Bv. Lvov et Av. Novichikhin, QUANTITATIVE INTERPRETATION OF THE EVAPORATION COEFFICIENTS FOR THE DECOMPOSITION OR SUBLIMATION OF SOME SUBSTANCES IN-VACUO, Thermochimica acta, 290(2), 1997, pp. 239-251
An attempt is made at a quantitative interpretation of the low evapora
tion coefficients alpha (10(-7)-10(-1)) observed under thermal decompo
sition of 12 different compounds (AgNO3, BaSO4, BaCO3, BN, Be3N2, Mg3N
2, AlN, GaN, ZnO, CdS, CdSe and NH4Cl) in vacuo under the conditions o
f free-surface evaporation after Langmuir. It is suggested that the ma
in reason for the low evaporation coefficients lies in the difference
between the true scheme of thermal decomposition of a given compound a
nd in assuming its direct decomposition to the products corresponding
to thermodynamic equilibrium. These differences consist, firstly, in a
primary gasification of all decomposition products, including low-vol
atile components (metals and metal oxides) and, secondly, in a partial
or total evolution of nitrogen, oxygen, sulfur, selenium and hydrogen
chloride in the form of free atoms or H + Cl mixture rather than of m
olecules. The interpretation of the evaporation coefficients a for ele
mental red phosphorus, arsenic and antimony is based on the assumption
that the sublimation of these substances occurs, accordingly, through
a primary formation of P-2 molecules, or of As-2 + As-4 and Sb-2 + Sb
-4 mixtures, rather than by direct formation of the P4, As-4, or Sb-4
molecules corresponding to thermodynamic equilibrium, as is generally
accepted. The proposed mechanisms of decomposition or sublimation are
validated by a comparison of the experimental and calculated activatio
n energies. (C) 1997 Elsevier Science B.V.