Fafu Liu

Effect of supersaturation on hillock of directional Growth of KDP crystals

KDP single crystals were grown in aqueous solution by using “point seeds” with a defined crystallographic direction of 59° to the Z axis. When hillock slopes on the (100) face of KDP crystals were measured within the supersaturation (σ) range of 0 < σ ≤ 0.06, the slope of hillocks with hollow cores depended nonlinearly on supersaturation. Below σ = 0.02, the hillock slope depended on supersaturation, but when σ was ≥ 0.02, the hillock slope increased more gradually and was less dependent on supersaturation. Hollow funnel-shaped growth dislocation on the (100) face of KDP crystals was observed at σ = 0.04, characterized by large holes with micro-steps and step bunching inside, the formation of which were analyzed. The result verified that the reversed growth appears to occur within hollow channels found on growth hillocks.

Refractive indices in the whole transmission range of partially deuterated KDP crystals

Refractive indices of partially deuterated potassium dihydrogen phosphate (DKDP) crystals with 55%, 70% and 80% deuterium contents were measured by auto-collimation method at 293 K between 0.254 to 1.529 μm. Dependence of refractive indices of DKDP on deuterium content show different trend in the infrared region as in uv-visible region. Dependence of n2 (the square of refractive index) on the mole fraction of deuterium shows a difference between pure KDP and partially deuterated KDP. The Sellmeier equations were obtained by the least square method. The non-critical phase matching angles calculated from the fitted formula were in good agreement with laser experiment results, by which the reliability of these Sellmeier equations was confirmed.

New annealing method to improve KD2PO4 crystal quality: learning from high temperature phase transition

The existence of a high temperature phase transition in KD2PO4 was confirmed by thermal analysis, dielectric analysis and Raman spectroscopy. The results showed that KD2PO4 presented only one high temperature phase transition at about Ttr = 430 K, which is much lower than the thermal decomposition temperature. The absence of Raman shift attributable to the dehydration product indicated that the HTPT had nothing to do with thermal decomposition. The structural change during the HTPT was discussed. The strong similarity between the HT-phase and tetragonal phase decreases the temperature of the HTPT in the KD2PO4 crystal. The hydrogen–deuterium exchange can be suppressed under a protective ambient environment, which helps improve the annealing temperature. A new annealing method was developed by using silicone oil as a protective ambient environment under higher temperature up to 403 K, thus improving the quality of KD2PO4 crystals.

Prismatic morphology of an ADP crystal grown in a defined crystallographic direction

Herein, ammonium dihydrogen phosphate (NH4H2PO4, ADP) crystals were grown in a defined crystallographic direction (θ = 90°, Φ = 45°) in an aqueous solution using the point seed rapid growth method. Ex situ atomic microscopy (AFM) measurements were implemented to observe the prismatic growth micro-morphology of ADP and Cr3+-doped ADP in the defined crystallographic direction. It was found that with the increasing supersaturation, the growth morphology changed in turn to elementary steps, macro steps, and 2D nuclei. Moreover, the differences between ADP and Cr3+-doped ADP grown in the defined crystallographic direction have been discussed. The influence of Cr3+ on the growth of ADP crystals has been explored.

Determination of deuteration level of K(H 1−x D x ) 2 PO 4 crystal

Neutron powder diffraction was applied to determine the deuteration level of K(H1−xDx)2PO4 (DKDP) crystals via Rietveld refinement method. Micro-Raman spectroscopy was employed using the neutron results to investigate the relation between deuteration level and PO4 vibration peak of DKDP crystal. The relative variation [Δν1 = ν1(KDP)−ν1(DKDP)] of the PO4 vibration peak was linearly well dependent on the deuteration level. Attenuated Total Reflectance-Infrared (ATR-IR) spectroscopy was also used to study the total relative variation [β(DKDP)-β(KDP) + ν1(DKDP)−ν1(KDP)] of β(O-H/D) and ν1(PO4) absorption band with the deuteration level of DKDP crystals. The IR results illustrate that two linear relationships existed between the deuteration level and the relative variations of the spectra. The two spectroscopic techniques can be combined and used to measure the degree of deuteration in the crystals of DKDP grown from solutions with deuteration level of less than 92%. ATR-IR spectroscopy is more suitable for measuring highly deuterated DKDP crystals.

Characterization of the three-dimensional residual stress distribution in SiC bulk crystals by neutron diffraction

Characterization of residual stress in bulk crystals is of vital importance since it can provide abundant information on the crystal growth and process techniques. In this paper, the residual stress in 4H-SiC bulk crystals was investigated by neutron diffraction. Detailed strain and stress calculations were performed using the Bragg equation and Hookes law. The (0004), (11−20) and (1−100) plane diffraction profiles were measured. The strain and stress in three directions were found to be on the order of 10−4–10−3 and 100–1000 MPa, respectively. Moreover, the stress distributions in different directions were anisotropic. Amongst the stresses in these three directions, the stresses in the direction were tensile and the maximum value was up to 2953 MPa, which indicated that there was a high risk of the grown crystals cracking in this direction.