Growth of sapphire profiles by EFG method and the use in structural analysis

J. Polák

Crytur spol. s. r. o., Palackého 175, Turnov, 51101

polak@crytur.cz

Single crystalline aluminum oxide - sapphire - belongs to one of the most important artificially produced material due to its unique physical and chemical properties. Sapphire has a high refractive index and a broad transmission band spanning the ultraviolet, visible and infrared bands. Sapphire is very hard material (Moh's 9) and has an extraordinary mechanical strength up to melting point 2053 °C, very good thermal conductivity, high electric resistivity and outstanding chemical resistance even to strong acids, bases and fluorination agents. All this makes sapphire a much sought-after material in industry and science.

Typically is sapphire grown as bulk crystal by Czochralski or Kyropoulos method. However crystal machining is difficult in comparison to other materials due to its high hardness. First growth of profiled single crystal sapphire was published in 1971 by La Belle and Mlavsky (TYCO laboratories) [1]. EFG (Edge-defined Film-fed Growth) method is in principle similar to older one (1953) of A. V. Stepanov for growth of shaped single crystals of metals [2]. Review of modern modifications of EFG and related methods was published [3][4]. Main advantages of EFG method are growth of near net shape decreasing production costs, possibility of growth of complicated shapes and large scale profiles, faster growth rate and also automated process control.

Crytur produces sapphire single crystals, rods, ribbons, tubes and fibers since 1978 and is the only EU manufacturer of the EFG sapphire today.

Sapphire tubes are used in various industrial and scientific applications demanding extraordinary material properties. High pressure, high temperature, together with harsh chemicals are the right reason to use sapphire tube instead of quartz or corundum ceramics. Tubes produced by EFG method can be designed to according to demands of specific applications like UV/VIS/IR measurements, nuclear magnetic (NMR) or X-ray (XRS) spectroscopy.

High pressure MNR tubes have to take hold of enough volume and typical outside diameter is 10 or 5 mm and wall thickness 1 - 1.5 mm. Pressures used during measurements are in range 10-150 MPa, the highest pressure require tubes with special collar design and post growth tube annealing [5].

In XRS high pressure XRS capillaries are used. Outside diameter of such a capillary is typically 1.57 mm (1/16" compatible with laboratory equipment) or 1.0 mm and wall thickness 0.2 - 0.4 mm are used. Crucial parameter for XRS is absorption coefficient of sapphire for Kα lines of RTG sources.

Table 1. Absorption coefficients and transmittance at important Kα lines

	Kα (keV)	µ/ρ (cm^2/g^-1)	d (cm)	τ
Cu	8,04	3,24E+01	0,10	0,0%
Mo	17,441	3,09E+00	0,10	29,2%
W	58,856	2,46E-01	0,10	90,7%

Figure 1. Absorption coefficients of Al₂O₃ - sapphire in dependence of radiation energy

The maximum pressure p_max is a function of the tensile strength τ of the material and the quotient of the outer and inner diameters d_o and d_i. According to [5] relation can be approximately expressed as

p_max = τ ln (d_o²/d_i²). (1)

Figure 2. Pressure test of sapphire capillary OD/ID 1.0/0.6 mm, tube burst at 1550 bar after 7.5 min

Due to possible presence of material defects like bubbles or material stress in sapphire cell the maximum operating pressure has to be verified by internal pressure test. Some tubes show time dependent pressure threshold, this process will be further studied. Also pressure test at elevated temperature will be important to set conditions which sapphire cell can withstand.

1. H. E. LaBelle, Jr., Nature, 216, (1967), 574.

2. A. V. Stepanov, Zh. Tech. Fiz., 29, (1959), 382 (in Russian).

3. P. I. Antonov, Progress in Crystal Growth and Characterization of Materials, 216, (2002), 63.

3. E. R. Dobrovinskaya, Sapphire: Material, Manufacturing, Applications, Springer. 2009.

5. M. R. Arnold, Journal of Magnetic Resonance, 161, (2003), 127.

My great acknowledgement for our colleagues M. Steinhart (University of Pardubice) and W. Kremer (University of Regensburg) for pressure tests of XRS and NMR sapphire cells.