Effect of Supersaturation and Temperature on the Growth Morphology of Ammonium Oxalate Monohydrate Crystals Obtained from Aqueous Solutions

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The experimental results of the effect of concentration of Mn(II) ions on the growth kinetics of different faces of ammonium oxalate monohydrate single crystals from aqueous solutions at a constant temperature and different predefined supersaturations are described and discussed. It was observed that: (1) at a given supersaturation σ, Mn(II) ions lead to a decrease in the growth rates of different faces of AO crystals, (2) the growth of a particular face of the crystals occurs above a critical supersaturation σ d but there is also another supersaturation barrier σ* when the rate abruptly increases with σ, (3) the values of σ d and σ* increase with increasing concentration of the impurity, and (4) the values of σ d depend on the growth kinetics of a face but those of σ* are independent of face growth kinetics. The experimental R(σ) data for different Mn(II) concentrations c i were analysed according to the model involving complex source of cooperating screw dislocations and concepts of instantaneous and time-dependent impurity adsorption. It was found that: (1) for a given face the differential heat of adsorption Q diff is higher during instantaneous impurity adsorption than that during time-dependent adsorption, and (2) the values of Q diff involved during instantaneous adsorption are related with face growth kinetics but those during time-dependent adsorption are independent of face growth kinetics.

Section: Introductionmentioning

confidence: 99%

Effect of Mn(II) ions on the growth of ammonium oxalate monohydrate crystals from aqueous solutions: II. Growth kinetics

Sangwal¹,

Mielniczek‐Brzóska²

2003

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“…At low supersaturations the steps of the macrospirals on the as-grown faces of the crystals grown both from solutions without impurities and in the presence of the impurities were oriented along low-index crystallographic directions, i.e. the macrospirals had a tendency to be polygonal [2,6]. With increasing supersaturation the macrospirals become rounded and better developed [6].…”

Section: Introductionmentioning

confidence: 96%

“…It was observed [2] that most of the faces appearing in the morphology are F faces and that macrospirals and growth hillocks appear on these faces. The authors also investigated [3][4][5][6] the effect of Cu(II), Fe(III) and Cr(III) ions on the growth of different faces of AO crystals.…”

Section: Introductionmentioning

confidence: 97%

“…It was observed [3,5,6] that all faces, composing in the growth morphology of the crystals grown from pure aqueous solutions, appear in the growth morphology in the presence of the above impurities. Moreover, as in the case of growth from aqueous solutions without impurities [2], macroscopic growth spirals are formed on the surfaces of AO crystals in the presence of Fe(III) [5] and Cr(III) impurities [6]. At low supersaturations the steps of the macrospirals on the as-grown faces of the crystals grown both from solutions without impurities and in the presence of the impurities were oriented along low-index crystallographic directions, i.e.…”

Section: Introductionmentioning

confidence: 97%

“…The authors investigated the growth kinetics [1] and growth morphology [2] of ammonium oxalate monohydrate [(NH 4 ) 2 C 2 O 4 ⋅H 2 O; AO] crystals from "pure'' aqueous solutions in relation to solution supersaturation and temperature. It was observed [2] that most of the faces appearing in the morphology are F faces and that macrospirals and growth hillocks appear on these faces.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Effect of Mn(II) ions on the growth of ammonium oxalate monohydrate crystals from aqueous solutions: I. Growth habit and surface morphology

Sangwal

Mielniczek‐Brzóska²,

Borc

2003

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The effect of concentration of Mn(II) ions on the growth habit and the surface micromorphology of different as-grown faces of ammonium oxalate monohydrate (AO) single crystals grown from aqueous solutions was studied at a constant temperature of 30°C and predefined supersaturations up to 20%. It was observed that the growth habit and the surface morphology of the crystals strongly depend on the supersaturation used for growth and the impurity concentration in the solution. The experimental results were analysed in terms of connected nets determined from different projections of the structure of AO crystals. Analysis of the observations revealed that: (1) the directions of connected nets corresponding to basic growth units composed of single (NH 4 ) 2 C 2 O 4 ⋅H 2 O molecules are in excellent agreement with the low-index crystallographic directions of the orientations of growth layers, (2) all faces appearing in the growth morphology of AO crystals are F faces, and (3) the {001} face growing from pure aqueous solutions is essentially a kinetically rough face but the presence of Mn(II) impurity leads to their appearance in the morphology due to increase in the strength of bonds of the connected nets composing the surface graph.

On the Effect of Cu(II) Impurity on the Growth Kinetics of Ammonium Oxalate Monohydrate Crystals from Aqueous Solutions

Sangwal¹,

Mielniczek‐Brzóska²

2001

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The effect of concentration of Cu(II) ions and solution supersaturation on growth rates of different faces of ammonium oxalate monohydrate (AO) single crystals is studied at 30 °C and supersaturations up to 10%. It was observed that the growth rates of the (001) face of the crystals decrease while those of the (100) and (010) faces increase with an increase in the concentration of the impurity. Instead of analysing the dependence of face growth rates R on impurity concentration ci for different supersaturations σ in terms of Cabrera‐Vermilyea and Kubota‐Mullin models, the dependence of R on σ for different ci were analysed using spiral growth theory involving cooperating dislocation source and multiple nucleation model. It was found that both models describe the experimental data on growth kinetics satisfactorily. Analysis of the data revealed that an increase in the face growth rate with impurity concentration is associated with a decrease in the free energy of the growing face (thermodynamic effect) while a decrease in face growth rate with an increase in the concentration of the impurity is a kinetic effect involving a reduction in the value of the kinetic coefficient β for the motion of steps on the surface. It was also found that usually both effects take place simultaneously but one of them dominates during the growth of a face. The dependences of surface free energy and kinetic coefficient on impurity concentration follow Freundlich adsorption isotherm.