Growing Crystals Like Rock Candy To Enable Next-Gen EV Chargers

Written by: Jonathan Valenzuela

Featured image by: Canva AI Generator

Imagine this: You’re driving in an unfamiliar place in your new electric vehicle (EV). You’re so close to your destination, but your car is on 1% of its battery’s life. Ugh! Even worse, your car can take up to 45 minutes to charge.

What if I told you the time lag wasn’t your car’s fault but the charger’s? More specifically, it’s the materials that are used to make the charger. By using special components made of ultra-wide band gap (UWBG) materials, you can cut your time at the charger down to less than 10 minutes [1].

Figure 1: Electric vehicle charger (credit: Unsplash).

Let’s learn more about the current materials and technologies we use for power electronics, such as EV chargers, and how the Materials for Advanced Technologies and Sustainability (MATS) Laboratory at Lehigh is working to advance these technologies.

Power electronics and the materials that make them

So what are power electronics? Put simply, they are a class of devices used to control and convert electrical power. They are used everywhere ‒ from our electrical grid to the phone in your pocket. These devices are used to optimize electrical power distribution and deliver power reliably. Their power control functions are enabled by the electrical properties of semiconductors.

A semiconductor is a type of material that can conduct electricity. It falls between a conductor (like a metal) and an insulator (like rubber), which can’t conduct electricity at all. This conducting behavior is related to the size of the band gap energy. The wider the band gap, the more insulating character of the material (Figure 2).

Figure 2: Band gap energy vs. class of material [2]. Semiconductors have a band gap energy that is in between strong electrical conductors, like metals, and poor electrical conductors, like rubber.

Silicon is a ubiquitous semiconductor in all of our electronic technologies, including power electronics. It has a band gap energy of 1.1-1.2 eV (Table 1), which, while acceptable for use in small devices, is completely impractical for use with larger electronic systems, like electric vehicles (EVs). For these systems, we need materials with a significantly larger band gap.

Table 1: Band gap energies of silicon, select wide band gap materials (SiC, GaN), and select ultra-wide band gap materials (cBN, AlN).

Wide band gap (WBG) semiconducting materials that are being used for large electrical systems are silicon carbide (SiC) and gallium nitride (GaN), which have a significantly larger band gap. This large band gap increases their tolerance to high voltages and temperatures, and ensures lower power losses. However, as technologies in the EV and consumer electronics industries continue to mature, so too will their need for higher operating voltages and lower losses. GaN and SiC are beginning to plateau in their voltage capacity and power loss containment [3][4][5], so new materials with even wider band gaps are necessary. For this, we need UWBG semiconductors.

What are UWBG semiconductors?

UWBG semiconductors are special because they have a very wide band gap, which means that it takes a lot more energy for electrons to move through the material. This makes them even better than WBG materials like SiC and GaN at handling high voltages and high temperatures without breaking down.  Thus, they are ideal for high power and high temperature applications. Within this class of materials, nitrides (materials that contain nitrogen) are prominent candidates [4]. Nitrides such as cubic boron nitride (cBN) and aluminum nitride (AlN) have been identified as prime candidates for the next generation of power electronics materials; their band gaps are 2X as large and have been predicted to be able to handle at least 10X more voltage than GaN or SiC [6]!

Like rock candy: Bulk crystal growth of UWBG semiconductor crystals

Almost all electronic devices require crystals with very low amounts of defects to operate properly, and their production must scale so that a lot of the material can be made cheaply. We currently do this with silicon for integrated circuits (Figure 3). To achieve the goal of scaling up the production of these energy-efficient power electronics, we must create systems for bulk growth of high-quality UWBG crystals at a low cost. Nitrides are especially challenging to synthesize, due to chemical barriers to bulk crystal growth. Current methods for producing nitride materials require extremely high temperatures (~3600 °F) and pressures (> 145,000 psi) to produce millimeter-sized grains [7], which are not scalable approaches. However, a technique identified for this crystal growth of UWBG materials which can be scalable is the flux method.

Figure 3: Ingots of single crystal, defect-free silicon. Ingots can measure almost 7 feet long! (credit: shutterstock)

“Flux method” may sound strange to you, but you’re actually probably very familiar with the technique; in principle, it is similar to the rock candy experiment in grade school [8]. To jog your memory a bit, your teacher would boil water and dissolve as much sugar into the water as possible. Then, as your sugar water cooled, you would dip a stick covered in tiny grains of sugar into the solution. Over the course of a few days, you would come back to see your tiny sugar crystals had become a sizable chunk of candy, ready to be eaten.

Figure 4: Sugar crystals after they’ve been left to grow in solution for a few days [8].

This method for growing sugar crystals can be used for growing UWBG semiconductors! A few parameters are changed, of course. Your “water” is the flux, which is a solution used to dissolve the atomic components of your crystals (in this case, we are dissolving nitrogen and another metal like aluminum or boron).  With a “sugar grain” (seed crystal) of your desired material (AlN or cBN), you control the temperature, cooling slowly to allow the atomic components to attach themselves to the seed crystal, yielding a large, high quality crystal. This crystal growth approach can also be done in much milder conditions, typically at < 2500 °F and around atmospheric pressure (15 psi) [9].

Crystal growth capabilities at Lehigh

The flux growth method for growth of UWBG semiconductors is a promising method for bulk crystal growth, but the technique has many challenges to overcome. Flux selection is a challenging task, as it must be carefully selected to allow for dissolution of all elements and be inert to the crystal growth system. The method also requires precise control of growth parameters to obtain high-quality crystals.

At Lehigh, the MATS Laboratory is developing advanced crystal growth technologies for flux crystal growth. Our group is investigating novel flux candidates, which will enable crystal growth of UWBG materials. We are also developing custom equipment for high control of growth parameters, which will translate to unique capabilities in bulk scale growth of UWBG materials in the future.

EVs are becoming a commonplace technology on the road, and there are major efforts being made to make this technology efficient to use, particularly with charging time. With the hard work coming from the MATS Lab at Lehigh, we’ll bring the hour-long wait for your EV charge down to just minutes, so you can get to your destinations faster.

References

1. T. Ilahi et al., “Design and Performance Analysis of Ultra-Wide Bandgap Power Devices-Based EV Fast Charger Using Bi-Directional Power Converters,” in IEEE Access, vol. 11, pp. 25285-25297, 2023, doi: 10.1109/ACCESS.2023.3255780. https://ieeexplore.ieee.org/document/10065480
2. Band gap – Energy Education. https://energyeducation.ca/encyclopedia/Band_gap.
3. Mingfei Xu and others, A review of ultrawide bandgap materials: properties, synthesis and devices, Oxford Open Materials Science, Volume 2, Issue 1, 2022, itac004, https://doi.org/10.1093/oxfmat/itac004
4. Wong, M.H., Bierwagen, O., Kaplar, R.J. et al. Ultrawide-bandgap semiconductors: An overview. Journal of Materials Research 36, 4601–4615 (2021). https://link.springer.com/article/10.1557/s43578-021-00458-1https://doi.org/10.1557/s43578-021-00458-1
5. Jiajie He 2020 IOP Conf. Ser.: Mater. Sci. Eng. 738 012009 https://iopscience.iop.org/article/10.1088/1757-899X/738/1/012009
6. Kaplar, Robert, et al. Ultra-Wide-Bandgap Semiconductors for Power Electronics. No. SAND2015-5644C. Sandia National Lab.(SNL-NM), Albuquerque, NM (United States), 2015. https://www.osti.gov/servlets/purl/1410755
7. Cubic boron nitride competing with diamond as a superhard engineering material – an overview. Sergio Neves Monteiro, Ana Lúcia Diegues Skury et al. Journal of Materials Research and Technology, 2, 1, 2013 https://www.sciencedirect.com/science/article/pii/S2238785413000057
8. Rock Candy Experiment. https://www.growingajeweledrose.com/2015/02/rock-candy-experiment.html.
9. Cryst. Growth Des. 2017, 17, 9, 4932–4935. Publication Date:August 1, 2017. https://doi.org/10.1021/acs.cgd.7b00871