Optical Materials 38 (2014) 204–210

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Optical Materials

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Mechanical, thermal and optical properties of the SPS-processedpolycrystalline Nd:YAG

http://dx.doi.org/10.1016/j.optmat.2014.10.0300925-3467/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +972 86461468; fax: +972 86479441.E-mail address: nfrage@bgu.ac.il (N. Frage).

M. Sokol, S. Kalabukhov, V. Kasiyan, A. Rothman, M.P. Dariel, N. Frage ⇑Department of Materials Engineering, Ben-Gurion University of the Negev, P.O.B 653, Beer-Sheva 84105, Israel

a r t i c l e i n f o

Article history:Received 25 June 2014Accepted 19 October 2014Available online 6 November 2014

Keywords:Transparent ceramicNd:YAGSpark plasma sinteringFluorescenceLifetime

a b s t r a c t

The present study deals with a comprehensive comparison of the mechanical and functional properties ofNd:YAG single crystals with those of the polycrystalline ceramics (PCs), undoped and LiF-doped,processed by Spark Plasma Sintering (SPS). The polycrystalline ceramics have higher mechanical proper-ties (hardness, bending strength and thermal shock resistance) than the single crystals. The optical trans-mittance of the LiF-doped PC Nd:YAG is significantly higher than that of the undoped one and is close tothat of the single crystal. With respect to other optical and thermal properties, i.e. refraction index,absorption coefficient, extinction ratio, thermo-optic coefficient, fluorescence and thermal conductivity,no significant differences were observed between the single crystals and the polycrystalline ceramic.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

The versatile, many faceted structural and functional propertiesof transparent ceramics have elicited great interest over the lastdecades. In most cases, the initial interest focused on singlecrystals of these materials. Single crystals are most often grownfrom the melt, a powerful but complex operation. It allows deter-mining the physical properties as a function of the crystal orienta-tion, a feature of great importance for crystals with a non-cubicsymmetry. The technique of single crystal growth has also severelimitations; it is a cumbersome technique, it often sets limitationon the composition and on the uniformity of the product, it entailselevated manufacturing costs and severely limits the dimensionsof the products. With the growing interest in the use of transparentceramics, in order to take advantage of their wide range of actualapplications, alternative manufacturing routes have been explored.Foremost of these approaches is the one based on powder process-ing that sets, in principle, no limitation on the composition inmulti-component compounds and no dimensional restrictions. Itsmain disadvantage vs. single crystal manufacture is the polycrys-talline nature of the product and the presence of grain boundariesand other structural imperfections (porosity, for instance) which,in certain cases, may impair the functionality of the product.

Considering this background, it is not surprising that thebeginnings of the present century have witnessed an explosion of

publications regarding the processing of transparent ceramicsbased on sintering of micro or nano-powders. The processedceramics comprised simple (MgO and Al2O3) and complex (MgAl2-

O4, garnet YAG and Nd:YAG) oxides, oxynitrides (ALON), ferro-electric (PLZT) ceramics, sulfides (ZnS). Processing was carriedout by a variety of techniques that included pressure-less sintering,hot pressing (HP), hot isostatic pressing (HIP), single and two-stageprocesses. Research focused on the influence of the raw materials,particularly powder particle size, morphology and purity, and onthe processing parameters. The great interest in these ceramics isdue to their wide range of applications, both civilian and military.Extensive reviews [1,2] provide an overview of the main process-ing approaches and of the optical and mechanical properties ofthe ceramics that were obtained. The present study focused onthe properties of the SPS-processed Nd:YAG polycrystallineceramics.

Yttrium aluminum garnet (YAG) and Nd:YAG are transparentceramics of great importance owing to their functional properties.They are suitable media for high power laser generation; they canbe fabricated in fiber form and be doped by a variety of dopingelements, each giving rise to specific applications.

Chaim et al. [3] carried out extensive studies on the densificationof polycrystalline YAG by SPS processing. Even though high densityceramics were obtained, their optical transparency was low in the15–20% range. Frage et al. [4] starting with nano-YAG powderdoped with 0.25% LiF succeeded in fabricating polycrystalline YAGwith close to the theoretical optical transmittance level. Palmeroet al. [5] studied the surface and mechanical properties of un-dopedpolycrystalline SPS-processed YAG at 1325 �C subsequently heat-

Fig. 2. Photograph of the mirror polished Nd:YAG samples. Left to right are Nd:YAGsingle crystal, Nd:YAG LiF-doped and Nd:YAG undoped.

M. Sokol et al. / Optical Materials 38 (2014) 204–210 205

treated at 900 �C at various temperatures. Transmittance valueswere in the best case of the order of 72%. The presence of the LiFdoping additive increases the grain size, but its effect on graingrowth and mechanical properties is substantially less dramaticthan for SPS-processed magnesium aluminate spinel [4].

No overall consensus is presently available regarding the neces-sity of the doping additive for obtaining elevated transmittancevalues. Even though SPS is unanimously considered a valuableprocessing technique, its use has not been ubiquitously espousedfor the fabrication of transparent ceramics, in general and for poly-crystalline YAG-derived ceramics, in particular. Moreover, there isa dearth of publications regarding the effect of the doping additiveand SPS processing technology on a wide range of optical, mechan-ical and thermal properties of YAG derived ceramics. The latter areof significant importance in connection with the functionality oflasing materials. In the present communication measured optical,mechanical and thermal properties of the SPS-processed Nd:YAGceramics (LiF-doped and undoped) are compared with those ofthe Nd:YAG single crystal.

2. Experimental procedure

2.1. Fabrication of Nd:YAG ceramics

Neodymium (1 at.%)-doped yttrium aluminum garnet powder(Nanocerox, Inc.) (Fig. 1) with d50 of 50 nm and impurities levelof 8S, 6Cl, 2Ca and 2Fe ppm was used for the SPS fabricatingpolycrystalline Nd:YAG samples.

For fabrication of the LiF-doped (0.25 wt.%) specimens, Nd:YAGand LiF powders (99.98%, Alfa Aesar) were pre-mixed.

The regimes of the SPS process for sample preparation werereported and discussed in our previous contribution [6,7]. Nd:YAGsingle crystal with 1.1 at.% Nd (Raicol Crystals Ltd.) was also testedfor comparison.

The pictures of the tested specimens are shown in Fig. 2.

2.2. Specimens characterization

2.2.1. Mechanical propertiesThe sintered Nd:YAG specimens were cut and mounted into

epoxy polymer, and then grounded and polished to optical level.The thickness after final polishing is about 2 mm. Micro hardnesswas measured by the Vickers method using Buehler micro-hardness tester (MMT-7) under 2000 g load. Three point bendingtests (ASTM standard C1161) were conducted on a LRX-Plus LLOYDinstrument (Lloyd Instruments). The elastic moduli of the

Fig. 1. SEM images of Nd:YAG powders.

specimens were established by the ‘‘pulse echo’’ method (LeCroy9304A). The relative density of the sintered specimens was deter-mined by the Archimedes technique. The thermal shock resistance(RT) was calculated for room temperature using the expression:

RT ¼krð1� mÞ

aEð1Þ

where k is thermal conductivity, r is the strength, m is the passionratio, a is the thermal expansion coefficient and E is the elasticmodulus.

2.2.2. Optical propertiesThe transparency (T) of the Nd:YAG samples was measured in

the 400–1100 nm range. The light source was an incandescent250 W tungsten halogen lamp. The light beam was mechanicallychopped at 140 Hz. A monochromator (Horiba FHR640) of1.2 nm/mm spectral dispersion (for 1200 grooves/mm grating)provided the spectral separation; a Si photo-detector at the exit slitwas used to measure the emerging light intensity. The signals wereamplified using a lock-in amplifier (SIGNAL RECOVERY, 1265 DSP),and the data was recorded using the LabVIEW computer program.

The refractive index (n) as a function of the wavelength was mea-sured based on the measured Brewster’s angle (hB) and the physicalrelation tan hB ¼ n in an ellipsometer apparatus (Alpha-SE, J.A.Woollam Co.).

The absorption coefficient (a) spectrum of samples was calcu-lated using the expression for transmission:

T ¼ ð1� RÞ2 expð�aLÞ1� R2ð�2aLÞ

ð2Þ

R ¼ ðn� 1Þ2

ðnþ 1Þ2ð3Þ

where T is the sample transmittance, R is the surface reflectivity, L isthe sample thickness, a is the absorption coefficient and n is therefractive index.

The extinction ratio (EC) of the samples was measured toquantify the residual strain within specimens as a result of thefabrication process. The extinction ratio was measured using aHeNe laser source (k = 632.8 nm, type 1507-2, JDSU), two polariz-ers (Type NT43-786, Edmund Optics) and a photodiode (amplifiedSilicon detector, Type PDA36A, Thorlabs). The setup is described inFig. 3. The extinction ratio defined as

EC ¼ 10� logðIjj=I?Þ ð4Þ

where Ijj is the maximum transmitted beam intensity with thepolarizers uncrossed, and I? is the beam intensity with the crossedpolarizers.

The thermo-optic coefficient (dn/dT) of the material indicates thechanges in the refractive index on changing the temperature. Thethermo-optic coefficient of the polycrystalline Nd:YAG samplewas calculated from the measured sample reflectivity at varioustemperatures using the expression

ChopperHeNe Laser

Vertical polarizer

Sample Horizontal polarizer

Detector Oscilloscope

Fig. 3. Schematic view of the extinction ratio measurement setup.

206 M. Sokol et al. / Optical Materials 38 (2014) 204–210

HeNe Laser Chopper

Detector 1

Detector 2

Beam splitter Sample

Heating system

Fig. 4. Schematic view of the thermo-optic coefficient measurement setup.

Oscilloscope

Detector

Sample

Prism

Nitrogen Laser

Focusing lenses

Lens

Fig. 5. Schematic view of the fluorescence lifetime measurement setup.

dndT

����T0

¼ n2ðT0Þ � 14

� 1RðT0Þ

dRdT

����T0

ð5Þ

where n is the refractive index, R is the reflectivity, T is the temper-ature, and T0 is a reference temperature.

The sample was optically polished to high flatness, yet mutuallywedged at approximately 1� to allow for spatial out-filtering of theback-surface reflections. The measurements conducted using aHeNe laser source laser (k = 632.8 nm, type 25LHP-111, MellesGriot), beam splitter and two photodiode detectors (0PD300, OphirOptronics). The sample was placed in a self-made thermo-cell withits temperature measured by a commercial T-type copper/copper–nickel thermocouple, and stabilized according to a preset programwith a model 91 PID Eurotherm temperature controller equipped

Table 1Mechanical properties of polycrystalline and single phase Nd:YAG.

Sample Bending strength(MPa)

Vickers hardness(HV)

Polycrystalline Nd:YAG LiF-doped 310 ± 28 1460 ± 30Polycrystalline Nd:YAG undoped 420 ± 18 1570 ± 30Single crystal Nd:YAG 200 ± 12 1320 ± 30

with MRC ITools software. The thermo-cell temperature wasadjusted by heating of an embedded hot plate. Heating wasperformed by driving a current through an inner resistance coil.The setup is shown in Fig. 4.

The florescence spectra of the samples was measured over thewavelength region from 1000 to 1100 nm with a 0.2 nm stepwidth and a 50 nm/min scan speed. The excitation source wasan 808 nm diode laser (CNI Optoelectronics Tech). The florescencesignal was measured by the same method described for thetransparency. The main florescence peak (at 1064 nm) wastreated in order determine the peak position and full width athalf maximum (FWHM).

The fluorescence lifetime (s) was calculated from the measureddecay rate of the 4F3/2 ?

4I11/2 transition (emission at wavelengthof 1064.1 nm). The sample was installed with an incidence equalto the Brewster angle in a front of a collective UV double-convexlens with a focal length of 150 mm (Fig. 5). The data was collectedby Si amplified detector with a 1064 nm wavelength filterproduced by Thorlabs Company. The excitation source was a337.1 nm nitrogen laser (Stanford Research Systems Company)with a pulse width of 3.5 ns.

The thermal expansion coefficient (a) and the liner thermalexpansion were evaluated between room temperature and1000 �C using a dilatometer (model UNITHERM™ 1252, ThermalExpansion System Anter Corporation, USA) in a stream of argonwith the heating rate of 2 �C/min. The test bars had £

6 mm � 20 mm size.The thermal conductivity (j) was measured as a function of

temperature from room temperature to 125 �C by the flash diffu-sivity method using a laser flash apparatus (LFA 457, Netzsch).The front face of the sample (£ = 6 mm, thickness = 1.5 mm) wascoated with a carbon thin layer and then irradiated by a short laserburst, the resulting rear face temperature was recorded andanalyzed. Thermal conductivity values were calculated using therelation j = c�CP�q, where c is the thermal diffusivity, CP is the spe-cific heat, measured by differential scanning calorimetry (SENSYSevo DSC, Setaram), and q is the bulk density of the sample.

3. Results and discussion

3.1. Mechanical properties

The values of hardness, bending strength and thermal shockresistance of the polycrystalline ceramics are higher than those

Poisson’sratio

Shear modulus(GPa)

Young modulus(GPa)

Thermal shockresistance (W m�1)

0.25 114 ± 5 286 ± 5 12940.25 114 ± 5 285 ± 5 15280.25 113 ± 5 284 ± 5 761

20

40

60

80

100 Polycrystalline Nd:YAG LiF-doped Polycrystalline Nd:YAG undoped Single crystal Nd:YAG

Tra

nsm

ittan

ce, %

Wavelength, nm

(b)

700 720 740 760 780 800 820 840 860 880 900400 500 600 700 800 900 1000 1100

20

40

60

80

100 Polycrystalline Nd:YAG LiF-doped Polycrystalline Nd:YAG undoped Single crystal Nd:YAG

Tra

nsm

ittan

ce, %

Wavelength, nm

(a)

Fig. 6. (a) Transmission spectrum of Nd:YAG ceramics. (b) Transmission spectrum in the 700–900 nm wavelength range.

400 500 600 700 800 900

1.82

1.83

1.84

1.85

1.86

1.87 Polycrystalline Nd:YAG LiF-doped Polycrystalline Nd:YAG undoped Single Crystal Nd:YAG

Ref

ract

ive

inde

x n

Wavelength λ , nm

Fig. 7. Refractive index of single crystal and polycrystalline LiF-doped Nd:YAG as afunction of wavelength.

M. Sokol et al. / Optical Materials 38 (2014) 204–210 207

the for Nd:YAG single crystal, while the Young modulus is thesame. Un-doped specimens display properties better than thoseof the LiF-doped samples do (see Table 1).

3.2. Optical transmittance

The optical transmittance in the visible and near-IR wavelengthrange for LiF-doped, undoped and single crystal Nd:YAG sampleare presented in Fig. 6.

400 500 600 700 800 900 1000 11000

2

4

6

8

10

12

14 Polycrystalline Nd:YAG LiF-doped Polycrystalline Nd:YAG undoped Single crystal Nd:YAG

(a)

Abs

orpt

ion

Coe

ffic

ient

, cm

-1

Wavelength, nm

Fig. 8. (a) Absorption coefficient spectrum of single crystal, polycrystalline Li

The spectrum illustrates significantly enhanced transmittancevalues for LiF-doped samples, especially for low wavelengths(<500 nm). The transmittance of the LiF-doped samples are onlyslightly lower than that for the Nd:YAG single crystal. It is also sig-nificant that the presence of the additive does not affect the Nd3+

dopant absorption peaks in the 700–900 nm range. The effect ofLiF additive on the transmittance was discussed in details in ourprevious contribution [7,8].

3.3. Refractive index

The refractive index of the Nd:YAG specimens in the 400–900 nm (Fig. 7) was evaluated based on the measured Brewster’sangle.

Only minor differences were detected between the refractiveindexes of different samples. This fact can be attributed to the iso-tropic cubic structure of the Nd:YAG and the uniform density of thesamples.

3.4. Absorption coefficient

The absorption coefficient of the samples for the 400–1100 nmwavelengths range (Fig. 8) was calculated according to Eq. (2). Theabsorption peaks at specific wavelengths are related to the corre-sponding energy transitions.

The absorption coefficient of the polycrystalline Nd:YAGdisplays the same dependence on the wavelength as that for thesingle crystal, especially around the 808 nm wavelength (Fig. 8b).

790 800 810 820 8300

2

4

6

8

10

12

14 Polycrystalline Nd:YAG LiF-doped Polycrystalline Nd:YAG undoped Single crystal Nd:YAG

(b)

Abs

orpt

ion

Coe

ffic

ient

, cm

-1

Wavelength, nm

F-doped and undoped Nd:YAG. (b) Expansion of the 790–830 nm range.

Table 2Extinction rations of Nd:YAG samples.

Sample Extinction ratio (dB)

Polycrystalline Nd:YAG LiF-doped 33.46 ± 3.09Polycrystalline Nd:YAG undoped 40.94 ± 3.05Single crystal Nd:YAG 34.93 ± 3.60

1.0000

1.0125

1.0250

1.0375

1.0500(b)(a)

Polycrystalline Nd:YAG LiF-doped

S2/S

1

Temperature ,OC

20 40 60 80 100 20 40 60 80 100

Polycrystalline Nd:YAG undoped

Temperature ,OC

Fig. 9. Reflectivity of the LiF-doped (a) and undoped (b) samples as a function oftemperature. The red line is a linear fit. (For interpretation of the references to colorin this figure legend, the reader is referred to the web version of this article.)

0.9750

0.9875

1.0000

1.0125

1.0250

Polycrystalline Nd:YAG LiF-doped

R/R

(0)

Temperature , OC

Slope = 1.16x10-5 8.49x10-6+-

20 40 60 80 100 20 40 60 80 100

Polycrystalline Nd:YAG undoped

Temperature , OC

Slope = 1.17x10-5 7.88x10-6+-

(b)(a)

Fig. 10. Normalized reflectivity of the LiF-doped (a) and undoped (b) as a functionof temperature. The dashed line is a linear fit.

1050 1060 1070 1080

Polycrystalline Nd:YAG LiF-doped Polycrystalline Nd:YAG undoped Single crystal Nd:YAG

Fluo

resc

ence

inte

nsity

(A

U)

Wavelength (nm)

Fig. 11. Fluorescence spectra at room temperature.

208 M. Sokol et al. / Optical Materials 38 (2014) 204–210

High absorption of light at this wavelength is important, mainlybecause the common pumping source (AlGaAs diode laser) for1064 nm Nd:YAG laser operates at 808 nm.

3.5. Extinction ratio

The homogeneity of the material and the presence of internalstrains lead to a depolarization and phase distortion of the laserbeam, resulting in higher cavity losses, low laser efficiency andpoor beam quality [9]. The extinction ratio was calculated fromthe results of the light intensity after crossing two crossed polariz-ers, as described in the experimental section. The results arepresented at Table 2.

The results indicate that the values of the extinction ratio of theSPS-processed ceramics are very close to those for the Nd:YAGsingle crystal.

3.6. The thermo-optic coefficient (dn/dT)

The thermo-optic coefficient of the polycrystalline LiF-dopedNd:YAG sample was calculated from the measured sample reflec-tivity in the 20–98 �C temperature range.

The results were treated in two steps. First, the ratio betweenthe measured reflected intensity (Fig. 9) from the sample (S1,detector 2) and the measured incident beam intensity (S2, detector1) was plotted versus temperature (Fig. 10).

The experimental results were fitted linearly and the fittedvalue of the lowest temperature was set as y0, namely the reflectiv-ity at room-temperature. Second, the sample reflectivity (S2/S1)was divided by the y0 value and was again plotted versus the tem-perature (Fig. 10).

Based on the linear fit slope of the graph SLOPE � 1RðT0Þ

dRdT

��T0

� �

and according to Eq. (5), the thermo-optic coefficient of the LiF-

doped sample is equal to dndT ¼ ð7:0� 1:4Þ � 10�6 deg�1. The

thermo-optic coefficient of the un-doped sample is equal to

dndT ¼ ð7:1� 1:3Þ � 10�6 deg�1. According to [10] the thermo-optic

coefficient for a single crystal is equal to dndT ¼ 7:3� 10�6 deg�1.

The values of the thermo-optic coefficients of the polycrystal-line and the single crystal Nd:YAG are very close and imply nosignificant difference in the thermal effect during the operationof lasers, based on polycrystalline or single crystal Nd:YAG.

3.7. The florescence spectra

The florescence spectra of the polycrystalline samples along tothe single crystal are presented at Fig. 11. All the samples showeda consistent profile of the florescence spectra with a relativelynarrow emission peak as presented in Table 3.

3.8. The fluorescence lifetime

The florescence decay as a function of time for various samplesis shown in Fig. 12. By treating the presented data, the decay rateand florescence lifetime for each specimen were estimated and thedata presented in Table 4.

According to the experimental data, all samples show very closelifetime values. The single crystal displays the lowest value of life-time, probably, due to a slightly higher Nd concentration. It is wellknown that the fluorescence lifetime decreases dramatically whenNd concentration exceeds 1% [11].

Table 3The location of the main florescence peak and FWHM.

Specimen Peak position (nm) FWHM (nm)

Polycrystalline Nd:YAG LiF-doped 1064.36 ± 0.03 1.17 ± 0.07Polycrystalline Nd:YAG undoped 1064.42 ± 0.03 1.22 ± 0.07Single crystal Nd:YAG 1064.32 ± 0.03 1.19 ± 0.07

350 400 450 500 550 600 650

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0 Polycrystalline Nd:YAG LiF-doped Polycrystalline Nd:YAG undoped Single crystal Nd:YAG Linear fit- Polycrystalline LiF-doped Linear fit- Polycrystalline Nd:YAG undoped Linear fit- Single crystal Nd:YAG

Arb

itrar

y un

its

Time (μsec)

Fig. 12. The normalized florescence intensity for various Nd:YAG specimens.

Table 4The decay rate and lifetime for the different specimens.

Specimen Nd (at.%) C ðl s�1Þ s ðl sÞ

Polycrystalline Nd:YAG LiF-doped 1 0.00423 236Polycrystalline Nd:YAG undoped 1 0.00425 235Single crystal Nd:YAG 1.1 0.00432 232

0 200 400 600 800 1000

0.000

0.002

0.004

0.006

0.008

0.010 Polycrystalline Nd:YAG LiF-doped. Polycrystalline Nd:YAG undoped. Single crystal Nd:YAG Brown at el. [12]

Temperature , OC

ΔL/L

o

Fig. 13. Linear thermal expansion data (DL/L0) as a function of temperature.

Table 5Values of the coefficients in the regressions.

Specimen a b � 103

Polycrystalline Nd:YAG LiF-doped 4.824 7.99Polycrystalline Nd:YAG undoped 4.868 7.99Single crystal Nd:YAG 5.074 7.83

0 200 400 600 800 10005

6

7

8

9

10

11

12 Polycrystalline Nd:YAG LiF-doped Polycrystalline Nd:YAG undoped Single crystal Nd:YAG

Temperature ,OC

α·10

6 , OC

-1

Fig. 14. Thermal expansion coefficient (a) as a function of temperature.

30 40 50 60 70 80 90 100 110 1200.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80 Polycrystalline Nd:YAG LiF-doped Polycrystalline Nd:YAG undoped Single crystal Nd:YAG Koechner W. [10]

Temperature ,OC

Cp,

J/gK

Fig. 15. Heat capacity dependence of temperature in LiF-doped, undoped and singlecrystal samples.

M. Sokol et al. / Optical Materials 38 (2014) 204–210 209

3.9. Thermal expansion

The linear thermal expansion DL/L0 of the ceramics (Fig. 13)was fitted by a polynome (6). The values of the coefficients in theexpression (6) are presented in Table 5.

DLL0¼ aþ bT þ cT2 þ dT3 ð6Þ

The thermal expansion coefficient a was obtained by taking thederivative of (6) with the respect to temperature T and shown inFig. 13.

The obtained data for both linear thermal expansion and ther-mal expansion coefficient are in a good agreement of valuesreported in the literature for room temperature [10] and for the0–500 K temperature range [12] (see Fig. 14).

c � 106 d � 102 R-square

�1.27 3.89 0.99�1.13 4.69 0.98�1.45 8.83 0.99

30 40 50 60 70 80 90 100 110 120

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6 Polycrystalline Nd:YAG LiF-doped Polycrystalline Nd:YAG undoped Single crystal Nd:YAG

The

rmal

dif

fusi

vity

, mm

2 /s

Temperature ,OC

Fig. 16. Thermal diffusivity in LiF-doped, undoped and single crystal samples.

30 40 50 60 70 80 90 100 110 1206.5

7.0

7.5

8.0

8.5

9.0 Polycrystalline Nd:YAG LiF-doped Polycrystalline Nd:YAG undoped Single crystal Nd:YAG Wang at el. [13]

Temperature ,OC

The

rmal

Con

duct

ivity

, W/m

K

Fig. 17. Temperature dependence of the thermal conductivity in LiF-doped,undoped and single crystal samples.

210 M. Sokol et al. / Optical Materials 38 (2014) 204–210

3.10. Heat capacity

The values of heat capacity of the different ceramics are veryclose (Fig. 15) and the differences may be attributed to the accu-racy of the measurements (<5%).

3.11. Thermal diffusivity and thermal conductivity

The values of thermal diffusivity and thermal conductivity cal-culated using data for thermal expansion coefficient are presentedin Figs. 15 and 16.

Thermal diffusivity and thermal conductivity of the singlecrystal (Figs. 16 and 17, respectively) are higher by 2–3% then thatof the LiF-doped ceramic. Thermal conductivity values for Nd:YAG(0.5%, Nd) single crystal according to Wang et al. [13] at 270–380 K

temperature range are in a good agreement with the obtainedvalues. Polycrystalline Nd:YAG without LiF addition displays thelowest values of thermal diffusivity and thermal conductivity.The latter is attributed to the relatively high fraction of grainboundary imperfections in this ceramic, as shown in our previouscontribution [6].

4. Conclusions

Polycrystalline Nd:YAG (undoped and LiF-doped) specimenswere processed by SPS. The mechanical, optical and thermalproperties of single crystal and of polycrystalline ceramics aredetermined and compared. The results indicate that the polycrys-talline samples display higher mechanical properties (hardness,tensile strength and thermal shock resistance) than their singlecrystal counterparts. The optical transmittance of the single crystalNd:YAG is the highest; the LiF-doped polycrystalline samples dis-played a significantly higher optical transmittance, in particularin the visible wavelength range, than the undoped ones. Withregard to other thermal and optical properties, namely refractionindex, absorption coefficient, extinction ratio, thermo-opticcoefficient, fluorescence, thermal conductivity, and thermal expan-sion no significant differences were detected between the singlecrystals and the polycrystalline ceramic samples.

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