the effect of impurities on the surface structure of evaporating i ce

J ournal of GlaciologYJ Vol. 10 , No. 59 , 197 1
THE EFFECT OF IMPURITIES ON THE S U RFACE
STRUCTURE OF EVAPORATING I CE
By
J. D.
CROSS*
(Inland Waters Branch , D epartm ent of Energy, Mines and R esources, Ottawa, Ontario,
Canada)
ABSTRACT . l\tlicrograph s of the surface of evaporating polycrysta llin e ice contain ing impurities a re
presented. Th e mi crogra phs show that impurities t hat en ter the ice lattice substitutiona Uy enh ance the
fin e structure o n th e surface, but impurities rejected by the la tti ce su ppress th e fine stru cture. I t is concl u ded
th at the observed fin e stru cture is not an artifact of th e machin e but a real p roperty of eva porating ice
dependent u pon the defects at the surface.
R ESUME. L'effet des impuretis sur la structure sllperjicielle de la glace ell cours d'evaporatioTl . D es micrographies
de la surface d ' un e glace polycrista llin e con tenant des impuretes son t presentees. L es m icrogra phi es mon o'em
que les impuretes qui penetrent d a ns le reseau cristallin de la glace favorise par compensation une structure
d e li ee en surface, mais que le rejet d es impu retes par le resea u crista lli n supprime cette struc ture fin e.
On co nclut q ue la structure deli ee observee n'est p as un artefact de !'appareilla ge ; il s'agit d ' une verita bl e
proprie te d e la surface en COUl'S d 'evapora tion sous la dependance d es defa uts exista nts en surface.
ZUSAMMENFASSUNG. Der Eilljluss VOIl Verschmut zwlgell my die Oberjliichenstruktur VOIl verdullstelldem Eis. Es
werden Mikroaufnahmen d er OberA ach e von verdu nstendem polykristallinem Eis mit Verschmutzungen
vorgel egt. Die Mikroa ufn ahmen zeigen, d ass Versch m utzungen, die in d as Eisgefllge eindringen, d ie
F einstruktur a uf d el' OberAach e wesentlich versta rken, dass dagegen Verschmutzungen, die vom Gefuge
a bgewiesen werde n, di e F einstruktur unterdru cken . Es wird geschl ossen, d ass die b eobachtete Feinst ruktur
nicht ein kunstlich es Ergebnis d er Appa ra tur ist ; sie ist vielmehr ein e sp ezifisch e Eigensch a ft verdunstend en
Eises, die von d en Defekten an d er OberAach e a bh a ngt.
I NTRODUCTION
R ecent studies of polycrystallin e ice surfaces with a scanning elec tron mi croscope
(Odencrantz and others, 1968; Cross, 1969 [a] , [b] ) have revealed a com pl ex surface structure
containing fin e filaments less than one micrometre thick . Such surface structure could
contribute to the electrification and nucleation processes in clouds (Cross and Speare, ' 969;
Ruskin , '969) . The existen ce of a highly disord ered surface laye r with a large concentration
of d efects has been indicated by d eformation studies (Muguruma, 1969 ), a nd J accard ( 1967 )
has reported that evaporation changes the electrical conductivity of ice, probably by producing d efects at the surface. Theoretical and experim ental studies of ice evaporating in a
controlled environm ent have been carried out by Gukhman and V olynets (1968 ) and ovikov
and Vagner ( 1968) . These studies d eal with the heat and mass transfer conditions for subliming
polycrystalline ice in the pressure range 10- ; to 10 5 N m - 2. The mass loss was found to be mainly
controlled by the radiant h eat flux to the specimen rather than by conduction and convection.
The evaporation was not confined to the geometric surface of the ice but occu rred over an
extended region , producing a rough surface. These repor ts support th e supposition that the
observed structure is a natura l structure for pure polyc rys talline ice related to the disordered
co nditions at the surface, and is not a n artifact of the mi croscope. If water containing ionic
impurities is frozen, the impuri ty ions are not generally incorporated into the ice lattice but
acc umulate at grain bo undaries. An1moni um and fluorid e ions are an exception. In small
concentrations these ions are incorporated substi tutionall y into the ice lattice and create
res pectively D and L Bj errum d efects. If the surface tr ucture of evaporating polycrystalline
ice is not an artifact of the microscope it should be sen itive to the a ddition of impurities to the
water forming the ice a nd a different behaviou r is to be exp ected in the influ ence of impurities
rej ected by the ice and those in corporated in the lattice. This investigati on was carried ou t in
ord er to check th e effect of impu rities on the surface tructure.
* P resen t a ddress : D epa rtm ent of E lectrica l Engin eering, Uni versit y of \\'a terl oo, \\'aterl oo, Onta ri o,
Canad a.
288
JO
RNAL
OF
GLACIOLOGY
EXPERIMENTAL PRO C ED U R E
All water used in this work was obtained from a commercial still producing water with a
resistivity of6 MQ cm. This distilled water will be referred to as pure water. Dilute solutions
of the following compounds were made with the pure water: NH 4 0H, HF, NH 4 Gl , NaG I,
and H G!. Polycrystalline ice samples were prepared from the solutions by attaching a drople t
(approximately 2 mm in diameter) to a fine glass fibre mounted on the standard scanning
electron microscop e specimen stub (Cross and Cross, 1968 ) and then freezing th e droplet at
- 50°C. The ice samples were then examined directly in the microscope without the use of
replicas. A frozen droplet in the microscope takes roughly thirty minutes to evaporate to the
point at which it falls from the fibre and is lost. Several sampl es of each type of ice were
studied and the micrographs presented here were chosen as being representative of the
appearance of each type of ice.
Fig. I (top left). The surface of pure /Jolycrystalline ice, ajier 10 lIlin evaporation. Instrument magnification X 2 200.
Fig . 2a (top right ). N H. OH doped ice, immediately after insertioll in the microscope. I llstrument magnification X 2700 .
Fig . 2b (bottom left ). N H. OH doped ice, after 10 min eva/Joration . Instrument magnification X 2600 .
Fig . 2C ( bottom right) . N H. OH doped ice, after 15 mill evaporation. Instrument magnification X 2600.
EFF ECT OF IMP U RITIE S ON THE
SU RFA CE ST R UCTU R E
28 9
R ESULTS
A typical micrograph of pure ice after ten m inu te in the m icro cope is hown in Figure I.
The surface has the fine tr ucture reported previously. Figure 2 show three micrographs
typical of dilute solutions of NH 40H. Concentrations of 10 part per million and 100 parts
p er million were studied and no significant difference was seen between them. Soon after
in ertion in the microscope the surface had crystalline features as shown in Figure 2a. As
evaporation proceed ed the comp lex structures shown in Figure 2b and c d eveloped.
Dilute solu tions of HF behaved somewhat simi larly, to the N H .OH solutions. Initiall y
the surface had a marked crystalline appearance a in F igure 3a and as the evaporation
Fig. 3a ( top left ). HF doped ice, immediately after insertion in the microscope. Instrument magnification X 700.
Fig. 3b ( top right ). HF doped ice, 10 p.p .m. after 10 min evaporation . Instrument magnification X 2 300 .
Fig. 3C ( bottom left). HF doped ice, 100 p.p .m. after 10 min evaporation. Instrument magnification X 2 300.
Fig. 4a ( bottom right). N H.CL doped ice, immediately after insertion in the microscope. Instrument magnification X 250.
JO U R
AL OF GLAC I OLOGY
proceed ed a com plex surface structure d eveloped but the structure varied with the concentration of HF. With a concentration o f 10 parts per million the a ppearance was a lmost indisting uishable from the NH 40H solu tions, as shown in Figure 3b. With 100 parts per
million of HF the surface structure was lower a nd less delicate as shown in Figu re 3c.
I ce mad e fmm dilute solu tions of NH 4 C l is initia lly crys talline in a ppearan ce as in
Figure 4a and evaporation reveal an extremely compl ex surface stru cture as in Figure 4b
a nd c. In creas ing the concen tration of NH 4C l from 10 parts per million to 100 parts per
mi llion produced a slightly more compact surface structure as in Figure 4G.
Fig. 4b ( top lift ) . N H. CI doped ice, 10 p.p.m. after 15 min evaporation. Instrument magnijication X J 200 .
Fig. 4c ( top right) . N H . CI doped ice, l OO p.p.7I!. after J 5 ",in evaporation . Instrument magnijication X 2 500.
Fig . .5a ( bottom teft) . H Ct doped ice 4 p.p.m. after 15 min evaporation. I nstrument magnijication X I 300.
Fig . 5b ( bottom right ) . H Ct doped ice J O p.p.m . aft er 10 min evaporation . I nstrument magnijication X 880.
EFFECT
OF
IMP U RITIE S ON
T HE
SU RFA CE
STRUCTURE
l ee doped with 4 parts per m illion and 40 parts per million of HCI was exam ined and all
'peeimens had a crystallin e appearance as shown in Figure 5a and b. No fibrou s surface
stru ctures were seen on any of th e H CI doped amples. A similar result was obtained with
ice cont aining 10 parts per million of ~aCI. A crystalline appearan ce as shown in Figure 6a
was typical of this ice and in som e cases at grain boundaries small spheroids of what appeared
10 be rej ec ted impurities co uld be seen a in Figure 6b.
F ig. 6a ( lift ). _VaGI doped ice 10 p.p.m . afler 10 min eva/)oralion. InslrlllneIlI magnificalion X J 300.
Fig . 6b (righl ) . NaGI doped ice 10 P./).III. ,uler 10 lIlin eva/)oTalion. IllslTwllenl magnificalion x 880.
DI SCL"SS ION
The micrographs show clearly that th e presence of small quantities of impurities modifies
the surface structure. Impurities that can be incorporated into th e lattice enhance the
urface structure as shown in Figures 2 , 3 and 4. Impurities that are rejected by th e ice
suppl'ess th e fibrous surface compl etely stru cture . Ammon ium ions and fluoride ions are
only incorporated in th e latti ce in small quantities, therefore when water containing 100
parts per million of NH 4 0H , HF or NH 4 CI is froz en som e of th ese impurities are rej ec ted
and acc umulate at the grain boundaries. In th e ca se of N H 4 0H the impurity is more volatil e
than the ice, and during evaporation th e concentration of impurity at grain boundari es will
r emain small. In th e case of HF and NH 4 C I evaporation will result in an accumulation of
impuriti es and th e surface stru cture in these ca es is less prominent. In the case of ~a C I and
HCI doping, the impuriti es are rejected and no surface tructure d evelops. The concl usion
to be drawn from these re ults is that the fibrou lII·face st ructure observed on evaporating
pol yerys talline ice is in some way a result of the large number of defects pre ent and it is
throug h the extra d efects introduced by the ions that enter the lattice subs titutio nall y tha t
th ese ion enhan ce the comp lex surface stru cture. Th e fine structure can not be attributed to
an artifact of the microscope and it is po sible that such a slrUCLUre may exist in ice evaporati ng
naLUraJl y.
AC K NOWLEDGEMENTS
The author wishes to thank Dr T. ~ a k aruma of the D epartment of En ergy, Mines and
R e ources for prov iding the solutions used in this work and the D epartment of AO'riculture
for use of the scanning elec tron microscope .
.\15. received ' 3 i\Iay 1970
JO U RNAL OF GLACIOLOGY
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