Rare Earth Elements consist of 17 metal elements, including 15 lanthanides, scandium, and yttrium. In recent 20 years, rare earth elements have made great contributions to the developments of high-tech industries. For example, they have been applied in functional ceramic.
Functional ceramic is a new type of ceramic material for meeting the developing needs of computer science, automatic control, sensing technology, bioengineering, environmental science and other fields. It can realize certain functions with direct effects and coupling effects of electricity, magnetism, sound, light, heat, and power. Because of its numerous types and various functions, functional ceramic has been widely used in electrical equipment, signal processing, sensor measurement, semiconductor devices, superconducting materials and other areas.
Functional ceramic is closely related to rare earths. It is known that most superconducting ceramics contains rare earth elements. For example, yttrium barium copper oxide (YBCO) is a kind of oxide ceramics with excellent high temperature superconductivity. With the addition of rare earths in the raw materials, the agglutinating property, density, intensity and other unique functions of functional ceramics will be greatly improved.
Applications of Rare Earths in Functional Ceramics
Rare earths for superconducting ceramics
Scientists have been studying on the functions and applications of high temperature superconducting ceramics with rare earth since 1987. According to the Japanese scientists’ research, replacing y in ybco with nd, sm, eu, gd and other light rare earths, the critical magnetic field strength and flux pinning force of Inbco will be remarkably improved. With special processing, the lnbco block can capture more than 10t magnetic field at 77k, which can substitute for the magnet of the maglev train.
Peking University did relating researches, in which yttrium, barium oxide and copper stratified evaporation were diffused with supporting base of ZrO2 under high temperature of 392℉. After heat processing with the temperature between 1472℉ to 1652℉, superconducting ceramics will have excellent temperature coefficient of metallic resistance. Japan Kagoshima University found that adding rare earth lanthanum in to Strontium oxide and Niobium oxide can produce a special ceramic film which has superconductivity quality at 255k.
Rare Earths for Piezoceramic
Lead titanate (PbTiO3) is a typical piezoelectric ceramic with mechanical energy-electric coupling effects. It can be applied in high temperature and high frequency conditions, since it has high Curie temperature (914℉) and low dielectric constant. However, this material is likely to have microcracks because of cubic/ tetragonal phase transition during the cooling process. With addition of rare earths, PbTiO3 can turn into re-PbTiO3 with better microstructure and higher density. Study has found that dielectric constant of re-PbTiO3 ceramics will reduce and piezoelectric anisotropy will increase due to the substitution of rare earth ions re3+. This material can be used in the manufacture ultrasonic transducer of high sensitivity and resolution, because ceramic’s dielectric constant and radial electromechanical coupling coefficient decrease and high-frequency resonance peak becomes simple.
By adding rare earth oxides such as La2O3, Sm2O3 and Nd2O3 into the piezoelectric ceramics, its sintering performance can strongly be improved and its electrical properties and piezoelectric properties will be more stable. The quality of PZT ceramics can be improved by adding 0.2%-0.5% rare earth oxide CeO2. Ceramic’s volume resistivity will increase, which is good for achieving high temperature and high electric field polarization. Besides, aging resistance of the ceramics will also be improved. PZT ceramics modified by rare earths have been widely used in high voltage generators, ultrasonic generators, underwater acoustic transducers and other devices.
Rare Earths for Conductive Ceramic
The yttrium stabilized zirconia ceramic with the addition of rare earth oxide Y2O3 has good thermal stability and chemical stability at high temperature. As a good oxygen ion conductor, this ceramic plays an important role in ion conductive ceramics. YSZ ceramic sensor has been used to measure the oxygen partial pressure in automobile exhaust gas. Besides it can effectively control air and fuel ratio which can remarkably save energy. Now it has been widely applied in industrial boilers, smelting furnace, incinerator and other combustion-based equipment. Precise ceramics with rare earth can also be used as SOFC battery bipolar plate, porous cathode and Porous anode material.
Rare Earths for Dielectric Ceramic
Dielectric ceramics are mainly used for ceramic capacitors and microwave dielectric elements. With the addition of La, Nd, Dy and other rare earth elements, the dielectric properties of the TiO2、MgTiO3、BaTiO3 will be remarkably improved. In the dielectric ceramic for thermal compensation capacitors, rare earth can be added to realize the improvement or adjustment of the dielectric constant, temperature coefficient and quality factor of the ceramic, and expand its application range. Microwave dielectric ceramic has a large category. The ceramic mixed with rare earth oxide BaO-Re2O3-TiO2 is a common material with various applications with the dielectric constant ε over 80.
Rare Earths for Sensitive Ceramic
Sensitive ceramic is one of the key functional ceramics, with the characteristic that it has sensitive reactions towards some external conditions such as voltage, gas composition, temperature, humidity and so on. It is also known as the sensor ceramic widely used in sensing elements because it can control the circuit or monitor the environment. There is a close relationship between rare earth and the properties of sensitive ceramic.
Transparent PLZT ceramic can be made when La2O3 is added into PZT. PLZT electro-optical ceramics has electro-optical effect, secondary electro-optic effect (Kerr effect), light scattering effect and optical memory effect. Among them, the Kerr effect is most commonly being used, such as shielding goggles for nuclear explosive radiation, windows for heavy bombers, optical communication modulators, holographic recording devices, etc. As PLZT electro-optical ceramic can change its optical properties with the use of electric field, it marks that ceramic material can be applied in the field of functional optics.
Pressure Sensitive Ceramic
According to the study on the effect of rare earth elements on the electrical properties of ZnO pressure-sensitive ceramics, varistor voltage vlma value was significantly improved after adding rare earth oxide la2o3 into the ceramic. When the addition increases from 0.1% to 10%, the non-linear coefficient α of the ceramic will decrease from 20 to 1, at which the ceramic is basically non-pressure sensitive. Therefore, low concentration of rare earth elements can increase the value of ZnO ceramic’s varistor voltage. However, high concentration of rare earth elements will made the ceramic with pressure sensitive characteristic.
Gas Sensitive Ceramic
Dalian University of Technology has studied on the function of ReFeO3 material with the addition of rare earth oxide Re2O3. According to their foundings, ultrafine grainification of materials is an important factor to improve the sensitivity of gas sensor. Besides, different kinds of rare earths have different impacts on the microstructures of the materials. It was found that the ReFeO3 series had higher sensitivity to ethanol and the order of sensitivity was ndfeo3> smfeo3> lafeo3. Besides, they have low sensitivity to gasoline and little reaction to other gases, therefore they have wide options.
Currently barium titanate (BaTiO3) is the most frequently studied and most widely used thermal ceramics. When the batio3 is mixed with rare earth elements such as la, ce, sm, dy, y, the resistivity of the ceramic is significantly reduced. Because Re3+ replaces part of Ba2+, leading to excess positive charge and forming weakly bound electrons with the effect of Ti4+. However, when the addition of rare earth exceeds a certain value (for example, Moles of La> 0.35%), the resistivity of the ceramic increases abruptly, and the ceramic can even become an insulator.
Humidity Sensitive Ceramic
There is a wide variety of wet sensitive ceramics. Their main additions of rare earths are lanthanum and its oxides, such as Sr1-xlaxSnO3, La2O3-TiO2, and La2O3-TiO2-V2O5. In order to further improve the sensitivity of humidity ceramic, as well as enhance its practical usage in terms of the stability, it is necessary to study on the properties of humidity sensitive ceramic with addition of rare earths.