What magical uses do rare earth elements have? (Ⅱ) ——About the application of scandium and yttrium

1.Application of Scandium

The element symbol of scandium is Sc, pronounced as kàng in Chinese, and its atomic weight is 44.9559.

Scandium is a transition metal at the top of the periodic table. It has high chemical activity and only reacts with oxygen at high temperatures, which can react with non-metallic elements such as nitrogen and sulfur, and form salts with acids and bases.

The first important application of scandium is the ‘scandium sodium lamp’, which is a metal halide electric light source. Some argon is filled in the bulb, and sodium iodide and scandium iodide powder are also contained in the bulb. When discharged at high voltage, the scandium ions and sodium ions emit a special spectrum, but the whole is white. Scandium sodium lamps have high luminous efficiency, good light color, energy saving, long life, and strong defogging ability. They are widely used in television shooting and squares, stadiums, and road lighting, known as the third generation of light sources. The luminous efficiency of scandium sodium lamps is about 4 times that of halogen tungsten lamps and 2 times that of high-pressure mercury lamps. And the color rendering performance is also good.

The second important application of scandium is solar cells. The efficiency of solar cells depends largely on the light absorption and carrier transport properties of the material. Due to its large bandgap and high light absorption coefficient, scandium materials can effectively absorb visible light and infrared light in the solar spectrum, thereby improving the photoelectric conversion efficiency of solar cells. In addition, scandium can also be compounded with other materials, such as silicon, to form a heterostructure, further improving the performance of solar cells. Therefore, the innovative application of scandium materials in solar cells is expected to contribute to the popularization of renewable energy.

The third important application of scandium is photo-catalysis. Scandium materials have a ‘high light absorption rate’ and ‘excellent photoelectric conversion performance’, which can effectively convert solar energy into chemical energy and drive photocatalytic reactions. For example, scandium materials can be used for ‘water decomposition to produce hydrogen’, converting solar energy into storable and usable hydrogen energy. For example, scandium materials can be used to prepare efficient photoelectric sensors to achieve highly sensitive detection and transmission of optical signals. For example, scandium materials can also be used in preparation of new optoelectronic devices, such as scandium-based photoelectric crystals and scandium-based light-emitting diodes, etc., consequently injecting new vitality into the development of the optoelectronic industry. For example, scandium oxide can ‘weaken hydrogen bonding forces’ and can be used as a catalyst for ‘dehydrogenation’ and ‘hydrogen cracking’.

The fourth important application of scandium is to improve the performance of alloys. For example, scandium has a very magical alloying effect with aluminum alloys. As long as a small amount of scandium is added to aluminum, Al3Sc will be generated, which will significantly change the structure and properties of the alloy. This ‘new generation of aluminum alloy material’ has a specific gravity close to that of aluminum, which also has a super crystal stability, super welding performance, super corrosion resistance and good strength at a high temperature of 900°C, and can also withstand neutron radiation; it has very attractive development prospects in aerospace, aviation, ships, nuclear reactors, light vehicles, high-speed trains, etc. For example, scandium titanium alloy and scandium magnesium alloy are ideal materials with high melting points, low specific gravity, and high strength. For example, scandium can also be used as an additive to high-temperature tungsten and chromium alloys to increase the service life of these materials. For example, scandium is also an excellent modifier for iron. A small amount of scandium can significantly improve the strength and hardness of cast iron.

The fifth important application of scandium is to stabilize the crystal lattice in ‘special ceramics’. For example, zirconium oxide ceramic materials have a very special property. The conductivity of this electrolyte increases with the temperature and oxygen concentration in the environment, and it can be used as a ‘sensitive oxygen content meter’. The unstable crystal structure of this ceramic material is a disadvantage, but the lattice of zirconium oxide ceramic materials with scandium oxide added is stable and can be put into practical use. Scandium oxide in ceramics plays a role in stabilizing the crystal lattice and making the structure dense as the concrete does. For example, silicon nitride, the high-temperature resistant engineering ceramic material, has the characteristics of super hardness, high-temperature resistance, corrosion resistance, small high and low-temperature impact deformation, small friction coefficient, and wear resistance. When using scandium oxide as a densification agent, it can further improve its high-temperature mechanical properties.

The sixth important application of scandium is as a catalyst for hydrocracking in the process of petroleum refining in the chemical industry. This property of scandium can help to make scandium oxide, zirconium oxide, and yttrium oxide together as an oxidant for hydrogen fuel cells and as an electrode material to have a better effect. In addition, the solid-state battery electrolyte made of lithium scandium indium chloride has good lithium ion conductivity and stable operation, which has contributed to the breakthrough progress of solid-state batteries.

The seventh important application of scandium is in the production of 'high-power laser crystals'. Now, ‘yttrium gallium scandium garnet’ is widely used to make high-power lasers. This solid laser has important applications in other industrial fields.

The eighth important application of scandium is its isotopes. Scandium can absorb neutrons in nuclear reactors to produce radioactive isotopes, which can be used as a gamma ray source for local tumor treatment and as a radioactive tracer.

2.Application of Yttrium

The element symbol of yttrium is Y, pronounced yǐ, and its atomic weight is 88.90585

The chemical properties of yttrium are closer to ‘heavy rare earth elements’, and yttrium minerals are often mixed with heavy rare earth element minerals.

The first important application of yttrium is to improve the performance of stainless steel. It is precisely because of the presence of a small amount of yttrium in FeCr alloy that its oxidation resistance and ductility are greatly improved. It is also because of the presence of a small amount of yttrium that rare earth magnesium alloys have the characteristics of high strength, high toughness, corrosion resistance, and high-temperature resistance, which also can replace some medium-strength aluminum alloys and be used in aircraft load-bearing components. This kind of alloy also has good thermal conductivity and electrical conductivity, and its application range is very wide. Similarly, adding a small amount of yttrium-rich rare earth to copper alloys and Al-Zr alloys can improve their many properties. Silicon nitride ceramic materials containing yttrium-scandium-aluminum can be used to manufacture engine components.

The second important application of yttrium is to make high-performance optical crystals. When using yttrium aluminum garnet （Y3AL5O13）single crystal in the production, high-end optical lenses will be of low absorption, high brightness, and no astigmatism. Neodymium yttrium aluminum garnet is also used to make laser crystals, which are widely used in solid lasers; among them, it is worth mentioning "high-power laser crystals", which can be used for laser processing, laser scalpels, laser weapons, etc. In addition, pure yttrium vanadate (YV O4) crystals are used to make optical fibers, and their propagation performance is ten times higher than that of silicon optical fibers.

The third important application of yttrium is to make ‘good primary color fluorescent powder’. For example, under electron excitation, europium oxide emits blue light; with europium as an activator, yttrium oxide, yttrium oxysulfide, yttrium vanadate can emit pure red light; with terbium as an activator, yttrium aluminum gallium garnet emits pure green light; these three materials are made into ‘phosphors’ working as ‘color units’ of displays. Each color unit can emit various expected colors with the impaction of electrons of different intensities. This color display technology gradually replaced the ‘liquid crystal display’ and ‘plasma display’ technologies.

The fourth major application of yttrium is a cracking catalyst in oil refining. Since both yttrium and scandium have the effect of weakening ‘hydrogen bonds’, small molecular light products are easily obtained during oil refining; similarly, yttrium is sensitive to hydrogen bonds, and yttrium-doped SrZrO3 is very sensitive to hydrogen concentration. The degree of hydrogen absorption can be reflected in the conductivity, so yttrium-doped SrZrO3 can be used as a gas sensor for ‘hydrogen content’.

The fifth important application of yttrium is to improve, fix, and densify the properties of the crystal lattice. For example, yttrium oxide is used to fix the zirconium oxide lattice to make the electrode of the hydrogen fuel cell without pulverization or degradation. For example, adding yttrium oxide to the structural ceramic ‘silicon nitride’ can densify the crystal structure and greatly improve the strength, toughness, wear resistance, impact resistance, and high-temperature resistance of the material. It can be used in gas turbine engines, other cutting tools, and wear-resistant parts. For example, yttrium-stabilized zirconia, commonly known as zirconium diamond, has excellent hardness and optical properties, which are very close to natural diamonds and may even be mistaken for real diamonds.

The above is an introduction to several important applications of the rare earth elements scandium and yttrium. Due to their extensive involvement in various fields such as electronics, petrochemicals, metallurgy, machinery, energy, light industry, environmental protection, and agriculture, we continue to increase our research efforts to ensure the rational use of rare earth elements and the sustainable development of related industries.