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Optics and Laser Technology

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Effect of shock waves on thermophysical properties of ADP and KDP crystals

A. Sivakumara, S. Suresha, S. Balachandarb, J. Thirupathya, J. Kalyana Sundarc,S.A. Martin Britto Dhasa,⁎

a Department of Physics, Abraham Panampara Research Center, Sacred Heart College, Tirupattur, Vellore, Tamil Nadu 635601, IndiabDepartment of Research and Development, AKSH Optifibre Private Limited, Bhiwadi, Rajasthan 301019, Indiac Department of Physics, Periyar University, Salem, Tamil Nadu 636011, India

H I G H L I G H T S

• Enhancement of thermophysical properties of ADP crystal is demonstrated.

• It is achieved by loading shock waves on the crystal.

• It enhances the applicability of a crystal in high power laser devices.

• Shock waves does not alter the thermophysical properties of KDP crystal.

• KDP is better crystal to be used in fast moving and vibrating devices.

A R T I C L E I N F O

Keywords:Shock tubeShock wavesThermophysical propertiesPhotoacoustic spectrometer

A B S T R A C T

Ammonium Dihydrogen Phosphate (ADP) and Potassium Dihydrogen Phosphate (KDP) crystals are grown byslow evaporation method at ambient temperature. The said crystals are utilized as a test specimen and subjectedto one-dimensional ‘loading of shock waves’ generated by rupturing a paper diaphragm with Mach number of 1.9using supersonic low energy table-top shock tube. Thermal diffusivity of crystal is measured using Photoacousticspectrometer (PAS) for the normal and shock loaded crystals, thermal conductivity and thermal effusivity arecomputed for the given volumetric specific heat capacity of the crystals. XRD characterization studies revealsthat KDP crystal has better immunity to shock wave than ADP crystal.

1. Introduction

The demands for crystals in bulk form or nano form has increaseddramatically during the past three decades to meet the requirement ofthe microelectronics and optoelectronic industrial applications. Thedemand of the crystals depends on the stability of performance of thosecrystals against accidental events such as vibration, pressure, mechan-ical or thermal shock [1,2]. In the case of technologically importantcrystals such as semiconductors, nonlinear optical crystals, piezo-electric crystals, etc., the internal heat generation within the crystalproduces a time varying temperature profile which significantly affectsthe quality of the outcome of those crystals. Therefore, the knowledgeon the changes in thermal transport properties is essential for the se-lection of material, thermal management, efficient design of opticalsystem etc. Since the above said properties may be altered by accidentalevents such as mechanical or thermal shocks, the study on the crystalsagainst such shocks is essential. In the present study, we investigated

the effect of the shock waves on the thermal transport properties ofpopular nonlinear optical crystals such as Potassium dihydrogenphosphate (KDP) and Ammonium dihydrogen phosphate (ADP). In thepresent context, shock wave is a sudden release of transient energycontaining high pressure, high temperature with a precise Machnumber having a pulse width of few microsecond which has potentialapplications in the fields of engineering, manufacturing, medical,agriculture, biological and scientific research [3,4]. Controlled andpredetermined strength of shock waves can be generated by a devicecalled shock tubes which can transfer the energy instantaneously andefficiently to introduce defects without explicitly destructing the crystal[5]. This can be treated as an accidental event on the crystal which maybe generated by dropping down the crystal or hitting an object on thecrystal. When shock waves propagate in the crystals, the morphological,grain sizes or even the structural changes may occur and followed bythe changes in physical or chemical properties of the crystal dependsthe strength of the shock which is denoted by the unit Mach number

https://doi.org/10.1016/j.optlastec.2018.10.001Received 5 September 2017; Received in revised form 3 February 2018; Accepted 1 October 2018

⁎ Corresponding author.E-mail address: martinbritto@shctpt.edu (S.A. Martin Britto Dhas).

Optics and Laser Technology 111 (2019) 284–289

0030-3992/ © 2018 Elsevier Ltd. All rights reserved.

T

[6,7]. Also impact of shock waves on mechanical and initiation ofchemical reaction are strongly coupled [8,9]. Recently it was demon-strated that the effect of shock wave response on quartz crystalline andsapphire crystals to tune the property of mechanoluminescence andelectrical resistivity [10]. Nachiketa Ray et al reported that the shockloaded aluminum alloy showed higher hardness [11] and Enqiang Linet al. studied theoretically the change of crystallographic orientationand the vacancy defect [12] and also the structural phase transition wasobserved by Kadu et al. [13]. In sapphire crystals shock waves createdplastic deformation, dislocations and twinning defects and is verysensitive to shock propagation direction [14].

Though, a large number of reports are available in the literatureregarding the structural and morphological changes due to loading ofshock waves, the effects of shock waves on thermophysical properties ofpiezo-electric, anti-ferro electric, electro optic, and non-linear opticalmaterials such as ADP and KDP are yet to be explored [15–27]. Ther-mophysical properties are playing an imperative role in the design ofnovel electronic device and high power laser systems. Especially,thermal diffusivity is the most important quantity to determine the heattransfer mechanism of the crystal and dynamic thermal measurements.Hence, the knowledge of thermo physical properties is essential in orderto save the device from thermal damage due to sudden heat exchangesduring high power laser irradiation [28,29].

In the present experiment, shock waves were generated by using atable-top shock tube which was indigenously fabricated in our labora-tory. The shock wave loaded crystals were characterized by XRD andphoto acoustic spectrometer before and after shock loading. To the bestof our knowledge, this is the first report in the literature to report theinvestigation of the effect of shock waves on thermo physical propertiesof ADP and KDP crystals. The details are discussed in the followingsections.

2. Experimental

2.1. Sample preparation

Single crystals of ADP and KDP were grown from saturated solutionsby slow evaporation method at room temperature. The saturated so-lutions of ADP and KDP are prepared with water as solvent then filteredand transferred to glass beaker and covered with polythene sheet withfew holes to evaporate slowly. ADP crystal with a dimension of32×24×8mm3 was obtained after 49 days. KDP crystal with a di-mension of 27×12×3.6mm3 was obtained after 41 days. The growncrystals (Fig. 1) were well shaped, transparent, colorless and stable.

2.2. Loading of shock waves on the crystal

Shock tube is a simple stay tool for producing predetermined shockwaves of required strength in the laboratory [30]. Shock tube consistsof three sections such as driver section, driven section and diaphragm

section which separates the driver and driven sections. When com-pressed-gas fills the driver section until diaphragm ruptures to createshock wave which travels into the driven section. The crystals areplaced in the sample holder at 2 cm away from the end of the drivensection as illustrated in Fig. 2. Couple of shock waves with 1.9 Machnumber was loaded on the crystals subsequently in a time period of tenminute. After each shock, the crystals were analyzed under opticalmicroscope to confirm that there is no visible damage on the surface ofthe crystals. Then, the crystals were undergone for the analysis, such asXRD and photo acoustic studies.

3. Results and discussion

3.1. Powder X-ray diffraction pattern

The crystals were subjected to X-ray diffraction studies of the samefaces before and after loading of shock waves. In both the cases, thenumbers of the peaks are same but the intensity of the peaks is reducedand small shift in position are observed. In the case of ADP, the peaksobtained after the shock is little shifted towards the lower angle and thefull width at half maximum also decreased around half of the initialvalue. From Fig. 3(a), it is clear that the grain size of the ADP crystal hasincreased from 4.9 to 9.40 nm after loading of shock waves whichevident from the full width half maximum of the peaks. Also, it is clearthat the splitting of the peaks vanished and become single and narrowpeaks which indicate the merging of two grains into one and also thereduction of d-spacing [31]. The powder XRD patterns of the KDPcrystal before and after loading of shock waves are shown in Fig. 3(b).In this case the obtained peaks are similar to the original peaks. Neithershift in the position nor in the peak broadening are observed, but thegrain size is slightly changed from 18.2 to 18.5 nm and the intensity ofthe peak after loading shock is considerably decreased. The reduction inthe intensity of the peaks aftershock loaded may be due to the mis-alignment of the crystalline plane of the grains due to the passage of theshock waves. Atomic bonds play vital role in the stability of a materialagainst extreme conditions such thermal or pressure shock waves [1,2].Since the hydrogen bonds have low bond strength, it can be easily re-sponded to the high pressure. Also KDP crystal has high bond energyand superior structure stability compare to ADP crystal because it hasfewer hydrogen bonds [32]. Therefore in the shock loading conditionsKDP crystal grain size could not be affected significantly. The aboveresults confirm that the KDP crystal has the better shock resistancecompared than ADP crystal.

3.2. Photo acoustic spectroscopic studies

Photoacoustic spectroscopy is a non-destructive tool to determinethe thermal diffusivity of solids. The principle of photoacoustic spec-troscopy is; when periodic electromagnetic radiation interacts withmatter, the molecules are excited and the de-excitation may occur by

Fig. 1. Photographs of as grown crystals (a) ADP crystal (b) KDP crystal.

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the emission of heat as non-radiative transition and this heat will beexchanged to the nearby medium. If the incident light is modulated,then there will be a temperature fluctuation in the medium followed bya pressure fluctuation. This pressure fluctuation can be detected by amicrophone and this signal contains the thermo physical property of thesurface of the material [28]. In the present work, thermal transportparameters such as thermal diffusivity, thermal conductivity and

thermal effusivity of the normal and shock loaded crystals were ob-tained by an indigenously developed photo acoustic spectrometer. Inthis photo acoustic spectrometer, a light beam from a 250W halogenlamp is well collimated by couple of lenses, modulated by mechanicalchopper and then focused on the sample (1mm thickness) which isplaced in acoustic free sample cell. The photo acoustic signal is detectedby a sensitive microphone and feed to the computer. The calibration of

Fig. 2. Schematic diagram of the experimental setup of loading shock waves on the crystals.

Fig. 3. XRD pattern of (a) ADP crystal before and after loading of shock waves (b) KDP crystal before and after loading of shock waves.

Fig. 4. Schematic diagram of Photoacoustic spectrometer.

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the spectrophotometer is performed using standard samples such asKDP crystal, BK7 and Quartz glasses. The thermophysical values ob-tained for ADP and KDP from this spectrometer are well agreed with thereported values [33,34]. The block diagram of the Photoacousticspectrometer is shown in Fig. 4.

Thermal diffusivity of the crystal is obtained from the photoacousticsignal amplitude by the curve fitting method [35–37]. Thermal con-ductivity and thermal effusivity are calculated from the following

relations:

= αρk cp (1)

= ρ αe cp (2)

where k, α ρ, , cp and e are the thermal conductivity, thermal diffusivity,density, specific heat capacity, and thermal effusivity of the crystalrespectively. The density of the ADP and KDP crystal are 1.799 g/cm3

Fig. 5. Photoacoustic signals (a) before shock and (b) after shock wave loaded on ADP crystal.

Fig. 6. Photoacoustic signals (a) before shock and (b) after shock wave loaded on KDP crystal.

Fig. 7. Graph plotted for Normalized Photoacoustic signal against square root of chopping frequency (a) ADP crystal and (b) KDP crystal.

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and 2.332 g/cm3 respectively and the specific heat capacities of ADPand KDP crystals at ambient temperature are 998 J/kg/K and 992 J/kg/K [33].

Thermo physical properties of crystals depend on the structure andgrain size. Though the crystalline structure is an inherent property of acrystal and can’t be changed, the grain size can be altered by variousmethods during its growth process and this grain size plays a key roleon phonon scattering and followed by thermophysical properties. Since,lower grain size materials have high structural complexity and as wellas large phonon scattering, thermal transport in the crystal is resistedby the grain boundaries. Consequently, larger grain size reduces thestructural complexity and phonon scattering and hence enhances thetransport of heat. Since the thermal conductivity and grain size of amaterial relation is linear, the increase of grain size leads to a decreasein the number of grain boundaries and hence the phonon scatteringreduces [38,39].

The variation of PAS recorded signals for the test samples withdifferent frequencies are shown in Figs. 5 and 6. The obtained photo-acoustic signal amplitude is decreased while increasing the choppingfrequency for before and after shock wave loaded conditions as well asthis is universal behavior of photoacoustic spectrometer measurements.Square root of chopping frequencies Vs Normalized amplitude for be-fore and after shock loaded test samples are shown in Fig. 7. Thermophysical values of ADP and KDP crystals before and after shock waveloaded is tabulated in Tables 1 and 2 respectively. It is clear fromTable 1 that thermal diffusivity, thermal conductivity, and thermal ef-fusivity of ADP crystal are increased when the shock wave applied. Itmay due to the increase in the grain size of the crystal during thepassage of the shock wave through the crystal and it is already evidentfrom the XRD results. The increase in thermal diffusivity of ADP crystalis a favorable property for the application where high power lasersystems and electronic devices [40,41].

Thermophysical values of the KDP crystal are given in Table 2. It isclear that thermo physical parameters of the KDP crystal are not sig-nificantly altered even shock waves loaded condition. Slight enhance-ment of thermophysical properties of KDP crystal for shock waveloaded conditions may be due to the slight increase in the grain size ofthe crystal from 18.2 to 18.5. It shows that KDP has highly stablecrystalline structure and high mechanical strength as it is further evi-denced by XRD results [42]. One can conclude from this result thatMach number 1.9 shock waves could not affect the thermo physicalproperties of KDP crystal and this crystal can be used in the moving andvibrating devices without losing its original thermophysical properties.

4. Conclusion

Good quality ADP and KDP single crystals were grown by the slowevaporation method at room temperature. The grown crystals aresubjected to shock waves of Mach number 1.9 produced by a shock tubeand the changes in thermophysical properties of crystals are reported.When shock waves exposed on the crystals, the thermophysical prop-erties of ADP crystal are significantly altered while the thermophysicalproperties of KDP crystal are slightly changed. The reasons for this ef-fect are probed by the XRD studies and the results reveal that thechanges in the grain size. From the above results, it is concluded thatKDP crystal has high shock resistance than ADP crystal. In other hand,since the thermal diffusivity of the ADP is increased, it enhances theapplicability of the crystal in high power laser systems and electronicdevices to reduce the thermal damage and KDP crystals can be a bettermaterial to use in moving and vibrating devices without losing its ori-ginal properties.

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Table 1Thermo physical properties of ADP crystal before and after shock applied.

S. No SamplenameADP

Thermaldiffusivity (α)×10−6 (m2/s)

Thermal effusivity(e)× 103

(J m−2 K−1 s−1/2)

Thermalconductivity (k)(Wm−1 K−1)

1 Beforeshock

0.678 1.463 1.204

2 Aftershock

1.552 2.214 2.758

Table 2Thermo physical properties of the KDP crystal before and after shock applied.

S. No SamplenameKDP

Thermaldiffusivity (α)×10−6 (m2/s)

Thermal effusivity(e)× 103

(J m−2 K−1 s−1/2)

Thermalconductivity (k)(Wm−1 K−1)

1 Beforeshock

1.055 2.376 2.441

2 Aftershock

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