Main Applications of Yttrium in Alloys and Phosphors

Introduction

Yttrium, with the chemical formula Y, is one of the rare earth elements due to its similar physical and chemical properties to other rare earth elements and early historical reasons. It is a silver, soft transition metal that is chemically similar to the lanthanide group, especially to the heavy rare earth element group, the atomic number of which is from 63 to 71. Yttrium’s electronic configuration is [Kr]5s²4d¹. It prefers to lose 3 electrons to form 8 electrons stable structure, so its oxidation state is +3. Y₂O₃ is one of the most common yttrium compounds used.

Yttrium has 31 ppm in the crust and is the 28^th most abundant element, about 26000 times more common than gold, Au. Yttrium is usually found with other lanthanides in rare-earth minerals as a by-product. Most yttrium comes from the following 3 sources:

1. Xenotime: a phosphate mineral that contains yttrium orthophosphate (YPO₄)
2. Monazite: a reddish-brown phosphate mineral that contains rare earth elements
3. Bastnaesite: a calcium fluoro-carbonate mineral with cerium, lanthanum, and yttrium

Yttrium is widely used in many areas such as phosphors in TV tubes, energy-efficient lighting, and fuel cells [2], metallurgy, ceramics, superconductors, and so on. This article will mostly focus on Yttrium Used in Alloys and Phosphors.

Yttrium Used as An Alloy Addictive

Yttrium is used in the alloy industries because of its deoxidizing, desulfurizing, denitrifying, or degassing effects, which are explained by its low thermodynamic oxidation potential [1]. Adding some amount of yttrium into, for example, Ni-20Cr alloy can improve its high-temperature oxidation resistance a lot. But the detailed reasons are still unknown though lots of hypotheses and research were done. There are two possible explanations:

1. Adding yttrium can reduce the alloys’ mass gain. (Mass gain means the total mass increase in an alloy due to the absorption of atoms or molecules from the environment. It may be caused by corrosion, oxidation, and precipitation.)
2. Yttrium addition enhances the alloys’ surface adherence.

Here, alumina alloy with yttrium addictive is taken as an example.

Fe-20Cr-4Al Alloy and Yttrium Implantation

Fe-20Cr-4Al is an alloy composed of 20% Cr, 4% Al, and Fe as the balance iron. It is often used in high-temperature applications such as combustion chambers or heat exchangers. It has good resistance to oxidation and corrosion at high temperatures.

Here are the steps on how we implant Yttrium into Fe-20Cr-4Al alloy:

Repeat hot and cold rolling with Fe-20Cr-4Al alloy to make a 0.5mm thick sheet. Use the implanter to implant yttrium ion into the alloy. Use the Rutherford backscattering spectroscopy (RBS) to accurately measure yttrium’s concentration in the alloy. Here we use 0.01% to 0.5% Y implanted alloys.

Experiment and Results Discussion

Use Fe-20Cr-4Al-(0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5)Y alloys and expose them under O₂ for 5h under 1200℃. Figure 1 shows a decrease in mass gain from Fe-20Cr-4Al to Fe-20Cr-4Al-0.1Y. After that, the mass gain increases again [1].

Figure 1: Mass gain change of Fe-20Cr-4Al-(0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.3)Y alloys under 5h at 1200℃ in O₂ [1].

Figure 2 shows these alloys' surface appearance after the O2 exposure. The Standard FeCrAl alloy’s surface starts to fall out.

From (b) to (h), they both form an oxide surface to protect the material in the middle. (b) and (c) still have slight spalls on their surface. As the yttrium concentration increases, the oxide surface forms better to protect itself. From 0.1Y to 0.5Y, their surfaces are much darker than those from 0Y to 0.05Y. By using X-ray diffraction to detect the surface of the alloys, we get the following observations [1].

(a) forms a very weak Al2O3 crystal surface structure. From (b) to (h), they all form strong Al2O3.

From (f) to (h), they also form very weakly weak Y3Al5O12 crystal structures.

Y₃Al₅O₁₂, also called yttria alumina garnet (YAG), is a synthetic material that has high-temperature, high-strength, and chemically stable characteristics. The forming of YAG may be one reason that mass gain increases from 0.1Y to 0.5Y. But this mass gain increase doesn’t mean the alloy's high-temperature oxidation resistance reduces. In fact, as Y concentration increases, the alloy shows better resistance to oxidation and corrosion at high temperature.

Figure 2: surface photos of FeCrAl-(0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.3)Y alloys under 5h at 1200℃ in O₂ [1]. (a) 0Y; (b) 0Y with purified; (c) 0.01Y; (d) 0.02Y; (e) 0.05Y; (f) 0.1Y; (g) 0.2Y; (h) 0.3Y

Yttrium Used in Phosphors

Phosphors are substances that receive radiation and emit light. The basic principle is the phosphor’s orbital electrons receive the radiation energy, become excited and go to higher orbitals. Finally, these electrons will go back to their ground states. The energy produced from this behavior will emit light.

Elements used in the phosphor directly affect the light that the phosphor emits. Because of yttrium’s stable, narrow and efficient red emission, Y2O3 is used in phosphors for color TVs, computer monitors, light-emitting diodes (LED), and x-ray-intensified screens [2]. 

General LED produces cold white light. Phosphor-converted warm white light emitting diodes (pc-WLEDs) is the new LED technology [3]. Nano-sized Y2O3 can add some red components in the phosphor to make the LED emit a warmer and high-quality light. 

Conclusion

Yttrium is one of the rare earth elements. Due to its unique properties, Y is widely used in phosphors and alloys. There are still many applications and yttrium compounds that are not mentioned today. Stanford Advanced Materials (SAM) provides different kinds of Yttrium. If you want more information about Yttrium or Yttrium compounds, you can provide your application information to our technical staff for advice.

Reference

1. Volkerts, B. D. (Ed.). (2010). Yttrium: Compounds, production, and applications: compounds, production and applications. Nova Science Publishers, Incorporated.
2. Zhang, K., Kleit, A. N., & Nieto, A. (2017). An economics strategy for criticality – application to rare earth element yttrium in new lighting technology and its sustainable availability. Renewable and Sustainable Energy Reviews, 77, 899–915. https://doi.org/10.1016/j.rser.2016.12.127
3. Petry, J., Komban, R., Gimmler, C., & Weller, H. (2022). Simple one-pot synthesis of luminescent europium doped yttrium oxide y2O3:eu nanodiscs for phosphor-converted warm white LEDs. Nanoscale Advances, 4(3), 858–864. https://doi.org/10.1039/d1na00831e