Growth Rates Of Different Faces Research Articles

Growing of SixGe1-xO2single crystals with α-quartz structure and their characterization

Crystals of α-Si x Ge 1-x O 2 solid solution is a new material with relatively high piezoelectric coefficients and temperature stability. Theoretical and experimental study on the development of reliable and reproducible method of α-Si x Ge 1-x O 2 crystal growth under hydrothermal conditions will be proposed in the report. Influence of the composition of solutions, T-P parameters, growth rates of different faces, capture and distribution of Si and Ge on morphology and internal structure of the crystals, and the basic crystallochemical characteristics and properties of the crystals have been investigated. The most important result of the performed investigations is an experimental proof of the growth of α-Si x Ge 1-x O 2 single crystals with α-quartz structure under conditions of the stable existence of trigonal quartz and tetragonal germanium oxide phases. The maximum GeO 2 content (38,78 mass%) has been attained while crystal growth in aqueous solutions of ammonium fluoride at temperature of 700°C and pressure of the order of 180MPa. Crystals grown in alkaline solutions at similar T-P parameters are characterized by the maximum GeO 2 content of 25 mass%. A temperature of the α-β polymorphous transition of α-Si x Ge 1-x O 2 with GeO 2 content of 38 mass% has reached 840°C (instead 573C of α-quartz). GeO 2 portion increases in α-Si x Ge 1-x O 2 crystals at temperature and growth rates rise. The highest GeO 2 content belongs to the growth sectors of positive and negative rhombohedra; the minimum - belongs to the growth sectors of hexagonal prism. A regular rise of the unit cell parameters and refraction indices are observed at GeO 2 portion increase in α-Si x Ge 1-x O 2 crystals. An appearance of new absorption bands was found in IR-spectra, caused by a formation of Si-O-Ge bridge bonds. A shifting of practically all bands in the IR-spectra of the combinational scattering for a distance of 10-15 cm -1 into the region of short-wave vibrations seems to be related to this very fact. The basic piezoelectric constants d 11 and d 14 of α-Si x Ge 1-x O 2 crystals have increased as compared to quartz by approximately two times. This work has been supported by the Russian Basic Research Foundation (Grant No 03-02-16613).

Growth and characterization of Si xGe 1−xO solid solution single crystals with quartz structure

Experimental investigations of the development of a reliable and reproducible growth method of Si x Ge 1− x O 2 solid solutions single crystals with quartz structure (α-Si x Ge 1− x O 2) have been performed under hydrothermal conditions. Effects resulting from solution compositions, T– P parameters and growth rates of different faces in morphology and internal structure of crystals as well as capturing and distribution of Si and Ge were also studied. The main crystallochemical characteristics and properties of the crystals have been fixed. α-Si x Ge 1− x O 2 crystals were studied by X-ray, refracting indexes determination methods, DTG-DTA, IR and Raman spectroscopy and they showed the high probability of the isomorphic substitutions of Si–Ge.

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

AbstractThe 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 σd but there is also another supersaturation barrier σ* when the rate abruptly increases with σ, (3) the values of σd and σ* increase with increasing concentration of the impurity, and (4) the values of σd 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 ci 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 Qdiff is higher during instantaneous impurity adsorption than that during time‐dependent adsorption, and (2) the values of Qdiff involved during instantaneous adsorption are related with face growth kinetics but those during time‐dependent adsorption are independent of face growth kinetics.

Effect of Cr(III) ions on the growth kinetics of ammonium oxalate monohydrate crystals from aqueous solutions

The effect of concentration of Cr(III) ions on the growth rates of different faces of ammonium oxalate monohydrate (AO) single crystals was studied at a constant temperature of 30°C and predefined supersaturations up to 8%. It was observed that: (1) at a given supersaturation Cr(III) 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 c, and (3) the value of σ c increases with an increase in the concentration of the impurity. The experimental R( σ) data for different Cr(III) concentrations c i were analysed according to the model involving complex source of cooperating screw dislocations. Analysis of the R( σ) data showed that, in general, the parameters related with the kinetic coefficient for step motion and interfacial energy initially decrease and then increase with an increase in c i, exhibiting a minimum at certain c i. This observation is associated with the changing activity of source of steps. The same data were then analysed from the standpoint of Cabrera–Vermilyea and Kubota–Mullin models using Langmuir and Freundlich adsorption isotherms. It was found that: (1) Freundlich isotherm describes the R( c i) dependences at given σ more reliably than Langmuir isotherm, and (2) the appearance of supersaturation barrier in the presence of Cr(III) ions during the growth of different faces of AO crystals is associated with the transition from Cabrera–Vermilyea to Kubota–Mullin mechanism.

Large-Scale Synthesis of Uniform Silver Nanowires Through a Soft, Self-Seeding, Polyol Process

A self-seeding process is used to generate bicrystalline silver nanowires by reducing AgNO3 with ethylene glycol at ∼160 °C. The key to the success of this solution-phase approach is the use of a polymeric “coordination reagent” capable of controlling the growth rates of different faces of silver. Uniform nanowires (see Figure) could be routinely synthesized with diameters of ∼60 nm and lengths up to ∼50 μm.

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

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.

Effect of Fe(III) ions on the growth kinetics of ammonium oxalate monohydrate crystals from aqueous solutions

The effect of concentration of Fe(III) ions and solution supersaturation on growth rates of different faces of ammonium oxalate monohydrate (AO) single crystals is studied at a constant temperature of 30°C and predefined supersaturations up to 8%. It was observed that, in contrast to the usually observed decrease in face growth rates of crystals with an increase in the concentration of an impurity, Fe(III) ions lead to an increase in the growth rates of different faces of AO crystals. The experimental data of the dependence of face growth rates R on supersaturation σ at different concentrations c i of Fe(III) were analysed from the standpoint of BCF surface diffusion theory and its modified version in the power-law form, spiral growth theory involving cooperating dislocation source and multiple nucleation model. It was found that the cooperating screw dislocation and multiple nucleation models represent the experimental data on growth kinetics from pure and impure solutions most satisfactorily. Analysis of the data according to these models revealed that the free energy of a growing face decreases with an increase in the concentration of Fe(III) ions, suggesting that the impurity facilitates the formation of kinks in growth steps. It was also found that the values of the differential heat of adsorption of Fe(III) impurity, obtained in the present study, are similar to those observed in the case of growth systems in which impurities lead to a decrease in the growth rate.

Growth promoting effect of organic impurities on growth kinetics of KAP and KDP crystals

The experimental results of the influence of a variety of organic impurities differing in their chemical behavior on the growth rates of different faces of KAP and KDP crystals are presented. It was observed that, with increasing additive concentration in practically all experiments, an impurity leads first to an increase and then a decrease in the growth rate, passing through a maximum. These features of the influence of an additive on crystal growth kinetics depend on the chemical nature of both the impurity and the crystal face. The initial growth promoting effect of additives is discussed in terms of the chemical nature of the materials used in the experiments and changes in the thermodynamic and kinetic parameters involved in the crystal growth models. The analysis suggests that the growth promoting effect is not connected with a decrease in the edge free energy in a simple manner.