Melting rare earth mineral powder into pots of molten goo to grow crystals for lasers has been a well understood process for decades, but now a supply chain dominated by China and the need to find alternate, more energy efficient processes is driving the industry to radical change.
The Czochralski technique is tried and tested and has been delivering crystals since the beginning of the laser era. This high temperature growth process involving tough heat resistance containers, sometimes made of iridium, takes rare earth minerals and other powders and heats them to about 2,000˚C to create a sort of molten fondu, to reliably produce crystals up to 300mm in length and a diameter of 100mm. A wide variety of products including neodymium-doped yttrium aluminium garnet (Nd:YAG) can be pulled from the melt with a seed rod. Northrop Grumman Synoptics’ largest product is the Nd:YAG crystal.
‘Our largest product is neodymium-doped yttrium aluminium garnet. You bring the growth station up to a temperature that would melt those powders and then you dip your seed rod into the melt. The seed is the crystal you’re growing and you slowly rotate that seed and you look to bring it up to diameter which are crystals that are three to four-inches in diameter,’ explains Scott Griffin, Northrop Grumman Synoptics’ director of sales and marketing. The major aerospace company, which uses the Czochralski technique extensively for its lasing crystals, is a supplier of synthetic laser crystals for solid state lasers.
The Czochralski growth process involves a growth station and rare earth mineral powder. These powders are mixed, but Griffin can’t talk about how they are mixed because, ‘our growth technology is proprietary, we can’t give a lot of details about that’. Depending on the crystal the time for the necessary growth to take place can be anywhere from two to six or eight weeks. Nd:YAG can take up to eight weeks if a large diameter is required. Synoptics has been growing Nd:YAG for close to 45 years. ‘There is not much more you can change on the periodic table, however we do look at development of new crystals for lasing and non-lasing applications,’ Griffin says.
Griffin’s company has 100 growth stations and as he explains the markets are going to drive how the company uses those. The market will determine how many of the stations are going to be growing this or that crystal. According to Griffin, every crystal has its own documented process for growing and ‘all of our growth stations are designed internally’. The amount of rare earths or powders that would be put into the growth station determines the size of the crystal that is grown. But ultimately it’s all about the market demand. ‘It [market demand] can change dramatically a couple of times throughout the year,’ says Griffin. While Nd:YAG is the standard product at Northrop Grumman Synoptics the company will also dope YAG with ytterbium and erbium and, according to Griffin, ‘all sorts of different rare earth minerals’.
But not all of the challenges with crystal growth are technical. The supply of rare earth minerals has become a major international issue with the vast majority of the minerals now only coming from China. It’s an issue Griffin has to deal with, because Synoptics gets all its rare earth minerals from China, explains Griffin. ‘It’s been a challenge to control costs’. The Chinese companies controlling this export have been making the most of their market dominant position. ‘There is nothing consistent in their pricing,’ Griffin, ‘We had one, terbium, for optic isolators, the pricing went up by 179 per cent,’ Griffin comments. The answer Synoptics has tried to implement is to negotiate longer blanket orders and to keep a lot of inventory. ‘To control that [pricing volatility] we keep a greater amount of inventory. Instead of a couple of months, we keep a year’s worth of supply on the shelf,’ says Griffin.
The long term answer is to find sources outside of China. ‘There is research being done, there is a mine in Australia and the US used to be a major source of rare earths,’ explains Griffin. While the minerals are called rare, Griffin points out that they are actually found all over the world and they do not really deserve to be described as rare. ‘Japan would be a huge source for rare earths, but they are all at the bottom of the Pacific Ocean,’ he adds. The US situation is not so difficult: a mine, which is not under the sea, that had been shut because of the once unbeatable price competition is now being reopened. ‘They are restarting the mine, but it could take time for this to be back at a production level’, says Griffin. Despite the rare earth mineral supply and pricing challenges the business case for the growth station melt remains a strong one. The business activities are focused on streamlining the process and finding cost reductions because, as Griffin explains, ‘lasing crystal processes meet all customer requirements for some time to come’. While for non-lasing crystals Synoptics is investigating new avenues, for a completely new product line. ‘We can’t divulge that and we have an active programme on right now that will take us in a different growth direction; it’s a very different growth direction, which is why we’re not saying what it is at this point. In terms of the standard crystals we’re not looking for different growth processes for what we’re doing now.’
It’s a reflection of laser’s widespread use across industry that a critical component, the lasing crystal, is the focus of efficiency cost reduction efforts and not research and development. A mature well understood component, crystals are as much a commodity as the not so rare earth minerals that are used to grow them.
