A laboratory furnace for obtaining crystals

Cabric & Janicijevic (2003) described a design for an air cooler in a laboratory crucible furnace, with the aim of regulating the crystallization rates in branchy Tamman test tubes. In the present paper, we show the development and improvement of the interior and exterior of the cooler, which better regulates different paths and rates of air ̄ow in a chamber furnace (Fig. 1). The setup serves to regulate simultaneous crystallization tests in different Tamman test tubes and on the surface of the melts, with the purpose of obtaining single crystals of compounds with a layered crystal structure [see Cabric in Table 3.1.-5 of Wilke & Bohm (1988)] in the range of melting points 373±1573 K. The procedure involves ®rstly an increase of the voltage until the substances are completely melted. Then, while a constant furnace voltage is maintained, a small air ̄ow is introduced through the cooler. As a result, solidi®cation starts in the test tube capillaries and on the surface of the melts. With the increase of air ̄ow, the crystallization front reaches the surfaces and the bottom of the melt. The shapes of the crystallization fronts and rates in each crucible are regulated by the path and the cross section of the air ̄ow (a), respectively (see Fig. 1). The crystallization rate can also be regulated by distance b, and the temperature gradients are regulated by distance c. The crystallization rate interval in each test tube is regulated by the cross section of the air ̄ow (d ), i.e. the position, and the length of the test tube (e) [see equation (2) of Cabric et al. (1990)]. The temperature gradient in the test tubes is regulated by distance f, i.e. by translational movement of the cooler. Different temperature gradients in the test tubes can be tested simultaneously using an inclined cooler. By varying the shape and dimensions of the cooler (i.e. `teeth and thresholds') and the number and positions of the mounting holes (i.e. `®neness of the sieve'), a set of `cold shelves' and `telescopic test sieves' can be modelled, for testing different numbers of test tubes, temperature gradients, front shapes and crystallization rate intervals. Tamman test tubes of various shapes and dimensions [see p. 591 of Wilke & Bohm (1988)] can be mounted in the mounting holes (and groove, for independent positioning of more test tubes) and thus tested simultaneously. Two coolers, with a set of Tamman test tubes, can be telescopically joined in the shape of the letter `U' and installed in a crucible furnace (`crystallization horseshoe'). The coolers can be joined into a rectilinear shape and installed in a tube furnace in a horizontal position (`crystallization bridge'). In a chamber furnace, several different coolers can be installed (a family group of `cold shelves'), thus increasing the number of simultaneous crystallization tests and enabling single crystals to be obtained quickly, using a low-budget, modular, movable, portable, and simple to make and handle device (with more shapes), i.e. a `crystallization shelf' for laboratory furnaces.