DOI: 10.1021/cg1010743
Thermophysical Properties of a New Nonlinear Optical Na3La9O3(BO3)8 Crystal
2010, Vol. 10 4965–4967
Jianxiu Zhang,† Guochun Zhang,† Yunge Li,† Yang Wu,†,‡ Peizhen Fu,† and Yicheng Wu*,† †
Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China, and ‡Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received August 16, 2010; Revised Manuscript Received September 5, 2010
ABSTRACT: New nonlinear crystal Na3La9O3(BO3)8 (NLBO) with good optical quality and weighing 45 g with size of 38 30 20 mm3 has been grown by the optimized top-seeded solution growth (TSSG) method. Thermophysical properties including melting point, specific heat, thermal expansion, thermal diffusion, and hardness were performed on the crystal to determine thermal behaviors along different crystallographic axes. The specific heat was measured to be 0.45-0.70 J 3 g-1 3 K-1 in the temperature range from 20 to 300 °C, which is lower than that of β-BaB2O4(BBO), LiB3O5 (LBO), and CsLiB6O10 (CLBO). The Vickers hardness were measured to be 1052.67 kg/mm2 for (100) and 1239.67 kg/mm2 for (001) faces, which are greater than most of the commonly used borate crystals such as BBO, LBO, CsB3O5 (CBO), BiB3O6 (BIBO), KBe2(BO3)F2 (KBBF), K2Al2B2O7 (KABO), etc. The thermal expansion coefficients were measured to be Rx = Ry = 7.8 10-6 K-1, and Rz = 15.3 10-6 K-1, resulting in weaker expansion anisotropy than those of BBO, LBO, BIBO, CBO, CLBO, KBBF, La2CaB10O19 (LCB), etc.
1. Introduction Borate-based nonlinear optical (NLO) crystals such as βBaB2O4(BBO), LiB3O5 (LBO), etc. have been used widely for frequency conversion in NLO devices and modern laser systems due to their excellent properties such as high laser damage threshold, high optical quality, high ultraviolet (UV) transparency, and good chemical stability. In 2001, a new promising borate NLO crystal Na3La9O3(BO3)8 (NLBO) was discovered in our group.1 NLBO crystals belong to the hexagonal system with space group P62m. Its coefficient of second harmonic generation (SHG) was measured to be ∼6 times larger than that of KH2PO4 (KDP) and optically transparent from 270 to 2100 nm. In addition, it has excellent physical chemical features such as good mechanical properties, chemically very stable, and free from moisture, which make it an attractive candidate for a wide range of frequency conversion applications in the visible and UV spectral regions. More recently, we compared the SHG performance of NLBO with that of LiB3O5 (LBO) under the same experimental conditions with the 1064 nm pumping source. A conversion efficiency of 58.3% at 532 nm was obtained for NLBO, whereas only 21.5% was obtained for LBO, indicating that NLBO is a highly attractive nonlinear material for frequency conversion of pulses into the visible and ultraviolet.2 Thermal loading on NLBO crystals will degrade NLO performance and may lead to crystal fracture. In this article, we thoroughly report the thermophysical properties of NLBO crystals including melting point, specific heat, thermal expansion, thermal diffusion, hardness, etc. These factors have a great influence on crystal growth and processing and greatly affect its possible NLO applications. 2. Experimental Section NLBO single crystal weighing 45 g with size of 38 30 20 mm3 shown as Figure 1 has been grown by the optimal TSSG method - the *To whom correspondence should be addressed. E-mail:
[email protected]. r 2010 American Chemical Society
Figure 1. As-grown NLBO crystal weighing 45 g with size of 38 30 20 mm3. top seed redipping technique. With this technique, the defects in the seed can be continuously reduced and prevented from extending into the as-grown crystal. The melting point was measured by a differential scanning calorimeter (NETZSCH DSC 404C) made by the NETZSCH Company in Germany at a heating rate of 10 K/min. The crystal bars used for thermal expansion measurements were polished and then annealed at 700 °C for 12 h. The thermal expansion of the NLBO crystal was measured in the temperature range from 30 to 500 °C by using a thermal dilatometer TMA (Perkin-Elemer). The specific heat measurement was performed on a small piece of NLBO crystal weighing 354.3 mg by the method of differential scanning calorimetry using a simultaneous thermal analyzer (NETZSCH STA 449C). The thermal diffusion coefficient was measured by the laser flash method using a laser flash apparatus (NETZSCH LFA 447 Nanoflash) in the temperature range from 30 to 290 °C. A Leitz ORTHOLUX-BK Micro Hardness tester was used to measure the Vickers hardness (Hv) of the NLBO crystal. Indentations were made with a Leitz ORTHOLUX-BK microhardness tester. The diagonals of the impressions were measured using a Leitz matallux II microscope with a calibrated ocular at magnification 500. The measurement was made at room Published on Web 09/29/2010
pubs.acs.org/crystal
4966
Crystal Growth & Design, Vol. 10, No. 11, 2010
Zhang et al.
Figure 3. Thermal expansion and density of the NLBO crystal. Table I. Thermal Expansion Coefficients of the Commonly Used Borate Crystals
Figure 2. The DSC (a), and specific heat curves (b), of the NLBO crystal. temperature and the identation time was kept at 10 s. The maximum load applied for NLBO crystal was 100 N.
3. Results and Discussion Figure 2a shows the DSC curve of the NLBO crystal. From this curve, a single sharp endothermic peak was observed in the temperature range between 1182 and 1187 °C. This peak exhibits the characteristics of a first-order phase transition with a peak temperature at 1185 °C, which was identified as the melting point of the crystal, a value that is higher than BBO (1095 °C),3 LBO (834 °C),4 BiB3O6 (BIBO) (726 °C),5 CsB3O5 (CBO) (842 °C),6 CsLiB6O10 (CLBO) (844.5 °C),7 KBe2(BO3)F2 (KBBF) (1100 °C),8 K2Al2B2O7 (KABO) (1109 °C),9 and BaAlBO3F2 (BABF) (973.6 °C),10 indicating that the thermal stability of NLBO is better than other borate crystals when it is used in laser generation. The thermal expansion of a crystal has a great influence on crystal growth and on its possible applications, so we measured the thermal expansion coefficients of NLBO. The thermal expansion coefficient [Rij] of a crystal is a symmetrical second rank tensor. For NLBO, the unique symmetry axis is a 6-fold axis along the crystallographic c axis. The axes of the crystallographic and crystallophysical coordinate systems in NLBO have the same direction. There are only two independent principal components of the thermal expansion coefficient tensor. These components are X =Y and Z. The values of X, Y, and Z can be obtained by measuring thermal expansion of the a-, b-, and c-oriented crystal samples. The thermal expansion ratio curves along the three crystallographic axes are shown in Figure 3. The thermal expansion ratio is almost linear over the entire measured temperature range from 25 to 500 °C. From Figure 3, we can see that the crystal exhibits only expansion when it is heated, which means that all the thermal expansion coefficients are positive. The mean positive linear thermal expansion coefficients in the temperature range from 25 to 500 °C, which were calculated according to the thermal expansion ratio curves to be Rx = Ry = 7.8 10-6 K-1, and Rz = 15.3 10-6 K-1, respectively. Table I lists the principal axis parameters Rx, Ry, and Rz directions for NLBO with comparison to other important borate crystals such as BBO, LBO, etc. The values of Rz/Rx for BBO (9.0), KBBF (5.05), and negative thermal expansion crystals such as LBO, BIBO, CLBO, etc., and the low value (1.96) for NLBO indicate that the thermal expansion anisotropy is relatively weaker and
expansion coefficients ( 10-6/°C) crystal
point group
Rx
Ry
Rz
ΔT (°C)
NLBO KABO BBO LBO BIBO CBO CLBO KBBF LCB
62m 6m2 32 3m na21 2 222 42d 32H
7.8 8.4 4.0 101 -28 20 20 19 8.7
7.8 7.7 4.0 31 54 11 20 19 8.4
15.3 16.5 36.0 -71 8 42 -22 96 2.3
25-500 25-3009 20-7003 25-6124 -200-30013 20-80014 25-60015 20-70016 25-30017
therefore optical-quality NLBO is relatively easier to obtain from the point of view of thermal expansion anisotropy. For NLBO the volume (bulk) expansion coefficient can be calculated from β = Rx þ Ry þ Rz = 30.9 10-6 K-1. Thermal stresses can be induced in crystals by temperature variation and the magnitude of the stress σ is, σ ¼ EaðT0 - Tf Þ ¼ EaΔT where E is the modulus of elasticity and a is the linear coefficient of thermal expansion. During crystal growth, anisotropic thermal expansion will cause large thermal stress in the crystal, especially when the crystal grows larger or when the temperature varies. When the NLBO crystal is heated above the temperature range of 50-600 °C, the maximum and minimum thermal expansion will occur in the direction of the c and a axis, respectively. The c axis dimension will lengthen to approximately two times that along the a axis since the thermal expansion coefficient along c axis is twice larger than along a axis. As a result, fractures are suggested to occur approximately perpendicular to the c axis if the cooling rate is more than 0.5 oC/day. The density of the crystal can be obtained by extrapolation of the measured thermal expansion. The density versus temperature curve of NLBO in the temperature range from 25 to 500 °C is shown in Figure 3. The density at 25 °C had been measured by using the buoyancy method with a resulting value of 5.102 g cm-3. It can be seen that the density of the crystal almost linearly decreases as the temperature increases and that the density is 5.038 g 3 cm-3 at 500 °C. Specific heat is one of the important factors that influence the crystal thermal properties. Figure 1b shows the specific heat vs temperature curves of the NLBO, BBO, LBO, and CLBO11 crystals. From the curves, it can be seen that the specific heat of NLBO is linear with temperature and increases smoothly from 0.45 to 0.70 J 3 g-1 3 K-1 in the measured temperature range from 20 to 300 °C. It has a lower value
Article
Crystal Growth & Design, Vol. 10, No. 11, 2010
4967
(Mohs: 5.0-5.5), KBBF (Mohs: 0-5.0), and KABO (Mohs: 5.5-6.5). 4. Conclusions The thermal properties of single crystal NLBO were carefully studied by measuring the melting point, thermal expansion, specific heat, thermal diffusion coefficient and hardness. The measured specific heat is lower than that of BBO, LBO, and CLBO. The measured hardness values are greater, and the thermal expansion anisotropy is weaker than most of the commonly used borate crystals such as BBO, LBO, BIBO, CBO, CLBO, KBBF, LCB, etc. Figure 4. Thermal diffusion coefficients of the NLBO crystal.
than the specific heat of BBO, LBO, and CLBO. The lower specific heats of NLBO and BBO would impose a larger internal temperature gradient than in LBO and CLBO when being irradiated by a pulsed laser beam. The thermal conductivity of a crystal is proportional to the specific heat. The low specific heat of NLBO also indicates that the cooling and rotation rates of NLBO should be lower than 0.2 °C/day and 10 rpm, respectively, to avoid accumulating the thermal stress during the crystal growth. The thermal conductivity of a laser crystal is of importance from both fundamental and applied perspectives. The service life of a laser crystal and the output beam quality of lasers could be greatly influenced by the thermal conductivity of the crystal. The crystal morphology, size, and growth period are also directly related to the thermal conductivity of the crystal.12 In our experiment, the thermal diffusion coefficients of NLBO along the a-, b-, and c-axes in the temperature range from 30-290 °C were measured directly and the thermal conductivity coefficients were calculated. The results are shown in Figure 4. From Figure 4, it can be seen that the thermal diffusion components decrease with increasing temperature. Thermal diffusion coefficients of the crystal along the a direction are 1.06 mm2 s-1 at 30 °C and 0.77 mm2 s-1 at 300 °C, along the b direction, they are 1.05 mm2 s-1 at 30 °C and 0.76 mm2 s-1 at 300 °C, and along the c direction, they are 0.92 mm2 s-1 at 30 °C and 0.0.73 mm2 s-1 at 300 °C, respectively. The measured Vickers hardness of NLBO were 1052.67 kg/mm2 (Mohs: 6.8) for (100) and 1239.67 kg/mm2 (Mohs: 7.2) for (001) faces, which are almost identical to YAl3(BO3)4(YAB) and SBBO crystals, whereas greater than those of BBO (Mohs: 4.0-4.8), LBO (Mohs: 5.5-6.0), CBO (Mohs: 4.2-6.1), BIBO
Acknowledgment. This work was supported by the National Natural Science Foundation of China under Grant No. 50802100.
References (1) Wu, Y. C.; Zhang, G. C.; Fu, P. Z.; Chen, C. T. Chinese Patent, Application No. 01134393.1, November 2, 2001, Publication No. CN052I010563. (2) Zhang, J. X.; Wang, G. L.; Liu, Z. L.; Wang, L. R.; Zhang, G. C.; Zhang, X.; Wu, Y.; Fu, P. Z.; Wu, Y. C. Opt. Exp. 2010, 18 (1), 237– 243. (3) Eimerl, D.; Davis, L.; Velsko, S.; Graham, E. K.; Zalkin, A. J. Appl. Phys. 1987, 62, 1968–1971. (4) Wei, L.; Guiqing, D.; Huang, Q.; An, Z.; Liang, J. K. J. Phys. D: Appl. Phys. 1990, 23, 1073–1077. (5) Brenier, A.; Kityk, I. V.; Majchrowski, A. Opt. Commun. 2002, 203, 125–132. (6) Kagebayashi, Y.; Mori, Y.; Sasaki, T. Bull. Mater. Sci. 1999, 22 (6), 971–973. (7) Yu, X. S.; Hu, Z. G. J. Cryst. Growth 2010, 312, 2415–2418. (8) Tang, D. Y.; Xia, Y. N.; Wu, B. C.; Chen, C. T. J. Cryst. Growth 2001, 222, 125–129. (9) Zhang, C. Q.; Wang, J. Y.; Hu, X. B.; Zhang, H. J. J. Syn. Cryst. 2005, 34 (5), 786–789. (10) Hu, Z. G.; Yoshimura, M.; Mori, Y. J. Cryst. Growth. 2004, 260, 287–290. (11) Chen, W. J.; Chen, J. Z. J. Syn. Cryst. 2003, 32 (2), 152–155. (12) Chen, Y. F.; Huang, T. M.; Kao, C. F. IEEE. J. Quantum Elect. 1997, 33 (8), 1424–1429. (13) Becker, P.; Bohaty, L. Cryst. Res. Technol. 1999, 36 (11), 1175– 1180. (14) Rimma, S. B.; Vladimir, S. F.; Julia, E. A.; Stanislav, K. F. Solid State Sci. 2002, 4 (1), 87–91. (15) Rimma, S.; Bubnova, P.; Stanislav, K. F. Phys. Status Solidi B 2008, 245 (11), 2469–2476. (16) Hong, H. P.; Lin, Q. Chem. Online 1994, 12, 51–52. (17) Jing, F. L.; Wu, Y. C.; Fu, P. Z.; Guo, R. Jpn. J. Appl. Phys. 2005, 44, 1812–1814.
