Use of Mo as the material of crucible for the crystal growth refractory oxides (> 1600⁰ C), gives the possibility, in some cases, replace the expensive iridium. Molybdenum is technologically better iridium. For example, the molybdenum crucible can be machined from billet, with any thickness and configuration of the walls and bottom of the crucible, which is impossible for iridium (in the case of large-sized products). However, the increased ability of molybdenum to oxidation at high temperatures imposes limitations on the environment in which heating is performed. Heating can be in a vacuum, inert or reducing atmosphere.

For use as a molybdenum as the material of the crucible, it is necessary to carry out selection of the optimum combination of the growth atmosphere and insulation material. In the first series of experiments a ceramic zirconium was used as insulation. In the second series (intermediate version), used a combination of zirconium and carbon insulation. In the third series, only carbon insulation was used.                               

The first series of experiments showed that the use of oxide thermal insulation in combination with an inert environment does not protect molybdenum against oxidation due to residual oxygen in the chamber, and the ceramic oxide as a source of adsorbed oxygen. Even it's a small amount, cause the flow of gas transport reaction associated with many possible volatile molybdenum oxides [1]. Transport reaction resulted in the precipitation of metallic molybdenum as an inner layer of zirconium insulation and at the crucible in the form of bundles of molybdenum needle (Fig1). 

                (а)                                   (b)                                  (c) 

Fig.1. Photo beams needles: (a) needle beams at the surface of the crucible; (b) image the beam at increasing x60; (c) is the same beam with increasing x250

  The second series showed that the use of carbon thermal insulation, as sources of reducing atmosphere are protects molybdenum partly. The action radius of protection is limited. The surface of the molybdenum remoted from carbon, still subjected to degradation with the formation of bundles of needles and needles on zirconium insulation.  Fig.2. shows that   growths at the crucible bottom are absent, and its number reaches a maximum on the top. 

                            (a)                                              (b)   

Fig.2. Photo of the surface of the crucible: (a) the general pattern of distribution of bundles of needles; (b) the beam close-up.

The third series of experiments showed complete protection against oxidation of the surface of the molybdenum from the oxidation and transport reaction. However, it also showed that heating above 2100⁰ C leads to another destruction  carbidization molybdenum surface. Carbidization is a formation of eutectic between molybdenum, and molybdenum carbide, with a melting point of about 2200⁰ C. At Fig.3 shown dislodging of the wall of the crucible, which destroys them, and reduces the life of the crucible.

                           (a)                                           (b)

Fig.3. Photos of the destruction of the crucible as a result of carbidization surface: (a) nodules on the stand; (b) traces of dislodging of the surface of the crucible.

Thus, the chosen combination of thermal insulation and protective atmosphere, allowing to use the molybdenum for growing oxide crystals. At the same time, it should be noted that: the melt should not interact with molybdenum; must not interact with CO, and its melting temperature should not exceed 2100⁰ C, or need to take special measures to solve the problem of carbidization. Which are: technological re-equipment of the thermal unit, and the maximum distance graphite-molybdenum.

1. D. Kostomarov, Kh. Bagdasarov, S. Kobzareva, and E. Antonov. Chemical Processes and Composition of the Gas and Solid Phases in the   Al2O3–Mo System in the Temperature Range 2327–2500 K . Crystallography Reports, 2008, Vol. 53, № 4.

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1. Features of YAG crystal obtaining by Czochralski method using Mo crucibles