Internal crystallization method

1.     Introduction

Single-crystalline eutectic oxide fibres are needed to reinforce metal, intermetallic and ceramic matrices to obtain high temperature composites. However, up to recent time, such fibres, which can be produced by crystallising the melts, have been too expensive to be used in structural composites. Well-known methods of fibre growth such as edge defined, film-fed growth (EFG) [[i]], laser heated pedestal growth (LHPG) [[ii],[iii]], and so called m-pulling-down method [[iv]] can be used for producing fibres only for special applications. A method to produce single-crystalline and eutectic oxide fibres based on the internal crystallization, that is crystallization of the oxide melt infiltrated into continuous channels made in an auxiliary matrix, i.e. molybdenum one [[v],[vi]], and then extraction of the fibres from the auxiliary matrix by i.e. chemical dissolution of it [[vii],[viii]] looks a promising starting point for the development of a fabrication technology of heat-resistant structural fibres. The method is relative simple and requires sufficiently small energy input into a real process. The method allows obtaining a variety of oxide fibres, such as sapphire [8 ] and YAG [[ix]] of a homogeneous crystallographic orientation, alumina-YAG eutectic [7], and single crystalline mullite [[x]].  

 

2.     The fundamentals of the internal crystallization method

The method of internal crystallization was developed in Laboratory of Reinforced Systems of Solid State Physics Institute initially to produce oxide-matrix/molybdenum-matrix composites, which were considered mainly as model ones. The concept of the method is illustrated in Figure 1.

Figure 1. Schematic of the internal crystallization method (ICM).

First of all a molybdenum carcass with continuous cylindrical channels is obtained by diffusion bonding of an assemblage of molybdenum wires and foils. The carcass is then put into contact with an oxide melt; capillary forces yield the infiltration of the channels with a melted fibre material. The oxide/molybdenum block is then moved to a cold zone of the furnace to crystallize the fibres. This is the basic scheme of the ICM [5 ,6], a variation of the basic scheme with single crystalline seeds at the top of the carcass (Figure 2) allows obtaining a fibre set of a homogeneous crystallographic orientation [8]. The final step in the obtaining fibres process is dissolution of molybdenum in an HCl/HNO3/H2O mixture.

Figure 2. A set of the molybdenum carcasses with seeds at the top.

A variety of the oxide fibres, such as sapphire [8 ] and YAG [9 ] of a homogeneous crystallographic orientation alumina-YAG eutectic [7 ], and single crystalline mullite [10 ] have been obtained by using the method. Some illustrations of the fibres obtained are presented in Figure 3 .

A bounded bundle of sapphire fibres.

 

A bounded bundle of sapphire fibres.

 

 

 

 

 

 

 

 

Sapphire fibres photographed in polarised light.

 

 

Single crystalline yttrium-aluminium garnet (YAG) fibre  

 

 

Mullite fibres photographed in polarised light.

Figure 3. Some fibres obtained by using the internal crystallisation method.

The infiltration and crystallization procedures are conducted in either commercially available universal crystallizing machine CRYSTAL or a special machine, CHIR, built up for the particular aim to fabricate ICM-fibres. CRYSTAL has the process zone with an 8 kHz-induction-heated-graphite-susceptor/molybdenum-crucible set-up. The zone limits the length of the molybdenum carcass by about 100 mm. The CHIR¢s process zone with a radiant tungsten heater allows dealing with molybdenum blocks up to about 250 mm. While dealing with 5 to 8 molybdenum blocks in one process, 120 to 150 g of fibres can be produced in CRYSTAL machine. Productivity rate of CHIR is about one order of the magnitudes higher. Because crystallizing a batch of the fibres by using ICM is actually similar to making bulk crystals, an expected cost of ICM fibres is of the same order of magnitudes as that of bulk crystals.

 

References  


[i]. H, E. LaBelle, Jr., A. I. Mlavsky,. Nature, 1967, 216, 574 - 575.

[ii] J.-M. Yang, J. M. Collins, Proc. 4th Intern. Conf. Composites Engineering (Ed.: D. Hui), ICCE, New Orleans, LO, 1997, p.1083.

[iii] H. E. Bates, Ceramic Eng. Sci. Proc., 1992, 13, 190 – 197.

[iv] D.-H. Yoon, T. Fukuda, J. Cryst. Growth, 1994, 144, 201 - 206.

[v] S. T. Mileiko, V. I. Kazmin, J. Mater. Sci., 1992, 27, 2165 - 2172.

[vi] S. T. Mileiko, Metal and Ceramic Based Composites, Elsevier, Amsterdam, 1997.

[vii] S. T. Mileiko, V. M. Kiiko, N. S. Sarkissyan, M. Yu. Starostin, S. I. Gvozdeva, A. A. Kolchin, G. K. Strukova, Compos. Sci. and Technol, 1999, 59, 1763 - 1772.

[viii] V. N. Kurlov, V. M. Kiiko, A. A .Kolchin, S. T. Mileiko, J. Cryst. Growth, 1999, 204, 499 - 504.