to 400 g of water per kilo of sorbent in 24 hours, which means that a ton of this unique material can be turned into a source of fresh water for a small community in an arid region.
Most of these pioneering R&D projects of the Institute of Catalysis have already been patented in this country.
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