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Growth of birefringent crystal α-BaB2O4 by Bridgman method
Materials Letters 57 (2003) 1141 – 1444 www.elsevier.com/locate/matlet
Growth of birefringent crystal a-BaB2O4 by Bridgman method Hongbing Chen a,*, Haiping Xia a, Jinhao Wang a, Jianli Zhang a, Xinmin Zhang a, Jiayue Xu b, Shiji Fan b a
Institute of Optoelectric Materials, Faculty of Information Science and Engineering, Ningbo University, Ningbo, Zhejiang 315211, PR China b Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 28 May 2002; accepted 10 June 2002
Abstract The growth of novel birefringent crystal a-BaB2O4 by the modified Bridgman method has been reported in this paper. By means of the optimum conditions such as stoichiometric feed materials, sealed platinum crucibles, [001] growth direction, growth rate less than 0.3 mm/h and convex or plane solid – liquid interface with temperature gradient 30 – 40 jC/cm, large-size crystals without macro-defects have been grown successfully. D 2002 Elsevier Science B.V. All rights reserved. PACS: 81.10.-h, 81.10.Fq Keywords: a-BaB2O4; Crystal growth; Bridgman method; Birefringent crystal
1. Introduction From the phase equilibrium in BaO – B2O3 system, the compound BaB2O4 is known to crystallize in two modifications, the low-temperature h-phase and the high-temperature a-phase. The transformation between the two phases occurs at the temperature of 925 jC under normal atmosphere [1]. The low-temperature phase BaB2O4 (h-BaB2O4) crystal is known as an excellent nonlinear optical crystal [2]. The hightemperature phase (a-BaB2O4) is a negative uniaxial crystal with a trigonal structure and is similar to hBaB2O4 in physical and chemical properties, but the nonlinear optical property is vanished because of the
* Corresponding author. E-mail address: [email protected] (H. Chen).
centric symmetry with its crystal structure. However, a-BaB2O4 crystal has been reported as a novel birefringent optical crystal since it has large birefringence over the broad transparent range from 189 to 3500 nm [3,4]. This crystal can be used as an excellent substitute for the birefringent crystals such as natural calcite, TiO2 and LiNbO3 in optical polarizing components and walk-off beam splitters, especially for highpower operations in the ultraviolet wave band. a-BaB2O4 crystal was initially grown by the Czochralski method [3], but there is no report about other techniques for the crystal growth so far. The main difficulties for Czochralski growth of the crystal are: (1) continuous composition change of the melts during growth because of serious volatilization of B2O3, (2) cracking in as-grown crystals due to the anisotropy of thermal expansion. Fan has applied a modified Bridgman process to grow large-size piezoelectric crystal
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 1 0 0 4 - 2
1442
H. Chen et al. / Materials Letters 57 (2003) 1141–1444
Li2B4O7 successfully [5]. Considering that a-BaB2O4 is melted congruently at moderate temperature, the vertical Bridgman method is a favorable technique to grow the crystal. The modified Bridgman technique was used to grow a-BaB2O4 crystals in our laboratory. In our process, sealed platinum crucibles and small temperature gradients in the Bridgman furnace were adopted to limit volatilization of B2O3 and cracking of as-grown crystals. The recent results of the Bridgman growth of a-BaB2O4 crystal are presented in this paper.
2. Experimental procedure The feed material for a-BaB2O4 crystal growth was synthesized with the high-purity BaCO3 (4 N) and H3BO3 (4 N). Taking into consideration the volatilization of B2O3 during the starting material was sintered, the molar percent of B2O3 in the starting material should be more than its stoichiometry. The molar ratio of BaCO3 to H3BO3 was adopted as 1:2.1 in order to make up for the volatilization of B2O3. The mixture was ground for 3 h in a mortar and then sintered at the temperature of 1160– 1200 jC for 12 h in a furnace. The polycrystalline bar obtained was identified to be a-BaB2O4 phase by X-ray powder diffraction and DTA analysis. Fig. 1 shows the scheme of a resistively heated vertical Bridgman furnace used for growing a-BaB2O4 single crystal. The furnace temperature of furnace was adjusted by a DWT-702 fine temperature controller with an accuracy of F 0.5 jC during the experiments. The temperature of the melting zone was usually controlled at 1175 – 1205 jC, which was 80– 110 jC higher than the melting point of crystal. The platinum crucible used in crystal growth was 25 mm in diameter and 200– 250 mm in length with a seed well of 10 mm in diameter at the conical bottom to hold the seed crystal. The crucible was installed in a refractory tube filled with Al2O3 powder to isolate it from external temperature fluctuations. In order to detect the axial temperatures along the crucible, the crucible was fitted with two Pt– Pt/Rh 10% thermocouples, an upper one and a lower one. Fig. 2 shows the axial temperature distribution measured by the lower thermocouple near the crucible. The furnace consists of three zones according to the temperature distribution. The solid –
Fig. 1. Scheme of vertical Bridgman furnace.
liquid interface will be positioned in the middle zone, where the temperature gradient is about 30 – 40 jC/cm. The feed material of 200 – 300 g was charged into the crucible for crystal growth. In order to obtain the seed crystal, the initial tries of the growth were done by spontaneous nucleation from the seed wells. Transparent single crystals with size of U10 40– 60 mm were chosen as the seeds after the crystals were oriented, cut and ground. The seeds were put in the seed wells, and then the feed materials were filled in the cylinder of crucibles. The assembled crucible was sealed in order to prevent the volatilization of the melt during crystal growth. After the furnace had been heated to the controlled temperature, the seeding process was performed by adjusting the crucible to such a position that only the top of the seed was melted. The feed material and the top of the seed were kept at the melting state for several hours so that a stable solid – liquid interface can be established on the top region of the seed. The temperature gradient across solid –liquid (s – l) interface was 30 –40 jC/cm. Growth process was driven by lowering the crucible at a rate less than 0.3 mm/h. To
H. Chen et al. / Materials Letters 57 (2003) 1141–1444
Fig. 2. Axial temperature profile in furnace.
prevent cracking resulting from thermal stress in the crystals, as-grown boule was heat-treated in the lower zone during growth. The furnace was cooled slowly to room temperature after the growth had finished. The crucible was stripped after taking out of the refractory tube and as-grown crystal was obtained.
3. Results and discussion The strict stoichiometry of the feed material is important for growing high-quality crystals. Compare to the Czochralski technique, the modified Bridgman process can avoid the volatilization of B2O3 effectively by sealing the crucible, so the composition of the melts is kept in constant during growth. In most cases, there are some white aggregations on the top of boules corresponding to the final portion of the melt to freeze. The aggregation layer is only 1 to 2 mm in thickness due to the deviation from the strict stoichiometry of the feed materials. Using the crystal fragments and the sintered polycrystalline bars with near stoichiometric composition as feed materials, respectively, the crystal growth experiments in the two cases were carried out for comparison. The result indicates
1443
that constitutional supercooling occurs easily for the melts with deviation from stoichiometric composition, while the crystal fragments are more suitable for growing the perfect crystals without the aggregations. The experiments show that the growth rate of aBaB2O4 crystal cannot be fast because of the high viscous melt. In the Bridgman growth, the convection of the melt is weaker due to the absence of seed rotation and the smaller temperature gradient. The fast growth causes a continuous or intermittent opaque region parallel to the growth direction in the crystals. The crystals will even become opaque because of the serious constitution supercooling if the growth rate is controlled too fast. It was confirmed that the growth rate should be less than 0.3 mm/h to obtain transparent crystals by Bridgman method. Another difficulty to grow large-size a-BaB2O4 crystals is the cracking occurring in the crystal growth process. In order to compare the crystal growth with different orientations, several crucibles with [100], [010] or [001] seeds were put in a Bridgman furnace in which the crystals in different orientations could be grown simultaneously at the same conditions. For [100], [010] growth, the crystals tend to crack along (001) cleavage face due to the anisotropy of thermal expansion, while transparent, crack-free crystals were obtained for [001] growth. The results indicate that the orientation [001] should be the optimum growth direction for large-size a-BaB2O4 crystals. The solid– liquid interface, where the crystallization takes place, is another important factor for growing high-quality crystals. The temperature gradient of solid– liquid interface in the Bridgman furnace is usually smaller than that in Czochralski growth, therefore the cracking is easier to be avoid during Bridgman growth. A moderate temperature gradient of solid– liquid interface is essential for the Bridgman crystal growth. Under smaller temperature gradient, the seeding operation becomes more difficult although the cracking can be avoided easily, while larger temperature gradient is convenient for the seeding but tends to bring about the cracking. In our process, the temperature gradient of the solid – liquid interface is controlled at 30 – 40 jC/cm. In addition, the shape of the solid – liquid interface can influence the quality of crystals. Coring in the crystals occurs more frequently as the solid– liquid interface is concave. When the solid – liquid interface is con-
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H. Chen et al. / Materials Letters 57 (2003) 1141–1444
4. Conclusion
Fig. 3. a-BaB2O4 crystal grown by Bridgman method.
trolled to be convex or plane, the quality of crystals is improved because the impurities can be easily expelled out during growth. By means of the optimum conditions such as stoichiometric feed materials, sealed platinum crucibles, [001] growth direction, growth rate less than 0.3 mm/h and convex or plane solid –liquid interface with temperature gradient 30– 40 jC/cm, a-BaB2O4 single crystals have been grown successfully by the described process. The as-grown crystals take the shape of the crucibles. A colorless and transparent crystal with the size of 25 mm in diameter by 55 mm in length is shown in Fig. 3. The crystal was examined to be free from scattering centers by a He – Ne laser. The optical properties are under investigation now.
The large-size a-BaB2O4 crystals without macrodefects have been grown successfully by the modified Bridgman process. The continuous composition change of the melts during growth can be avoided because the volatilization of B2O3 is limited by sealed platinum crucibles. The cracking of as-grown crystals can be decreased due to the lower thermal stress under the small temperature gradient. The results prove that the modified Bridgman process is promising for the growth of large-size a-BaB2O4 single crystals with high quality.
Acknowledgements This work is supported by the Doctor Science Foundation of Ningbo City under Grant no. 01J20300-14.
References [1] K.H. Hubner, Neues Jahrb. Mineral. Monatsh. (1969) 325. [2] C.T. Chen, Y.C. Wu, R.K. Li, J. Cryst. Growth 99 (1990) 790. [3] G. Zhou, J. Xu, X. Chen, H. Zhong, S. Wang, K. Xu, P. Deng, F. Gan, J. Cryst. Growth 191 (1998) 517. [4] G. Zhou, J. Xu, X. Chen, W. Chen, H. Li, P. Deng, F. Gan, J. Synth. Cryst. 29 (5) (2000) 68. [5] S. Fan, G. Shen, W. Wang, J. Li, X. Le, Bridgman growth of Li2B4O7 crystals, J. Cryst. Growth 99 (1990) 811.
Growth of birefringent crystal a-BaB2O4 by Bridgman method Hongbing Chen a,*, Haiping Xia a, Jinhao Wang a, Jianli Zhang a, Xinmin Zhang a, Jiayue Xu b, Shiji Fan b a
Institute of Optoelectric Materials, Faculty of Information Science and Engineering, Ningbo University, Ningbo, Zhejiang 315211, PR China b Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 28 May 2002; accepted 10 June 2002
Abstract The growth of novel birefringent crystal a-BaB2O4 by the modified Bridgman method has been reported in this paper. By means of the optimum conditions such as stoichiometric feed materials, sealed platinum crucibles, [001] growth direction, growth rate less than 0.3 mm/h and convex or plane solid – liquid interface with temperature gradient 30 – 40 jC/cm, large-size crystals without macro-defects have been grown successfully. D 2002 Elsevier Science B.V. All rights reserved. PACS: 81.10.-h, 81.10.Fq Keywords: a-BaB2O4; Crystal growth; Bridgman method; Birefringent crystal
1. Introduction From the phase equilibrium in BaO – B2O3 system, the compound BaB2O4 is known to crystallize in two modifications, the low-temperature h-phase and the high-temperature a-phase. The transformation between the two phases occurs at the temperature of 925 jC under normal atmosphere [1]. The low-temperature phase BaB2O4 (h-BaB2O4) crystal is known as an excellent nonlinear optical crystal [2]. The hightemperature phase (a-BaB2O4) is a negative uniaxial crystal with a trigonal structure and is similar to hBaB2O4 in physical and chemical properties, but the nonlinear optical property is vanished because of the
* Corresponding author. E-mail address: [email protected] (H. Chen).
centric symmetry with its crystal structure. However, a-BaB2O4 crystal has been reported as a novel birefringent optical crystal since it has large birefringence over the broad transparent range from 189 to 3500 nm [3,4]. This crystal can be used as an excellent substitute for the birefringent crystals such as natural calcite, TiO2 and LiNbO3 in optical polarizing components and walk-off beam splitters, especially for highpower operations in the ultraviolet wave band. a-BaB2O4 crystal was initially grown by the Czochralski method [3], but there is no report about other techniques for the crystal growth so far. The main difficulties for Czochralski growth of the crystal are: (1) continuous composition change of the melts during growth because of serious volatilization of B2O3, (2) cracking in as-grown crystals due to the anisotropy of thermal expansion. Fan has applied a modified Bridgman process to grow large-size piezoelectric crystal
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 1 0 0 4 - 2
1442
H. Chen et al. / Materials Letters 57 (2003) 1141–1444
Li2B4O7 successfully [5]. Considering that a-BaB2O4 is melted congruently at moderate temperature, the vertical Bridgman method is a favorable technique to grow the crystal. The modified Bridgman technique was used to grow a-BaB2O4 crystals in our laboratory. In our process, sealed platinum crucibles and small temperature gradients in the Bridgman furnace were adopted to limit volatilization of B2O3 and cracking of as-grown crystals. The recent results of the Bridgman growth of a-BaB2O4 crystal are presented in this paper.
2. Experimental procedure The feed material for a-BaB2O4 crystal growth was synthesized with the high-purity BaCO3 (4 N) and H3BO3 (4 N). Taking into consideration the volatilization of B2O3 during the starting material was sintered, the molar percent of B2O3 in the starting material should be more than its stoichiometry. The molar ratio of BaCO3 to H3BO3 was adopted as 1:2.1 in order to make up for the volatilization of B2O3. The mixture was ground for 3 h in a mortar and then sintered at the temperature of 1160– 1200 jC for 12 h in a furnace. The polycrystalline bar obtained was identified to be a-BaB2O4 phase by X-ray powder diffraction and DTA analysis. Fig. 1 shows the scheme of a resistively heated vertical Bridgman furnace used for growing a-BaB2O4 single crystal. The furnace temperature of furnace was adjusted by a DWT-702 fine temperature controller with an accuracy of F 0.5 jC during the experiments. The temperature of the melting zone was usually controlled at 1175 – 1205 jC, which was 80– 110 jC higher than the melting point of crystal. The platinum crucible used in crystal growth was 25 mm in diameter and 200– 250 mm in length with a seed well of 10 mm in diameter at the conical bottom to hold the seed crystal. The crucible was installed in a refractory tube filled with Al2O3 powder to isolate it from external temperature fluctuations. In order to detect the axial temperatures along the crucible, the crucible was fitted with two Pt– Pt/Rh 10% thermocouples, an upper one and a lower one. Fig. 2 shows the axial temperature distribution measured by the lower thermocouple near the crucible. The furnace consists of three zones according to the temperature distribution. The solid –
Fig. 1. Scheme of vertical Bridgman furnace.
liquid interface will be positioned in the middle zone, where the temperature gradient is about 30 – 40 jC/cm. The feed material of 200 – 300 g was charged into the crucible for crystal growth. In order to obtain the seed crystal, the initial tries of the growth were done by spontaneous nucleation from the seed wells. Transparent single crystals with size of U10 40– 60 mm were chosen as the seeds after the crystals were oriented, cut and ground. The seeds were put in the seed wells, and then the feed materials were filled in the cylinder of crucibles. The assembled crucible was sealed in order to prevent the volatilization of the melt during crystal growth. After the furnace had been heated to the controlled temperature, the seeding process was performed by adjusting the crucible to such a position that only the top of the seed was melted. The feed material and the top of the seed were kept at the melting state for several hours so that a stable solid – liquid interface can be established on the top region of the seed. The temperature gradient across solid –liquid (s – l) interface was 30 –40 jC/cm. Growth process was driven by lowering the crucible at a rate less than 0.3 mm/h. To
H. Chen et al. / Materials Letters 57 (2003) 1141–1444
Fig. 2. Axial temperature profile in furnace.
prevent cracking resulting from thermal stress in the crystals, as-grown boule was heat-treated in the lower zone during growth. The furnace was cooled slowly to room temperature after the growth had finished. The crucible was stripped after taking out of the refractory tube and as-grown crystal was obtained.
3. Results and discussion The strict stoichiometry of the feed material is important for growing high-quality crystals. Compare to the Czochralski technique, the modified Bridgman process can avoid the volatilization of B2O3 effectively by sealing the crucible, so the composition of the melts is kept in constant during growth. In most cases, there are some white aggregations on the top of boules corresponding to the final portion of the melt to freeze. The aggregation layer is only 1 to 2 mm in thickness due to the deviation from the strict stoichiometry of the feed materials. Using the crystal fragments and the sintered polycrystalline bars with near stoichiometric composition as feed materials, respectively, the crystal growth experiments in the two cases were carried out for comparison. The result indicates
1443
that constitutional supercooling occurs easily for the melts with deviation from stoichiometric composition, while the crystal fragments are more suitable for growing the perfect crystals without the aggregations. The experiments show that the growth rate of aBaB2O4 crystal cannot be fast because of the high viscous melt. In the Bridgman growth, the convection of the melt is weaker due to the absence of seed rotation and the smaller temperature gradient. The fast growth causes a continuous or intermittent opaque region parallel to the growth direction in the crystals. The crystals will even become opaque because of the serious constitution supercooling if the growth rate is controlled too fast. It was confirmed that the growth rate should be less than 0.3 mm/h to obtain transparent crystals by Bridgman method. Another difficulty to grow large-size a-BaB2O4 crystals is the cracking occurring in the crystal growth process. In order to compare the crystal growth with different orientations, several crucibles with [100], [010] or [001] seeds were put in a Bridgman furnace in which the crystals in different orientations could be grown simultaneously at the same conditions. For [100], [010] growth, the crystals tend to crack along (001) cleavage face due to the anisotropy of thermal expansion, while transparent, crack-free crystals were obtained for [001] growth. The results indicate that the orientation [001] should be the optimum growth direction for large-size a-BaB2O4 crystals. The solid– liquid interface, where the crystallization takes place, is another important factor for growing high-quality crystals. The temperature gradient of solid– liquid interface in the Bridgman furnace is usually smaller than that in Czochralski growth, therefore the cracking is easier to be avoid during Bridgman growth. A moderate temperature gradient of solid– liquid interface is essential for the Bridgman crystal growth. Under smaller temperature gradient, the seeding operation becomes more difficult although the cracking can be avoided easily, while larger temperature gradient is convenient for the seeding but tends to bring about the cracking. In our process, the temperature gradient of the solid – liquid interface is controlled at 30 – 40 jC/cm. In addition, the shape of the solid – liquid interface can influence the quality of crystals. Coring in the crystals occurs more frequently as the solid– liquid interface is concave. When the solid – liquid interface is con-
1444
H. Chen et al. / Materials Letters 57 (2003) 1141–1444
4. Conclusion
Fig. 3. a-BaB2O4 crystal grown by Bridgman method.
trolled to be convex or plane, the quality of crystals is improved because the impurities can be easily expelled out during growth. By means of the optimum conditions such as stoichiometric feed materials, sealed platinum crucibles, [001] growth direction, growth rate less than 0.3 mm/h and convex or plane solid –liquid interface with temperature gradient 30– 40 jC/cm, a-BaB2O4 single crystals have been grown successfully by the described process. The as-grown crystals take the shape of the crucibles. A colorless and transparent crystal with the size of 25 mm in diameter by 55 mm in length is shown in Fig. 3. The crystal was examined to be free from scattering centers by a He – Ne laser. The optical properties are under investigation now.
The large-size a-BaB2O4 crystals without macrodefects have been grown successfully by the modified Bridgman process. The continuous composition change of the melts during growth can be avoided because the volatilization of B2O3 is limited by sealed platinum crucibles. The cracking of as-grown crystals can be decreased due to the lower thermal stress under the small temperature gradient. The results prove that the modified Bridgman process is promising for the growth of large-size a-BaB2O4 single crystals with high quality.
Acknowledgements This work is supported by the Doctor Science Foundation of Ningbo City under Grant no. 01J20300-14.
References [1] K.H. Hubner, Neues Jahrb. Mineral. Monatsh. (1969) 325. [2] C.T. Chen, Y.C. Wu, R.K. Li, J. Cryst. Growth 99 (1990) 790. [3] G. Zhou, J. Xu, X. Chen, H. Zhong, S. Wang, K. Xu, P. Deng, F. Gan, J. Cryst. Growth 191 (1998) 517. [4] G. Zhou, J. Xu, X. Chen, W. Chen, H. Li, P. Deng, F. Gan, J. Synth. Cryst. 29 (5) (2000) 68. [5] S. Fan, G. Shen, W. Wang, J. Li, X. Le, Bridgman growth of Li2B4O7 crystals, J. Cryst. Growth 99 (1990) 811.