Kyropoulos method

One method we might use is to cool the melt to incipient nuclei-formatlon. toss in the seed-crystal, and allow the melt to freeze into a single crystal. This is the KYROPOULOS method which we will discuss in detail later. Alas, this method only works for a few systems, notably alkali halides (cubic) and the like. We find that we can use a seed-crystal to grow single crystals, but only if we use it under carefully defined conditions. A modified K5rropoulos method has been used for many years to form single-crystal sapphire up to 13.0 inches in dieimeter. Plates cut from such crystals are used as windows and substrates for all sorts of integrated circuits, as well as watch "crystals". 

Figure 3.6 Methods for crystal growth from melt (a) Czochralski method (b) Kyropoulos method (c) Bridgman-Stockbarger method and (d) Vemeuil method.

In the Kyropoulos method, the top of the melt is only slightly above its melting temperature and a seed crystal, which is cooled externally by attaching it to a water cooled heat sink, is placed in contact with the liquid to induce crystallization. The crystal may be rotated and continues to grow in the melt (Figures 37a and b). The size and quality of the crystal produced depends on the temperature gradient induced by the external coolant and the degree to which the temperature of the furnace can be controlled. 

Note that we have reversed the temperature gradient within the furnace in 4.2.22. and that the top is cooler then the bottom, where the melt is first formed. In other variations of the Kyropoulos method, we raise the melt through the freezing point of the melt, by raising it past the baffles of the furnace. Another possible method is one where we use a single- zone fiimaee, stabilize the melt, lower the fumaee temperature to ineipient... 

Potassium lithium niobate KjLijNbjOis KLN Kyropoulos method... 

In the Kyropoulos method, the crystal-liquid interface moves into the melt as... 

KTN (potassium-tantalite-niobate) film crystallinity, 379 crystallographic matching, 382 grain size analyses, 382 highly-oriented, 378 hysteresis loops of, 384 polarization, 384 KTN films, oriented crystallization, 381 effect of thin Seed layer, 381 electrical properties, 383 orientation on substrate, 380 KTN precursors, 378 Kyropoulos method, 378... 

The main difference between the methods of Kyropoulos (see Figure 7.19a) and Czochralski (see Figure 7.19b) is that in the former the seed is permitted to grow into the melt, while in the latter the seed is withdrawn at a rate that keeps the solid-liquid interface more or less in a constant position. Pull rates depend on the temperature gradient at the crystal-melt interface and can vary from 1 to 40mmh . The steeper the gradient the faster the growth rate and, hence, the faster the permissible rate of withdrawal. 

The methods of seed-crystal pulling from the melt (the Kyropoulos and Czochral-ski techniques) have a number of inherent merits as compared to the normal crystallization in a crucible. First, there is the possibility of rotation of the crystal pulled that provides for intensive mixing of the melt before the crystaUization front Secondly, there are no problems associated with crystal adhesion. But, at the same time, these methods are very sophisticated from the technical standpoint... 

Facilities of the ROST type (Fig. 14.2) are also adapted for the modified Kyropoulos technique (with a changeable level of the melt in the crucible) and the automated continuous growth method (with constant melt level) in its two versions for crystal puUirig continuous and discrete. 

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