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The crystallisation of strontium and barium molybdates and tungstates from lithium chloride melts by continuous cooling. The kinetics of crystal growth in alumina crucibles
Author(s) -
Packter A.,
Roy B. N.
Publication year - 1975
Publication title -
kristall und technik
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.377
H-Index - 64
eISSN - 1521-4079
pISSN - 0023-4753
DOI - 10.1002/crat.19750100407
Subject(s) - molybdate , crystal growth , barium , crystallite , crystal (programming language) , crystallization , tungstate , analytical chemistry (journal) , diffusion , strontium , materials science , mineralogy , chemistry , crystallography , inorganic chemistry , thermodynamics , chromatography , physics , organic chemistry , computer science , programming language
Strontium and barium molybdate tungstate solutions in lithium chloride melts were crystallised in alumina crucibles; saturated solutions were cooled from initial temperature T 0 = 800 °C at cooling rates R T = 40°, 100° and 200°C hr −1 . The kinetics of the crystal growth were studied by chemical analysis of the solution and by optical microscopy measurements of the crystals. – Heterogeneous nuclei first grew to small crystallites (lenghts l from about 0.02 to 0.12 cm) during prolonged induction periods: the main crystal growth then started after the development of some excess solute concentration Δ C = 0.3 Δ C tot to Δ C tot (where Δ C tot was the total excess solute concentration that developed on cooling to the eutectic temperature) at temperature T . The main crystal growth, at grown times τ = 0 to τ*, was diffusion rate‐controlled. The average crystal lenghts generally varied with growth time according to the relation,where R c (= Å T ) is the rate of development of excess solute concentration.k Δ is the diffusion rate constant at temperatures near TK is ϱf N (where N is the crystal number): for the first forty percent crystallisation, this relation simplified toThe intial growth rates and l τ 2 /τ ratios depended not on cooling rates but on the Δ C values (at the start of growth). The kinetics of crystallisation of barium molybdate (where Δ C = Δ C tot were similar to those for crystallisation from supersaturated solutions onto seeds at temperature T ; for these crystallisationswhere I D (α) is the Nielsen function for diffusion‐controlled growth. The last thirty percent growth was rate‐controlled by the rate of development of excess solute concentration only; in this range, crystal lenghts varied with growth time according to the relation,