Izvestia Academia Of Science USSR
1981, v.17, N 3, p. 466 -469 . ( offprint rus.)
For the processing of metallurgical and blast-furnace slugs, as well as the production of cement and stone casting, crystallization processes in silicate melts are of great importance. A number of papers [1-8] have been published in which the question of the relationship between the rate and mechanism of crystal growth with the composition of the melt is investigated. Such studies require the attainment of high temperatures (1070-2070 K.), which significantly increases the difficulty of observing the phenomena of crystallization.
In , the maximum values of the linear crystal growth rates were measured. For ≈ 100 melts of the CaO -Al2O3-SiO2 system, an attempt was made to interpret the results obtained from the point of view of the theory of continuous growth on the basis of the formula 
v = K exp (-ED / RT) [1-exp (-ΔHΔT /RTLT)] (1)
where K is a constant (with respect to temperature), ED is the activation energy of diffusion, ΔH is the melting enthalpy, and ΔT = TL-T is supercooling. This formula, but the opinion of many authors, comparatively well describes the dependence of the growth rate on temperature. Since a direct experimental determination of ED for silicate melts is difficult, it is of interest to estimate it from the data on the dependence of lg (v) on 1 / T.
Based on these considerations, a study was made of the nonequilibrium crystallization in the pseudo-binary system of the gehlenite-dicalcium silicate (Ca2Al2SiO7-Ca2SiO4) (later on in abbreviated notation: C2AS - C2S) in the encompassing region of compositions of significance For the production of cements and slag-stone. The main results of the work were Data on the linear growth rates and induction periods of nucleation from supercooling, as well as on the growth patterns of crystals. As methods of investigation, high-temperature microscopy and X-ray phase analysis of samples obtained using a specially designed low-inertia high-temperature furnace with operating temperature up to 2470 K were used.
High-temperature microscopy was used to study six compositions with liquidus temperatures below 1920 K, since, taking into account the need for overheating of the melts by ≈ 100 K, this was the limiting temperature for the Pt + 30% Rh alloy heater. Fig.1 presents nucleation periods as a function of temperature. The nucleation was observed in volume of the melt as well on the surface of the heater and melt.
In the region of 10-30% of C2S, two different types of crystalline formations may appear and grow, each of them with the own temperature of melting and different forms of growth at the same temperatures for the same composition. Heterogeneous nucleation on the surface of the heater causes the appearance of crystals with the leading phase of βH-C2S, and then crystals C2AS may appear. With homogeneous nucleation, which occurs at a lower temperature than the heterogeneous one, gehlenite crystals show up in volume. During the transition through the eutectic composition (~ 34% C2S), crystalline formations with the leading phase βH-C2S were generated in the C2S field.
X-ray phase analysis showed that when crystals grew with the leading phase of C2AS, the crystalline formations also contained βH-C2S.
Fig.1 Dependence of induction time of nucleation from temperature in the system
gehlenite - calcium disilicate. Melt composition: a -0, b - 10, c -
20, d - 30, e - 35, f -45, (weight % of C2S).
1 - heterogenic, 2 - homogeny nucleation..
Fig. 2 Dependence of linear growth rate (μm/s) of crystal
from temperature. Melt composition: a -0, b - 10, c - 20, d - 30, e - 35, f -45,
(weight % of C2S). Leading phase: 1: 1 - C2AS,
2 - βH-C2S.
The intensities of the characteristic peaks of both phases were approximately the same for the same composition of the initial melt. Hence the conclusion was made that crystalline formations of the stoichiometric composition grow in the melt, which are formed by the mechanism of cooperative eutectic crystallization .
Cooling of the samples containing crystalline formations with the leading phase of βH-C2S was accompanied by a polymorphic transition to the γ-form of dicalcium silicate at 373-673 K, the more active the larger the content of βH-C2S in the sample. Quenching in distilled water managed to prevent this transition for all compositions, except for pure C2S.
Knowing the temperature regimes of the appearance of two kinds of crystalline formations, it was possible to measure the dependence of the linear growth rates on temperature. The results of the measurement are shown in Fig. 2. The temperature of "equilibrium" of βH-C2S crystals with a melt in the field of gehlenite, about 1650 K, sharply increases, beginning with the composition of 30% C2S. This temperature is much lower than the melting point of the eutectic, which according to the literature  is 1818 K (supercooling phenomenon).
From Fig. 2, then for large supercooling, when the term in square brackets in equation (1) is close to one, in the first approximation a simple exponential law is valid. The table gives data on the effective energies of diffusion activation. For those temperature regions where the dependence of lg v on 1 / T asymptotically approaches a straight line.
|Fig.3 . Growth of gehlenite (sphere) and calcium disilicate (dendrite) in the melt of 30% C2S under 1520 K. Scale 70:1.||Fig.4 Growth of calcium disilicate in melt of in the melt of 30% C2S under 1620 K. Scale 130:1.|
There are three regions of supercooling, for which various mechanisms of the surface reaction are realized and, accordingly, different crystal growth forms . At low supercooling, the mechanism of layer-by-layer growth is realized, as a result of which good cut crystals grow. With increasing supercooling after the transition region, the mechanism of continuous growth is realized, when on the surface of the crystals all points are accessible for embedding new structural elements.
|Content, %||Leading phase||T, K||Activation energy, kcal/mol|
|100||0||C2AS||1623 - 1423||68|
|90||10||C2AS||1523 - 1373||64|
|80||20||C2AS||1473 - 1373||73|
|70||30||C2AS||1473 - 1323||87|
|65||35||βH-C2S||1423 - 1123||66|
|55||45||βH-C2S||1523 - 1373||54|
|55||45||βH-C2S||1373 - 1273||108|
Observations with the aid of a high-temperature microscope showed that for gehlenite crystals, the layered growth region extends to supercooling of ≈30-50 K, after which the crystal edges begin to round off and, starting from ΔT ≈ 150 K, the growth of the spheres occurs (Fig. 3). Even for the smallest supercooling (≈5 K) achievable with of a high-temperature microscope the layered growth of faceted dicalcium silicate crystals was not observed. The main elements of the crystalline formations were spheres with a diameter of 10-20 μm, which had a tangential connection with each other (Fig.4) and formed more or less symmetrical structures.
Such a growth mechanism is probably due to the fact that the dependence of the growth rate on the radius of curvature of the surface has a sharp maximum at rmax <5 μm, and for some reason, not clear for the time being , needles do not grow. Such a situation should lead to the observed pattern, when after a random perturbation of the growth of the surface with the formation of a protuberance with a radius greater than the critical one, it rapidly grows to rmax, and then the growth slows down sharply. The presence of a certain symmetry is caused by the fact that the origin of a new sphere is most likely in the direction of the faces with large crystallographic indices, since there the velocity of the surface reaction is maximal. In large supercooling, dicalcium silicate grew in the form of dendrites.
Data on linear growth rates and induction periods of crystal nucleation as a function of temperature and composition in a pseudo-binary system of gelenite-dicalcium silicate are presented. A metastable growth region of dicalcium silicate crystals in a gehlenite field with a melting point lower than the melting point of the eutectic (supercooling phenomenon) is observed.
Institute of Chemistry of Silicates named I.V.Grebenschikova
Academy of Sciences of the USSR
Received by the Editor 19.III.1980.