Study On The Cracking Of KDP Crystals By Experimental And Simulation Method

Owing to their manifold excellent performances such as piezoelectric property,dielectric property,electrooptical effect and nonlinear optical property,crystals of potassium dihydrogen phosphate(KDP)and its isomorphs have been the subject of a variety of investigations for over 80 years.In the past researches,people have achieved remarkable effects on studying the crystal structure and the growth mechanism,the growth velocity has increased markedly and their optical quality has improved.These achievements make KDP crystals better satisfy the demands of the inertial confinement fusion configuration.However,although KDP crystals have the excellent properties such as the wide transmittance,large nonlinear optical coefficient,high laser-induced damage thresholds and good refractive-index homogeneity,they also have shortcomings,such as temperature sensitivity,high brittleness,deliquescence and softness,which limits their application and decreases the output efficiency.Crystal cracking occurs as stresses in the crystal exceeds its fracture strength.To better understand the crystal cracking.Firstly,it is important to study the source of stress and the stress distribution.Secondly,it is also necessary to obtain the exact mechanical parameters of KDP crystals.Due to the temperature sensitivity of KDP crystals,we deem that the temperature difference during the crystal growth and processing has a vital effect to the crystal cracking.The thermal stress resulted from the temperature non-uniformity is real-time,stress distribution changes with the difference of thermal field at different moments.Thus,numerical simulation,as a method that can adjust the setting parameters repeatedly and realize the transient calculation,is widely used to predicate the thermal field and stress distribution of crystals during the growth and cooling process.Combining the experiment and numerical simulation,this paper aims at studying the relationship between temperature difference,the stress and crystal cracking of KDP crystals with different scales.In addition,the bending strength and fracture toughness of KDP crystals have been experimental measured,with considering factors that would affect the mechanical properties.The main contents of this paper are as follows:1.The phenomenon of the cracking of KDP z-cut seed crystals with a series of sizes(36?50 mm)due to the temperature difference of the growth solution was explored in detail.Thermal stress generated in the KDP seed crystal at the moment of cracking was calculated using the finite-element method.Temperature differences leading to cracking for heated samples were nearly twice those of cooled specimens.For the cooled samples,the cracks occurred mainly along the(100),(110)and(001)crystal planes and expanded from the sample periphery to the interior spaces,and the opposite crack propagation direction was displayed in the heated samples,with cracks observed mainly along the(110)plane.More importantly,the temperature differences were observed to decrease with an increase in the sample size.In addition,the calculated results indicated that the field distribution of tensile stress is coincident with the points of crack initiation.The maximum stresses calculated were found to be greater than the fracture strength of 6.67 MPa for the KDP crystal along the[100]direction,and the direction of large stresses was found to be normal to the fracture surface.It is interesting that the maximum stress in fracture crystals decreases with an increase in the crystal size as cracks are generated.2.The cooling experiments on KDP crystals with the sizes of 30?40 mm were carried out to investigate the crystal cracking caused by temperature-nonuniformity.Temperature and stress distributions on crystals during the cooling process were simulated by the finite-element method.The results show that cracking occurs in the early period(10?150 s).Deep main cracks and tiny cracks both appear on the experimental crystals.Main cracks usually originate from crystal periphery and propagate to the interior with a direction that parallels to(100)or(001)plane.Tiny cracks are just on the crystal surface.Cracks that parallel to(001)plane appear in the samples with a large height-width ratio.The simulation results reveal that crystals with the sizes of 30?40 mm likely crack when a difference between the maximum temperature and minimum temperature inside is greater than 4?.The maximum stress at the moment of cracking for each sample is 4.76 MPa,5.36 MPa,7.69 MPa,5.74 MPa,respectively,which is close to 6.67 MPa(i.e.,the fracture strength of KDP crystal).The location of the maximum stress is in the middle point of the bottom edge or prism edge,which is in accord with the crack origination.3.Bade on the environment temperature measured around the growth tank,transient numerical calculations were carried out to predict the evolutions of temperature and thermal stress in traditional grown large-size KDP crystals during the taking out process,with considering two ways that used to accomplish the crystal extraction.The influence of the crystal size and the temperature difference between the crystal and environment on the stresses inside the KDP crystals were also investigated in detail.Results indicate that,in the case of extracting solution,the maximum stress at the crystal periphery is larger than that in the interior,it is probably to produce cracks from the outside surface of crystals.In the case of retaining solution,the maximum stress in the crystal internal body is larger,it has a big possibility to originate cracks in the inner.In both two processes of taking out crystals,the highest stress transfers from the crystal periphery to the internal part from the early to the later stage.Both the increased crystal sizes and temperature differences between crystals and environment at the end of crystal growth are factors to trigger the crystal cracking.The maximum stress in crystals in the case of retaining solution is less than that in the case of extracting solution,which brings about a decreased potential of cracking.Thus,retaining solution for a period of time after the growth was completed,such as 96h,is suggested to be adopted to accomplish the crystal extraction.4.We have simulated the stress distribution in large-scale KDP crystals in dimension of 200 mm to 500 mm,caused by the thermal expansion coefficient no uniformity.The relationship between the stress and crystal cracking was also illustrated.In the simulation,we took the cap region existing or not,the thermal expansion coefficient variation pattern and variation range,the thermal expansion coefficient variation in one direction into consideration.Results indicated that,comparing to the case of considering the cap region,results from without considering the cap region were more consistent with the experimental observation.In the case without considering the cap region,the maximum stress was located at the interface edge between the cap region and transparent region,this position was exactly the crack origination in the real cracked crystals.The maximum stress was positive related to the thermal expansion coefficient no uniformity,especially the thermal expansion coefficient variation in x direction.With the enlargement of crystal sizes,the maximum stress increased and also the risk of crystals cracking.5.The bending strength and fracture toughness related to KDP crystals cracking were experimental measured.The bending strength was measured by the three point bending test along the[001]direction and multi-directions in the(001)plane.The fracture toughness was measured by the single edge notched beam(SENB)method.And the effect of temperature,sample orientations,annealing and loading rate have been considered.Results indicated that,anisotropy existed both in the bending strength and fracture toughness.The[001]direction had the biggest bending strength and fracture toughness.The bending strength of multi-directions in(001)plane differenced slightly.The Weibull moduli calculated for all samples were similar,which meant that the variability of bending strengths were nearly equal.The relationship of fracture toughness on different orientations was concluded as follows:x-cut>Type ?>Type ?>z-cut.With the increase in temperature,the fracture toughness of crystals increased and reached its maximum value at 160?;then,it decreased,indicating that 160 ? is the temperature at which brittle-to-ductile transition occurs.Using an optical microscope,it was observed that the fractured surfaces of the samples become gradually rough with the increase in temperature and shows the significant increased roughness at 160?.This may offer a viewpoint for understanding the dependence of fracture toughness on temperature,and also verify the brittle-to-ductile transition at 160 ?.Furthermore,annealing and decrease in the loading rate were conducive to the significant increase in the fracture toughness of KDP crystals.