Rare earth metal, also known as rare earth elements, is the periodic table Ⅲ B group scandium, yttrium, lanthanide 17 kinds of elements in general, commonly used R or RE said. From 1794 found the first rare earth element yttrium, to find the natural rare earth elements in 1972, after 178 years, people only 17 kinds of rare earth elements all found in nature. Rare earth metal luster between silver and iron. Rare earth metal chemical activity is very strong.
Rare earth metal luster between silver and iron. The impurity content has a great effect on their properties, so the physical properties contained in the literature are often significantly different. Lanthanum is superconductor at 6K. Most rare earth metals exhibit paramagnetism, and gadolinium has stronger ferromagnetism at 0 ° C than iron. Terbium, dysprosium, holmium, erbium, etc. also show ferromagnetism at low temperatures. The low melting point of lanthanum and cerium and the high vapor pressure of samarium, europium and ytterbium show a great difference in the physical properties of rare earth metals. The thermal neutron absorption cross section of samarium, europium and gadolinium is widely used in nuclear reactor control materials such as cadmium and boron. Rare earth metals have plasticity, with samarium and ytterbium as the best. In addition to ytterbium, the yttrium group has a higher hardness than cerium group.
Rare earth metal chemical activity is very strong. When reacted with oxygen, a highly stable R2O3 type oxide (R represents a rare earth metal) is produced. Cerium, praseodymium, terbium also produce CeO2, Pr6O11, PrO2, Tb4O7, TbO2 type oxide. Their standard heat generation and standard free enthalpy negative values are greater than those of calcium, aluminum and magnesium oxides. The melting point of the rare earth metal oxide is above 2000 ° C. Europium has the largest atomic radius, the most active nature, and the loss of metallic luster at room temperature immediately upon exposure to air, and soon oxidation into powder. Lanthanum, cerium, praseodymium, neodymium are also easy to oxidize, produce oxide film on the surface. Metal yttrium, gadolinium, lutetium corrosion resistance, can be a long time to maintain its metallic luster. Rare earth metals can react with water at different rates. Europium reacts violently with cold water to release hydrogen. Cerium group of rare earth metals at room temperature and water reaction is slow, the temperature is increased to respond faster. Yttrium group rare earth metal is more stable. Rare earth metal reacts with halogen at high temperatures to produce +2, +3, + 4 valent halides. The anhydrous halide has a strong water absorption and is easily hydrolyzed to produce ROX (X represents a halogen) type oxyhalide. Rare earth metals can react with boron, carbon, sulfur, phosphorus, hydrogen and nitrogen to form the corresponding compounds. Rare earth metal alloys such as lanthanum nickel alloy (LaNi5) has a large capacity of hydrogen absorption, is a good hydrogen storage material.
Rare earth metals and their alloys in the steel from the role of deoxidation desulfurization, both the content can be reduced to less than 0.001%, and change the shape of inclusions, fine grain, thereby improving the steel processing performance, improve strength, toughness , Corrosion resistance and oxidation resistance. Rare earth metals and their alloys are used in the manufacture of ductile iron, high-strength gray cast iron and vermicular graphite cast iron to change the form of graphite in cast iron, improve the casting process and improve the mechanical properties of cast iron (alloy steel, cast iron). In the bronze and brass smelting to add a small amount of rare earth metals can improve the strength of the alloy, elongation, heat resistance and conductivity. In the casting of aluminum-silicon alloy to add 1 ~ 1.5% of the rare earth metal, can improve the high temperature strength. In the aluminum alloy wire to add rare earth metal, can improve the tensile strength and corrosion resistance. Fe-Cr-Al electrothermal alloy by adding 0.3% of rare earth metals, can improve the antioxidant capacity, increase the resistivity and high temperature strength. Titanium and its alloys to add rare earth metal can refine the grain, reduce the creep rate, improve the high temperature corrosion resistance.
