ACADMEICIAN LUKIRSKIY'S DISCOVERY OF IMPORTANT SURFACE TENSION EFFECTS

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP80-00809A000600350591-7
• Release Decision: RIPPUB
• Original Classification: C
• Document Page Count: 3
• Document Creation Date: December 22, 2016
• Document Release Date: August 22, 2011
• Sequence Number: 591
• Publication Date: October 24, 1950
• Content Type: REPORT

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nn G uNTIAI.
CENTRAL INTELLIGENCE AGENCY REPORT
INFORMATION FROM
FOREIGN DOCUMENTS OR RADIO BROADCASTS
COUNTRY USSR
SUBJECT Scientific .. Physics, surface tension
HOW
PUBLISHED .Monthly periodical
WHERE
PUBLISHED Moscow
DATE
PUBLISHED Jun 1950
LANGUAGE Russian
TNI/ DOCO NIN? CONTAINI INFORMATION AI/RVINI TNI NATIONAL DI-INII
A, I. C., II AND It, Al ANINDID. TIITI TNANINIOIION ON MIAIIYIIATION
AN
I
IIINODOCTION OF TNI0 CONY II P10NIIIT00. II !AO.
NT TLAN, IN ART
SO(RCE Priroda, No 6, 1950, pp 58-60.
CD NO.
DATE OF
DATE DIST. Oct 1950
NO. OF PAGES 3
SUPPLEMENT TO
REPORT NO.
THIS IS UNEVALUATED INFORMATION
ACADEMICIAN LUKIRBKIY?B DISCOVERY
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;n several experiments with acicular monocrystals, it has been noted that
the form of the point changes when these ends are heated to quite high tempera-
tures (below the melting point) however). The supposition was that surface ten-
sion forces were responsible for this change. Since surface tension of mono-
crystals depenp* on crystallographic orientation, we should expect'that the
monocrystal surice would not return from the pointed to a regular spherical
surface.
Academician P. I. Lukirskiy (Doklady Akademii Nauk SSSR, Vol XLVI, 300,
1945) set up a number of experiments to clarify this problem. The experiments
were made on quite large monocrystals. Small spheres with diameters ranging
from one to several centimeters were cut from rock salt monocrystals, These
were then carefully polished, and heated in a furnace for several hours at vari-
ous temperatures (for different tests) within the interval 720-7600 C. Study
of the monocrystals after.heating established that the monocrystallic spheres
took on the form of an irregular hexoctahedron (48 faces) after each such ex-
periment. This effect was obtained after any number of tests with a given
sphere; if the polyhedron obtained was again polished and heated, the same
.figure was reproduced. A slight difference of the lengths of the crystallic
axes was observed in the figure obtained from the sphere. For an initial
sphere diameter of several centimeters, this difference was only of the order
of 10 microns. It was also established that all edges and vertices of the
polyhedron obtained were slightly rounded off, so that the transition from
one surface area to another was relatively smooth, The phenomenon discovered
by P. I. Lukirskiy was explained by the action of surface tension forces.
It was noted above that surface tension in monocrystals depends on the
ery.tallographic Briemtetien selected. Consequently, surface tension of a
monocryetallic sphere will not be the same at every point of its surface. It
is further known that if ataoq of the crystal could move freely, as in the
case of a liquid, for exanple,',they would try to occupy a position at which
surface energy would be minimum. By keeping the temperature of the crystal
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high, we make po: ..ble the freest and most intensive displacement of its atoms,
but nol to the degree that holds for a liquid. Because of complete isotropy in
liquid , the equilibrium figure is a sphere; but other figures represent equili-
brium for crystals, Undoubtedly, the hexoctahedron of the rock salt crystal ob-
?nl.ned is more "in equilibrium" than a sphere of the same monocrystal. However,
as Lu] rskiy pointed out, there is till a possibility that the figure obtained
i5 only intermediate and that a final form ev?n more in equilibrium could be ob-
tained.. This might occur if there were several solutions to this problem of the
free energy minimum, i.e., if several equilibrium forms existed. The last sup-
position, however, has not been confirmed as yet; despite prolonged heating, no
f:;rther change in form was obtained,
With regard to the mechanism governing the formation of equilibrial fig-
ures, P 1. Lukirskiy noted that this ferm.tion might come about through re-
peated eva~pration and condensation or as a result of "creep" of surface atoms.
The latter is most probable, since in this case atoms with lower energies could
participate in the process.
A very curious phenomena -- straightening out or leveling of the surface --
should take place if creep of atoms is the correct hypothesis. if some face of
a ciyetal is made rough, it should become smooth and shiny after being heated
for several hours at temperatures providing intensive creep. P. I. Lukirskiy?s
experiments showed that this phenomena actually occurred,
The results obtained by P. I. Lukirskiy caused S. V. Starodubtsev and
N. I. Timokhina (DAN, Vol LXII, p 619, 1948) to study sintering of monocrys-
talli"c powders, where surface tension is assumed to be.of great importance.
This assumption seems quite plausible; the role of surface tension in monocrys-
tallic powders should be considerably greater than in the case of large mono-
crystals, since the surface in the first case is much larger for identical vol-
imie.
Crystallic powders sinter when the packed powder is heated at a temperature
somewhat below the melting point, i.e., the same conditions under which Lukir-
skiy investigated rock salt monocrystals. Starodubtsev and Timokhina investi-
gated sintering of/ powders consisting of rock salt crystals (NaCl) and powders
containing KI crystals. The powders were prepared in the following way: The
salt monocrystals yere crushed an4,then divided into separate fractions with
respect to the size of the crystals. The fraction consisting of particles
whose linear dimensions varied from 0.1 to 0.075 cm was selected for the ex-
periments. These particles were examined both before and after the experi-
ment in a microscope and were subjected to microphotography when' necessary.
In the experiment itself, the particles were placed in crucibles and heated
for 2 hours at 5000 C. The sintered specimen was then again destroyed by ap-
plication of the proper mechanical load. In the powder thus obtained, there
were a great many crystals which had grown together fsrostok," literally,
clustepj ,
Inspection of the particles which had grown together revealed that they
were connected with each other by quite narrow bridges, having a characteris-
tic circular cross section. The width of the bridges was several times less
an the dimensions of the particles themselves. This held for particles of
k.C1 crystals and KI particles, In studying the different connecting bridges,
Starodubtsev and Timokhina discovered "embryonic clusters," having the form of
thin threads from 300 to 20 microns long, among the couples destroyed by the
application of a mechanical load. The smallest diameter of such Aservable
,ousters was 20 microns.
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The same authors used another variation of the experimert to study sinter-
ing at the very beginning of the process. A small NaC1 crystal was placed on
one of the planes of a rock salt crystal. The gap between the parallel faces
was 20 microns. Both crystal.- were placed can a quartz elate, which was heated
at 6000 C, Study of the gap during heating revealed that cone shaped projec-
tions grew from the face of one crystal to the face of the others. Then thread-
shaped clusters, which changed into connecting bridges, formed from these cone-
shaped projections, It was found that the rate of formation of the clusters
was temperature-dependent; and was 3 microns/hr at 6000 for a gap of 20 microns.
The experiments also showed that the thread-like clusters frequently do not con-
r:.ect the crystals along the shortest distance. The direction of thread egress
depends on crystallographic orientation.
In connectiea with these experiments, we cite some considerations on the
mechanism of the sintering process which were borrowed from a book by Ya. I.
rr?enkel'' (Introduction to the Theory efMetal-, Gostekhi.zdat, Moscow/Leningrad,
19ES1. If*all_crystais of a powder were instantaneously melted and transformed
into drops, all drops would flow together because of the action of surface ten-
ior forces. Crystals, just as liquids, although to a considerably lesser deg,
,aree, have fluidity, provided either by plastic deformation or diffusion flow.
"Acr he drawn
We can therefore expect that the particles of a uhtystallic pow.-
together in one mass, a unique conglomerate, at quite high temperatures, when
the fluidity of the crystals becomes more noticeable: Of course, considerably
more time is required than in the case of liquid drops, since the viscosity of
crystals even at high temperatures is considerably greater than the viscosity
of liquid drops. If, for example, the viscosity of heated crystals iz 1010
times that of drops, sintering will be 1010 times slower than fusing of drops.
Considering the time for fusing of drops to be i0'.5 seconds, we obtain a rate
of growth of. the connecting bridges between drops of the order of 5.106 microns
sec; or:~2?lO10 microns/hr for gaps of the order of 50 microns. This figure would
yield a'rate of growth of the connecting bridges between crystals of the order
of 2 microns/hr, which is close to the value obtained in Starodubtsev and Timok-
hina's experiments.
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