Molybdenum and tungsten products for production of leucosapphires

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POLEMA JSC is the leading global manufacturer of chromium, molybdenum, tungsten products, metal powders and composites. The enterprise has great production facilities and analytic base, plants of isostatic molding, high-temperature sintering, extrusion, rolling mills, up-to-date machine pool producing unique large-size rolled stock and refractory metal products.

The delivery of heat shields and refractory metal crucibles for growth setup furnaces was started in 1998.  The enterprise extended the products’ range and increased volume of crucibles production, large-size molybdenum rolled stock and ??70W30 alloy and tungsten products. POLEMA introduced extra capacities into operation for production of large-size tungsten crucibles. It also expands production of molybdenum large-size thin-sheet rolled stock and furnaces under customers’ drawings.

Molybdenum products:

• powders;
• sheets;
• plates;
• discs;
• electrodes;
• rods,
• crucibles;
• cups,
• shields and etc.

The grades of sheets and plates are as follows:

• sheets of M-PM grade, RF Standard 17431-72;
• molybdenum rolled sheet of high precision, Spec. 14-22-149-2001.

Rolled stock is produced with the thickness of 0,3 - 45 mm. It’s characterized by high purity in reference to interstitial and metal impurities (table 1), high plasticity and homogeneous structure.

Molybdenum sheets are supplied annealed for inner stress relieving unless otherwise specified by the customer. Table 2 contains typical strength values of rolled stock under room temperature.

Example values of mechanical characteristics of M-PM molybdenum rolled stock

Molybdenum rolled sheet is applied for flat and round heat screens. The sheets are recommended to be treated with local gas-plasma or inductive heating under the temperatures depending on rolling thickness and deformation degree during screen forming at bending press.  E.g. the recommended temperature range for the sheet with thickness of 0,5mm is about >100-3000°?, the sheet of 1mm - >150-3500°?, the sheet of 2 mm - >270-4700°?.

Due to the development of plants for monocrystal growth, 700 mm thin-sheet and wide rolled stock will be needed. To meet the customers’ demands POLEMA JSC is going to purchase new 12 or 20 stand of rolls for thin sheets with the width of 700mm and minimum corrugation. It’s also considering purchasing of equipment for precise sheet processing by water jet or laser cutting. Each variant has its merits and demerits and we’d be glad to get customers’ recommendations.

Adoption of these equipment will allow to expand rolled stock nomenclature, improve its quality and reduce costs.

POLEMA JSC is developing production of formed parts made of zirconium oxides stabilized with yttrium. It ensures that all components for plants furnaces could be purchased at one and the same enterprise. It will have a positive impact on logistics of plants production.

Molybdenum rods, electrodes.

Chemical composition. Up-to-date methods of powder metallurgy ensure high purity of molybdenum in reference to metal and interstitial impurities (C,N,O,H), proper microstructure for heat-resistant constructive materials applied in electrical engineering and other plants operating under high pressure.

The products have perfect plasticity due to low content of metal and interstitial impurities, fine grain structure.

The rods are produced heat treated, in annealed condition for inner stress reliving unless otherwise specified by the customer.

Full re- crystallization structure is not permitted in the rods of 40 mm in diameter unless special requirements to re-crystallization annealing and structure properties are specified by the customer.

The rods are produced ground, turned and dross purified (without machining), with the diameter above 3,18-80 mm.

POLEMA JSC produces rolled sheet and rods made of ML-PM lanthanized molybdenum, age-hardened with lanthanum oxide, ODS – Mo. Lanthanum oxide serves as a barrier for dislocation displacement in deformation process. It stabilizes alloy structure, ensures strength and creep resistance under high temperatures. The material has low cold-shortness threshold, high plasticity in re-crystallized condition, high strength under 1600 °? and high deformation resistance (creep) at long-term strength test under 1800 °?. The material has good welding properties.

The company supplies molybdenum-tungsten alloy products of MW30-PM grade (crucibles, supports, abutments) for furnaces. Hardened alloy MW30 has high re-crystallization temperature, strength and resistance to creep under high temperatures.

Tungsten rolled stock and crucibles.

Tungsten is characterized by maximum interatomic bonds, the highest melting temperature among refractory metals  - 3420 °?, high density of 19,3 g/cm³,  high strength, thermal conductivity, hardness (HV30 >460), resistance to creep and long-term strength. Refined tungsten has higher re-crystallization temperature of 1350 °C in comparison with molybdenum, lower steam pressure and evaporation rate under temperatures of 1800-2500 °?, lower coefficient of thermal expansion (4,2·10^-6 ??? 20 °?) and low electric resistance (0,05·10^-6 Om·m) [1].

Development of sapphire growth plants of optical quality is connected with constructive methods of equipment production (crucibles, shields) for ensuring of high purity process.

Up-to-date methods of powder metallurgy ensure high purity of tungsten in reference to metal and interstitial impurities (C,N,O,H), fine grain structure for furnaces and other appliances operating under extremely high temperatures.

POLEMA JSC produces rolled sheet with the thickness of 0,5 – 45 mm and tungsten crucibles of pure raw materials – internal tungsten powder with the purity above 99,98%. Sapphire crystals raised in tungsten furnaces are of higher purity than those raised in plants with tungsten-molybdenum crucibles and molybdenum shields [3].

Shape and dimension of crucibles is specified as per customers’ needs. Roughness of machined crucibles is about Rz 6,3 max. Typical dimensions of TW-PM crucibles applied by Russian and foreign partners of POLEMA, mm:

Ø200?240, Ø240?300, Ø250?350-400, Ø330?350, Ø330?450, Ø338?460.

The dimension of crucibles is constantly growing during last years.

Image 1. Change of maximum crucibles dimension in 2012 –  the predictable crucible dimension.

The most perspective crucible dimension is of 400-420 mm and 500 mm.

To meet the demands of growth setups for large-diameter crucibles, POLEMA is purchasing new advanced hydrostatic press by 2012. Pool of furnace and cutting equipment applied for crucible production will be updated.

During exploitation process in leucosapphire growing plant defects are formed on the surface of inner and outer crucible faces:

1. local molybdenum deposition at outer face the source of which is molybdenum equipment;
2. local tungsten re-crystallization at inner crucible face with scoring and microfissure.

The studies aiming at revealing of structure and characteristics of re-crystallization and microfissure distribution have shown that the cause is considered to be local overheating of crucibles, mechanical influence of crystal on crucible walls during its movements, heat stress due to high temperature gradient  according to crucible height.

The images 2-4 show typical defects of waste W crucible. The researches were held by POLEMA with the help of electron microscope JSM-3690 and X-ray add-on.

Image 2. Deposited Mo layer at the outer crucible face.	Image 3. Al2O3 impurities at the inner crucible face.
Image 4. Crack.

The target of crucible defects reduction during exploitation is acute. The amount of defects greatly differ at different enterprises.

Development of refractory metal production for furnaces of sapphire growth plants is the priority direction of POLEMA JSC. The enterprise extends co-operation with internal and foreign companies in the sphere of delivery of equipment and sets for furnaces, improvement of equipment construction and structural researches.

References:

1. E.M. Savitsky, G.S. Burkhanov. Physical metallurgy of refractory metals and alloys. Science, M, 1967.
2. Eskov E.V., Ignatov A.Y., Postolov V.S., Filimonov A.S. Optical sapphire, technical requirements and technology of crystal growth. North Caucasus State Technical University, Materials of the conference “Chemistry of solids”, 2006.
3. D.I. Bletskan, V.J. Bratus, A.R. Lukyanchuk, V.T. Masluk, O.A. Parlag. Uzhgorod OH, JSC "Tehnokristall”,  the Institute of semiconductor physics NAS, the Institute of electronic physics NAS, Ukraine, Letters to TPJ, 2008, volume 34, edit.14.

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