PLANSEE makes use of creep-resistant WVM to construct heaters for the growth of sapphires. This is because WVM-made heaters can successfully operate for twice as long as traditional tungsten heaters.
The growth of sapphires typically takes place at high temperatures of approximately 2,100 °C. In the Kyropolous process, rod heaters made of pure tungsten are employed as standard, but these heaters must be switched after only a small number of cycles as a result of the high levels of deformation and embrittlement. The heater rods, in some extreme instances, can even short circuit when they come into contact with the shielding.
WVM - An Innovative Material
WVM is made up of pure tungsten with trace quantities of potassium (30-70 ppm). The inclusion of potassium offers WVM a creep resistance multiple times that of pure tungsten. A recent sag test carried out by PLANSEE offered evidence of this capability.
Sag Test and Results
PLANSEE carried out a simulation of the use of WVM at high temperatures, and under a variety of differing mechanical loads. As can be seen in Figure 1, due to the presence of potassium, which offers greater stability and a coarser crystalline structure than pure tungsten, WVM sags at just 25% of the rate of pure tungsten. Additionally, this property had a positive effect on the material’s creep resistance.
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Figure 1. WVM sags four times less than pure tungsten
The below table details the main physical properties of WVM:
| WVM key facts |
| Melting point |
3,420 °C / 3,693 K |
| Purity |
99.97 % |
| Coefficient of thermal expansion at 20 °C |
4.2•10-6 m/(m•K) |
| Thermal conductivity |
164 W/(m•K) |
| Electrical conductivity at 20 °C |
18•106 [1/Ω•m)] |
The comparison of key characteristics of WVM with pure tungsten is shown in the following table:
| Property |
WVM |
| Recrystallization temperature |
Much better than pure tungsten (2000 °C at 99.99% deformation level) |
| Ductility |
Much better than pure tungsten |
| Stability at high temperatures, creep resistance |
Better than pure tungsten |
| Machinability/workability |
Better than pure tungsten |
| Fineness of grain |
Better than pure tungsten |
| Thermal conductivity |
Similar to pure tungsten |
| Electron work function |
Similar to pure tungsten |
WVM Heaters for Sapphire Growth
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Figure 2. Sheet Heater
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Figure 3. Mesh Heater
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Figure 4. Rod Heater
Mesh heaters, sheet heaters and rod heaters built with WVM offer improved dimensional stability, particularly in processes involving high temperatures, such as sapphire growth. Heaters produced with WVM are able to last two to three times longer than those made from traditional tungsten, which need to be replaced after just a few cycles.
WVM shares the 99.97% purity of pure PLANSEE tungsten, and there is no risk of contamination of the furnace atmosphere, as the potassium content is firmly bound in WVM. WVM, like pure tungsten, is an appropriate material for exceedingly sensitive high-vacuum practices.
Non-Deformable and Ultrapure Tungsten Crucibles
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Figure 5. Pressed sintered tungsten crucible
PLANSEE can deliver more than simply hot zones, particularly with hard-wearing heaters, for the growth of sapphires. The company also produces the complementary pressed-sintered tungsten crucibles with an exceptionally smooth surface for easy removal of the sapphire, without risk of damage to the surface of the crucible.
The crucible’s remarkably even surface ensures that there is minimal need for costly and time-consuming reconditioning of the crucible surface, and makes it less susceptible to corrosion when coming into contact with the sapphire melt.
In addition to this, tungsten crucibles from PLANSEE offered increased consistency, with great dimensional stability, even at high and rapidly varying temperatures. The company ensures the production of pure sapphire through their strong emphasis on a production process free of contaminants. All production processes take place internally, from the metal powder, right through to the completed crucible.
Conclusion
PLANSEE has over 90 years of experience in the industry of processing refractory metals, and can supply more than just exception quality of materials. The company is also ideally placed to produce products based on individual customer requirements.
As well as heaters, PLANSEE also produces non-deformable shielding layers, as well as energy-efficient hot zones for high-temperature furnaces. PLANSEE engineers work to support customers from the initial drawings of furnace constituents, right through to the completed product.
This information has been sourced, reviewed and adapted from materials provided by PLANSEE.
For more information on this source, please visit PLANSEE.