STUDIES OF AGEING PROCESSES IN ICE-FORMING AEROSOLS WITH SILVER IODIDE
A.G. Shilin, V.N. Ivanov, A.V. Savchenko, A.I. Fedorenko
Federal State Budgetary Institution “Research and Production Association “Typhoon”
URGENCY OF THE PROBLEM
When airborne or ground-based ice-forming aerosol generators are used for weather modification, rather a long time
period passes before the aerosol comes into the target zone. During this time the ice-forming aerosol activity
decreases by several orders of the value. This drop depends both on the environmental conditions and the composition
of aerosol particles used . At present, such effects are not taken into account, and the main characteristic of the
generators is the total amount of ice-forming active particles obtained immediately at the moment of aerosol formation.
As a result, one and the same ice-forming composition or generator is used under absolutely different conditions.
The goal of the work is in determining the mechanism and peculiarities of AgI-containing aerosols ageing processes,
on the basis of which the development of compounds and generators designed for different conditions of usage will be possible.
INSTRUMENTATION METHODS AND EQUIPMENT
For the generation of aerosol three pyrotechnic compounds were used (Table 1).
* The percentage of AgJ in combustion products (here and below the data of a thermodynamic calculation with the use of
program  are given).
||Compound No. 1
||Compound No. 2
||Compound No. 3
||KJ 12 % ( 8.79 % J )
||NH4JO3 35% (21.6% J)
||C7O2H5J, n-Iodobenzoic acid 18% (8.44% J)
||KJ 12% (Coincides with the iodizating admixture)
||Amorphous boron 5 %
|AgJ in combustion products*
|Soluble components in aerosol
||8.67% (5.65% of compounds containing J-)
||19.05 % (No soluble J- compounds)
||No hygroscopic constituent
The compositions studied contain equal amounts of silver. They are calculated so that the AgJ content in the aerosol
formed should be almost equal. The composition of aerosols as to other components is cardinally different.
In the first case, the hygroscopic constituent of the combustion products is presented by different soluble components,
including those forming J- ions. In the second case there are no soluble components of J, in the third case there is no
hygroscopic constituent in the aerosol.
During the experiment, a 200-mg portion of a pyrotechnic compound was ignited in the air blown combustion chamber with a
profiled nozzle and a rupturing diaphragm (Figure 1). The computed pressure of the bursting compound is equal to 10 bar.
It provides sufficiently reproducible conditions of aerosol combustion and condensation for compounds with different
The experiment was made in the 12 m3 chamber with controlled temperature and humidity. The aerosol formed was
kept at 20oC during 100 minutes with periodically measured ice- forming activity and aerosol particles spectrum.
The measurements of aerosol ice-forming properties (characteristics) were made at minus 10oC in the climatic chamber
"Feutron 3001-01" containing a supercooled fog. For measurements of aerosol particles spectrum the spectrometers
"Electrostatic Classifier TSI 3080" and "Laser Aerosol Spectrometer (LAS) 3340" were used.
The variations of ice-forming activity with time at different humidity for the compounds under study are given in the graphs (Figure 2).
Figure 2. Dynamics of changes in aerosol ice-forming activity with time at 20% and 80% relative humidity.
When analyzing the information obtained, one should keep in mind that simultaneously with changing aerosol ice-forming
properties its spectrum changes as well. Nevertheless, even at this stage of the analysis a similar change of activity
for all the compounds and experimental conditions, except compound No.1 at 80%-humidity, should be noted.
The variation of aerosol spectra during the experiment is shown in Figure 3 (instrumentation data are in red, the
interpolation results – in black).
During the experiment aerosol undergoes significant changes: compound No.1 changed its modal sizes from 105 nm
(at the concentration of 2E+5 particles/cm3) to 124 nm (7.8E+2), compound No.2 from 91 nm (1.8E+5)
to 131 nm (7.4E+2), compound No.3 from 94 nm (1.5E+5) to 105 nm (2.2E+2). Thus, in the first two cases some integration
of particles took place, in the third case the shape of the spectrum remained the same.
In this situation for estimation of aerosol (S) ice nucleation properties it would be desirable to compare their
changes with those of aerosol spectrum. According to some theories, the ice nucleation properties are connected with
the presence of active (nucleation) sites, the number and activity of which are proportional to the particle surface area.
To estimate such changes the following relationship can be applied:
Where N is the total number of ice-forming active nuclei per volume unit at the time moment t;
S - is average coefficient density of active centers characterizing a nucleation event under the conditions set;
– is a mean diameter of an aerosol particles measured in the i-th range;
– is the number of particles per volume unit measured in the i-th range of sizes.
The calculation results of S variations with time (1) are given in the graph (Figure 4). From the graph it is seen that for
aerosols without soluble compounds of J the parameter S does not change with time and is constant at any humidity.
In other words, aerosol particles of this composition do not undergo ageing processes. Aerosols with soluble
J- compounds are an exception of this case. For them after 60 minutes of being in the medium with 80% humidity S sharply
Figure 4. Calculation results of S variation with time.
RESULTS AND DISCUSSION
Aerosol of composition compound No. 1 contains potassium iodide – a substance with high hygroscopicity, that absorbs water at the medium humidity higher than 60%. Based on the theories connecting nucleation centers with the existence of structural defects of an ice-forming compound, the removal of defects should result in a reduction of crystallization centers number or a drop of their activity. In its turn, the existence of a crystal with structural defects is less energetically advantageous as compared to a crystal with a regular crystal lattice, if the recrystallization process leading to the formation of a less active ice-forming active crystal is possible. The possibility of realization of this process significantly favors the presence of J- ions (or NH4+) in the particle hydrated shell due to the formation of soluble complexes of AgJõ1-x (or Ag(NH3)2+) having the transportation function. Thus, when differentiating the processes of ice-forming aerosol formation and its functioning in the medium, the following conditions should be fulfilled:
- At the thermocondensational way of aerosol formation in plasma the existence of excessive iodide content is necessary for blocking thermal decomposition and oxidizing of silver iodide;
- No J- should be present in an aerosol particle at the condition of its long existence in a medium with high humidity;
- An additional condition of a high ice-forming efficiency based on the mechanism of “condensation with subsequent freezing“ is in the presence of a hygroscopic component in the particle .
When developing pyrotechnic modification means, the problem of introduction of additional iodine and ensuring the availability of a hygroscopic component can be solved, for example, by the introduction of organic iodine-containing components and the substances with high hygroscopicity not containing iodine and not leading during combustion to the formation of soluble iodine complexes. In the case of liquid-fuelled generators, when the solution of AgJ is injected into the fuel, the use of soluble iodides as an iodizing admixture is a necessary condition. In the opposite case, the solution to be used cannot be obtained. Only NH4J can be the most promising iodide complex, that at combustion can be oxidized up to the formation of elementary iodine. But at its use one should take into account the following::
- Under a certain regime of combustion controlled by the ratio of an oxidizer and a deoxidizing agent, flame temperature, etc., complete combustion of the NH4 group and the transition of excessive iodine into the non-ionic form can be possible;
- The deviations from the preset regime will result in the penetration of iodine in ionic form into an aerosol particle and a rapid drop of aerosol ice-forming activity.
- In this case, the situation will be additionally more aggravated by the presence of ammonium ions also leading to the formation of silver complexes in the hygroscopic part as compared to cations of alkali metals being neutral in this connection..
- Shilin A.G., Drofa A.S., Ivanov V.N., Savchenko A.V., Shilin V.A. Experimental Studies of Silver Iodide Pyrotechnic Aerosol Ice Forming Efficiency Dynamics. 19th International Conference “Nucleation and Atmospheric Aerosols”. AIP Conf. Proceeding, New York, 2013, V 1527, p. 945-949.
- Cruise D. R., Theoretical Computations of Equilibrium Composition. Thermodynamic Properties, and Performance Characteristics of Propellant Systems. NWC TP 6037, Naval Weapons Center, China Lake, CA 93555-6001, 1991.
- Isono K., Ishizaka Y. On ice nucleating properties of different faces of silver iodide crystals.// J.Recher.Atmos., 1986, v.3, N 1-2, p.139-140.
- Laaksonen A., A Combined Theory of Heterogeneous Nucleation and Adsorption of Vapors on Solid Surfaces. 19th International Conference “Nucleation and Atmospheric Aerosols”. AIP Conf. Proceeding, New York, 2013, V 1527, p. 270-273.