Growth Of Potassium Dihydrogen Phosphate Crystals Research Articles

Numerical simulations of flow and mass transfer during large-scale potassium dihydrogen phosphate crystal growth via three-dimensional motion growth method

A novel solution-based crystal growth method, namely, the three-dimensional motion growth method (3D MGM) had been suggested by our team to effectively utilize convection for simultaneous enhancement of morphological stability and mass transfer. To evaluate this new method and link it to practice, numerical simulations of flow and mass transfer during large-scale potassium dihydrogen phosphate (KDP) crystal growth via this new method are carried out. The supersaturation field on the crystal surface is presented as functions of translational distance and crystal size. The relationships between directions of solution flow adjacent to crystal surface and surface shear stress are summarized and discussed. Results indicate that 3D MGM benefits the large-scale crystal growth by improving the morphological stability and crystal quality.

Numerical simulation of the hydrodynamics and mass transfer in 3D spiral motion system for KDP crystal growth

A numerical study of the unsteady turbulent flow and mass transfer involved in the potassium dihydrogen phosphate crystal growth from an aqueous solution with a three-dimensional spiral motion configuration was performed using the standard k–ε model with an enhanced wall treatment. The three-dimensional spiral motion is composed of a uniform circular motion on the horizontal plane and a reciprocating motion in the vertical direction. The simulation indicates that the crystal spiral motion will drive the mainstream solution rotating around the axis of the growth vessel and form an oscillatory flow near the crystal, which results in a periodical reversal of the supersaturation distribution on the crystal faces and effectively suppresses the morphological instability of the crystal surfaces. With the increase of the orbital radius R or the characteristic rotation rate ω, the line speed V0 of crystal in the horizontal plane will increase, leading to a decline of the thickness of the diffusional boundary layer and an increase of the time-averaged supersaturation on the crystal faces. At the same time, the homogeneity of the surface supersaturation will be improved too, which would benefit the morphological stability of the crystal surfaces. However, when the crystal size increases, the time-averaged supersaturation on the crystal faces will decrease and its homogeneity will deteriorate. Therefore, with crystal growth, increasing the orbital radius, R, and reducing the period, T1, of the crystal circular motion are necessary.

Step bunching in potassium dihydrogen phosphate crystal growth: Phenomenology

We have developed a real-time phase-shifting interferometer capable of imaging interfacial morphology with a depth resolution of approximately 25 Å, with a lateral resolution of approximately 0.5 μm across a field of view of 2 × 2mm2, and with time resolution of 0.1 s. The method is applied in situ to the (101) face of potassium dihydrogen phosphate crystals growing from an aqueous solution. We image the formation and evolution of solution-flow-induced step bunches and determine their characteristic wavelength to be λc = 45 μm. This wavelength is within the range predicted by a stability theory on the basis of the balance between the diffusion interaction between steps and capillarity. The value of λc suggests that step–step interactions are the likely major factor for instability.

The kinetics of potassium dihydrogen phosphate crystal growth from aqueous solution

The kinetics of growth of KDP crystals from aqueous solution has been examined at various supersaturations and temperatures, when stable supersaturated solutions have been injected with KDP seed crystals. The growth rate has been determined from measurements of the increase in the weight of a crystal. In the unagitated system, the growth rate of KDP follows a parabolic equation with respect to very low supersaturations, and a first-order equation with respect to higher degrees of supersaturation. The activation energy for growth in the temperature range 30-50 degrees C is calculated as 3.8+or-0.04 kcal mole-1.