The phenomenon of the cracking of KDP z-cut seed crystals with a series of sizes due to the temperature difference of the growth solution was explored in detail.

The results of the temperature dependence of fracture toughness of KDP crystals with four orientations show that the brittle-to-ductile transition occurs at 160 °C.

The thermal stress evolution in large-scale KDP crystals during the extraction process has been simulated by the finite element method.

To further elucidate the relationship between the growth stress and cracking of KDP (KH₂PO₄, potassium dihydrogen phosphate) crystals of different sizes, a three-dimensional finite element calculation was conducted to analyze the growth stress of KDP single crystals grown from Z-plate seeds with varying cooling rates. The mismatch in the coefficient of thermal expansion (CTE), between the cap region and its close vicinity, and among the transparent region, was taken into account. The results indicate that when the cap region is a solid region (when the seed was regenerated with a cooling rate of 0.1 °C/day), the difference in material properties between the cap region and its close vicinity, especially the CTE mismatch along the a-axis, is the main reason of the high stresses. When the cap region is a box-like structure filled with solution (when the seed was regenerated with a cooling rate of 0.3 °C/day), the calculated stress is in proportion to the CTE gradient of the transparent region. Under both models, the stresses induced from an incremental CTE value (from the cap region to the growth front) are greater than those calculated from a diminishing CTE value, implying that the impurities reduce the CTE of KDP crystals, causing the crystals to crack more easily. Despite the maximum stresses inside the crystals changing slightly with an increase in crystal size, the decreased fracture stress of large brittle crystals leads to a higher cracking risk in a large-sized crystal.

The bending strength anisotropy on the (001) plane or between the (100) and (001) planes of KDP crystals was investigated via a three-point bending test. Fracture surfaces were observed. Moreover, the defect distribution of the KDP samples was also analyzed. The results indicated that the bending strength along the [001] direction was higher than that along multiple directions on the (001) plane, showing pronounced anisotropy. However, the bending strengths along different directions on the (001) plane were similar. For the fractured samples with different orientations, multiple fracture surfaces for the (001), (100), (110), (10), (310) planes were observed. It was also deduced from the linear elastic fracture mechanism that the surface or internal defects with a dimension of 0.1 mm existed in the samples.