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Rare earth information

1. Cerium

Cerium is the most abundant of the rare earths. It is characterized chemically by having two valence states, the +3 cerous and +4 ceric states. The ceric state is the only non-trivalent rare earth ion stable in aqueous solutions. It is, therefore, strongly acidic. It is also a strong oxidizer. The cerous state closely resembles the other trivalent rare earths. Cerium salts that have been characterized include carbonates, nitrates, chlorides, fluorides, carbonates and sulfides.

The numerous commercial applications for cerium include metallurgy, glass and glass polishing, ceramics, catalysts, and in phosphors.

In steel manufacturing it is used to remove free oxygen and sulfur by forming stable oxysulfides and by tying up undesirable trace elements, such as lead and antimony.

It is considered to be the most efficient glass polishing agent for precision optical polishing. It is also used to decolor glass by keeping iron in its ferrous state. The ability of cerium-doped glass to block out ultra violet light is utilized in the manufacturing of medical glassware and aerospace windows. It is also used to prevent polymers from darkening in sunlight and to suppress discoloration of television glass. It is applied to optical components to improve performance.

Cerium is also used in a variety of ceramics, including dental compositions and as a phase stabilizer in zirconia-based products.

Ceria plays several catalytic roles. In catalytic converters it acts as a stabilizer for the high surface area alumina, as a promoter of the water-gas shift reaction, as an oxygen storage component and as an enhancer of the NOX reduction capability of Rhodium. Cerium is added to the dominant catalyst for the production of styrene from ethylbenezene to improve styrene formation. It is used in FCC catalysts containing zeolites to provide both catalytic reactivity in the reactor and thermal stability in the regenerator.

The role of cerium in phosphors is not as the emitting atom, but as a "sensitizer."

2. Lanthanum

Lanthanum is the first element in the rare earth or lanthanide series. It is the model for all the other trivalent rare earths. After cerium, it is the second most abundant of the rare earths.

Lanthanum-rich lanthanide compositions have been used extensively for cracking reactions in FCC catalysts, especially to manufacture low-octane fuel for heavy crude oil.

It is utilized in green phosphors based on the aluminate (La0.4Ce0.45Tb0.15)PO4.

Lanthanide zirconates are used for their catalytic and conductivity properties and lanthanum stabilized zirconia has useful electical and mechanical properties.

It is utilized in laser crystals based on the yttrium-lanthanum-fluoride (YLF) composition.

3. Dysprosium

Dysprosium is most commonly used as in neodymium-iron-boron high strength permanent magnets. While it has one of the highest magnetic moments of any of the rare earths (10.6µB), this has not resulted in an ability to perform on its own as a practical alternative to neodymium compositions. It is however now an essential additive in NdFeB production.

It is also used in special ceramic compositions based on BaTiO formulations.

Recent research has examined the use of dysprosium in dysprosium-iron-garnet (DyIG) and silicon implanted with dysprosium and holmium to form donor centers.

4. Erbium

Erbium has application in glass coloring, as an amplifier in fiber optics, and in lasers for medical and dental use.

The ion has a very narrow absorption band coloring erbium salts pink. It is therefore used in eyeware and decorative glassware. It can neutralize discoloring impurities such as ferric ions and produce a neutral gray shade. It is used in a variety of glass products for this purpose.

It is particularly useful as an amplifier for fiber optic data transfer. Erbium lases at the wavelength required to provide an efficient optical method of amplification, 1.55 microns. As shown below under recent research, many interesting developments are occurring in this area.

Lasers based on Er:YAG are ideally suited for surgical applications because of its ability to deliver energy without thermal build-up in tissue .

5. Europium

Europium is utilized primarily for its unique luminescent behavior. Excitation of the Europium atom by absorption of ultra violet radiation can result in specific energy level transitions within the atom creating an emission of visible radiation.

In energy efficient fluorescent lighting, Europium provides not only the necessary red, but also the blue. Several commercial blue phosphors are based on Europium. See additional discussion of phophors.

Its luminesence is also valuable in medical, surgical and biochemical applications.

6. Gadolinium

Gadolinium is utilized for both its high magnetic moment (7.94µB) and in phosphors and scintillator material.

When complexed with EDTA ligands, it is used as an injectable contrast agent for patients undergoing magnetic resonance imaging. With its high magnetic moment, gadolinium can reduce relaxation times and thereby enhance signal intensity.

The extra stable half-full 4f electron shell with no low lying energy levels creates applications as an inert phosphor host. Gadolinium can therefore act as hosts for x-ray cassettes and in scintillator materials for computer tomography.

7. Holmium

Holmium has the highest magnetic moment (10.6µB) of any naturally occurring element. Because of this it has been used to create the highest known magnetic fields by placing it within high strength magnets as a pole piece or magnetic flux concentrator.

This magnetic property also has value in yttrium-iron-garnet (YIG) lasers for microwave equipment.

Holmium lases at a human eye safe 2.08 microns allowing its use in a variety of medical and dental applications in both yttrium-aluminum-garnet (YAG) and yttrium-lanthanum-fluoride (YLF) solid state lasers. The wavelength allows for use in silica fibers designed for shorter wavelengths while still providing the cutting strength of longer wave length equipment.

8. Lutetium

Lutetium is the last member of the rare earth series. Unlike most rare earths it lacks a magnetic moment. It also has the smallest metallic radius of any rare earth. It is perhaps the least naturally abundant of the lanthanides.

It is the ideal host for x-ray phosphors because it produces the densest known white material, lutetium tantalate (LuTaO4).

It is utilized as a dopant in matching lattice parameters of certain substrate garnet crystals, such as indium-gallium-garnet (IGG) crystals due its lack of a magnetic moment.

9. Neodymium

After cerium and lanthanum, neodymium is the most abundant of the rare earths. It shows similiar charteristics to the other trivalent lanthanides.

Primary applications include lasers, glass coloring and tinting, dielectrics and, most importantly, as the fundemental basis for neodymium-iron-boron (Nd2Fe14B) permanent magnets.

The neodymium-based magnet was first introduced in 1982 simultaneously by Sumitomo Specialty Metals (Japan) and General Motors (USA) and commercialized in 1986. It is used extensively in the automotive industry with many applications including starter motors, brake systems, seat adjusters and car stereo speakers. Its largest application is in the voice coil motors used in computer disk drives. Other uses include medical magnetic imaging equipment. For more on neodymium magnets, see Magnetic Products.

Neodymium has a strong absorption band centered at 580 nm, which is very close to the human eye's maximum level of sensitivity making it useful in protective lenses for welding goggles. It is also used in CRT displays to enhance contrast between reds and greens. It is highly valued in glass manufacturing for its attractive purple coloring to glass.

Neodymium is included in many formulations of barium titanate, used as dielectric coatings and in multi-layer capacitors essential to electronic equipment.

Ytrrium-aluminum-garnet (YAG) solid state lasers utilize neodymium because it has optimal absorption and emitting wavelengths. Nd-based YAG lasers are used in various medical applications, drilling, welding and material processing.

10. Praseodymium

Praseodymium resembles the typical trivalent rare earths, however, it will exhibit a +4 state when stabilized in a zirconia host. The element is found in most all light rare earth derivatives.

It is highly valued for ceramics as a bright yellow pigment in praseodymium doped zirconia because of its optimum reflectance at 560 nm.

Much research is being done on its optical properties for use in amplification of telecommunication systems, including as a doping agent in fluoride fibers.

It is also used in the scintillator for medical CAT scans.

11. Samarium

Samarium is primarily utilized in the production of samarium-cobalt (Sm2Co17) permanent magnets. It is also used in laser applications and for its dielectric properties.

Samarium-cobalt magnets replaced the more expensive platinum-cobalt magnets in the early 1970s. While now overshadowed by the less expensive neodymium-iron-boron magnet, they are still valued for their ability to function at high temeratures. They are utilized in lightweight electronic equipment where size or space is a limiting factor and where functionality at high temperature is a concern. Applications include electronic watches, aeospace equipment, microwave technology and servomotors. For more information on either samarium-cobalt or neodymium-iron-boron magnets, see Magnetic Products.

Because of its weak spectral absorption band samarium is used in the filter glass on Nd:YAG solid state lasers to surround the laser rod to improve efficiency by absorbing stray emissions.

Samarium forms stable titanate compounds with useful dielectric properties suitable for coatings and in capacitors at microwave frequencies.

12. Terbium

Terbium is primarily used in phosphors, particularly in fluorescent lamps and as the high intensity green emitter used in projection televisions, such as the yttrium-aluminum-garnet (Tb:YAG) variety.

Terbium responds efficiently to x-ray excitation and is, therefore, used as an x-ray phosphor.

Terbium alloys are also used in magneto-optic recording films, such as Tb-Fe-Co.

13. Ytterbium

Ytterbium is being applied to numerous fiber amplifier and fiber optic technologies and in various lasing applications. It has a single dominant absorption band at 985 in the infra-red making it useful in silicon photocells to directly convert radiant energy to electricity.

Ytterbium metal increases its electrical resistance when subjected to very high stresses. This property is used in stress gauges for monitoring ground deformations from earthquakes and nuclear explosions.

It is also used as in thermal barrier system bond coatings on nickel, iron and other transitional metal alloy substrates.

14. Yttrium

Yttrium has the highest thermo-dynamic affinity for oxygen of any element. This characteristic is the basis for many of its applications. While not part of the rare earth series, it resembles the heavy rare earths which are sometimes referred to as the yttrics for this reason.

Another unique characteristic derives from its ability to form crystals with useful properties.

Some of the many applications of yttrium include in ceramics for crucibles for molten reactive metals, in florescent lighting phosphors, computer displays and automotive fuel consumption sensors.

Yttria stabilized zirconium oxide are used in high temperature applications, such as in thermal plasma sprays to protect aerospace high temperature surfaces.

Crystals of the yttrium-iron-garnet (YIG) variety are essential to microwave communication equipment.

The phosphor Eu:Y2O2S creates the red color in televisions.

Crystals of the yttrium-aluminum-garnet (YAG) variety are utilized with neodymium in a number of laser applications.

Yttria can also increase the strength of metallic alloys.

15. Zirconium

Zirconium is primarily used in it's oxide or zirconia form. Zirconium dioxide has a high melting point (2,700 C) and a low thermal conductivity. Its polymorphism, however, restricts its widespread use in ceramic industry. During a heating process, zirconia will undergo a phase transformation process. The change in volume associated with this transformation makes the usage of pure zirconia in many applications impossible. Addition of some oxides, such as CaO, MgO, and Y2O3, into the zirconia structure in a certain degree results in a solid solution, which is a cubic form and has no phase transformation during heating and cooling. This solid solution material is termed as stabilized zirconia, a valuable refractory. Stabilized zirconia is used as a grinding media and engineering ceramics due to its increased hardness and high thermal shock resistivity. Stabilized zirconia is also used in applications such as oxygen sensors and solid oxide fuel cells due to its high oxygen ion conductivity.

16. Calcium

Calcium metal is widely used in metallurgical, chemical and other industries. It is mainly used as a reducing agent in the melting of metals, a deoxidant for alloys, a dehydrating agent for oil, as well as a desulphurizer or a decarburizer for iron and ferroalloys. It is also used in the production of calcium battery and maintenance-free lead-calcium-tin rechargeable battery.

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