Single- and multi-hole baffles—a heat transfer and fluid flow control for hydrothermal growth

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Abstract

Hydrothermal solution growth is an important technique to grow high-quality single crystals. An autoclave, a closed cylindrical low-carbon steel vessel, is used to grow the large size crystals. A baffle located at the median height divides an autoclave into two chambers. Previous numerical studies indicate that the buoyancy-driven flow within an autoclave is very sensitive to the temperatures on the wall established by the heating. Baffles are required to establish the conditions for growth of high quality single crystals. This paper presents a numerical study of the effects of various baffles on the fluid flow and temperature fields in an industry-size autoclave. A wide range of baffle area openings for a single-hole baffle from 2 to 25%, together with five multi-hole baffles, is investigated. Computational results indicate that changing the baffle hole opening and the number of holes on a baffle are effective ways to control the temperature uniformity in the growing chamber. With a single-hole baffle, a smaller opening leads to a weaker flow field and a more uniform temperature profile. A multi-hole baffle establishes a more uniform temperature in the upper chamber than a single-hole baffle of the same area opening. The number of holes of a multi-hole baffle shows significant effects while the hole arrangements, however, affect the thermal condition only in the near-baffle region.

Introduction

Hydrothermal growth, the industry method of preference for producing high quality single crystals for resonators and surface acoustic wave devices [1], [2], [3], is usually carried out in a closed, cylindrical steel container called an autoclave. Autoclaves are separated into a lower and an upper chamber by a baffle with a small opening and filled with aqueous solution (Fig. 1). The raw material, small pieces of quartz crystals packed into the lower chamber, is dissolved. High quality seed crystal plates are hung in the upper chamber and quartz is deposited at a very low rate from the solution. Two independently controlled heaters maintain the temperatures of the two chambers, a higher temperature TH in the lower chamber and a lower temperature TL in the upper chamber. The transport of nutrient from the dissolving chamber to the growing chamber is through the natural convection flow induced by the temperature differential, ΔT=TH-TL [4], [5].

Natural convection of solution has strong effects on the growth morphology and quality of the grown crystals [6], [7]. Therefore, extensive studies have been carried out to investigate the fluid flow and heat transfer in industry-size hydrothermal autoclaves [8], [9]. A recent numerical study by the present authors found that the flow and thus the temperature fields in the autoclave are significantly different between cases with and without a baffle [9]. It is generally believed that the baffle design has a strong influence on the crystal growth but no study (experimental and/or numerical) of these effects has been reported. This paper presents a systematic study of the effects of the baffle designs on the fluid flow and temperature fields in an industry-size autoclave.

Section snippets

Physical model and mathematical formulation

A simplified physical model for the fluid flow and heat transfer in an industry-size autoclave is shown in Fig. 2. Only the aqueous solution inside the autoclave is considered. The geometry of the model is from an actual industry autoclave with diameter D=0.33m, aspect ratio H/D=10. Isothermal boundary conditions are employed for the walls of both the lower and upper chambers with temperatures TH=410°C and TL=400°C, respectively. Correspondingly the flow driving temperature differential is ΔT=TH

Single-hole baffles

Since a baffle is used to limit the flow and to separate the two chambers into the two temperature zones, baffles with a hole-opening larger than 25% by area are not recommended in practice. On the other hand, for baffle hole area openings smaller than 2% the thickness of the baffle will come into play. When the diameter of the baffle hole is comparable to the thickness of the baffle, fluid and heat exchange rates are significantly reduced. The hydrothermal process cannot be maintained as a

Conclusions

This paper examines the fluid flow and heat transfer in industry-size autoclaves with various baffles for hydrothermal crystal growth. Single-hole baffles with the hole area opening ranging from 2 to 25% and multi-hole baffles with a fixed 15% hole opening and various numbers of holes and hole arrangements have been investigated. Five kinds of multi-hole baffles, including one 4-hole baffle, three 8-hole baffles, and one 16-hole baffle, are proposed and studied. It is found that with a

Acknowledgments

This research is partially founded by National Science Foundation CRCD Project, Grant #9980325. The authors also give thanks to Sawyer Research Products, Inc. and College of Engineering in the University of Akron.

References (11)

  • R.A. Laudise et al.

    Hydrothermal crystal growth

  • H. Li et al.

    J. Crystal Growth

    (2003)
  • H. Li et al.

    J. Crystal Growth

    (2004)
  • P. Vigoureus, Quartz Oscillators and their Applications, Eyre and Spottiswoode Limited, 1939, p....
  • C.Z. Rosen et al.

    Piezoelectricity

    (1992)
There are more references available in the full text version of this article.

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