Lanthanum chromate (LaCrO3) was developed in the 1960s with the need for electrodes of magnetohydrodynamic (MHD) generators. Because in the MHD generator, the hot electrically conductive gas passes through a pipe with a magnetic field. At an appropriate angle, the electromotive force of the air flow in the magnetic field can generate a voltage between the electrodes installed on both sides of the pipeline. The temperature of the gas must be close to 2000°C, and potassium is used to activate the gas to cause conduction. Therefore, the electrode material must have electronic conductivity and be resistant to potassium corrosion at high temperatures. At the same time, considering the cooling process, the material must withstand a high temperature of 1500℃ for 10000h. LaCrO3 has a melting point of 2500°C, high conductivity (about 100S/m at 1400°C), and corrosion resistance, so it is an ideal material. However, LaCrO3 is currently not used much in MHD systems, but is mostly used for special heating elements.
Lanthanum chromate is a cubic crystal with a perovskite structure and is a typical ionic crystal. Because part of La3+ or Cr2+ can be replaced by Ca2+, Mg2+, Sr2+, etc., a solid solution can be formed, which greatly improves the conductivity.
In an oxidizing atmosphere, the replacement of part of La3+ by Ca2+ causes insufficient positive charge and generates part of Cr4+.
As Cr3+ becomes Cr4+, electronic conductivity is formed. In an oxidizing atmosphere, there is basically no change in the electrical properties and structure of the material before and after high-temperature volatilization. On the contrary, its conductivity increases slightly after oxidation.
2. Manufacturing process of lanthanum chromate conductive ceramics
The manufacturing process of lanthanum chromate conductive ceramics is similar to other new ceramics. It has higher requirements on the purity, fineness, particle shape and gradation of raw materials.
The formulation of lanthanum chromate ceramics can be formulated as La(MexCr1-x)O3, where Me can be Mg, Al, Ca, Sr, Fe, etc.
The biggest disadvantage of LaCrO3 is that Cr2O3 is volatile, which is more serious when it is above 1000°C. Therefore, in formulas, +2 alkaline earth metal ions (such as Mg2+, Ca3+, Sr2+, etc.) are often added to replace part of La3+ to obtain semiconductors
La1-x(Ca,Sr)CrO3(x=0~0.12), compared with pure LaCrO3, its performance is greatly improved. The purity of La2O3 and Cr2O3 should be chemically pure, with molar ratio, first calcined in the air at 900℃ for 2h to remove the adsorbed water and decompose part of the hydration products. Ca is added in the form of CaCO3 and then prepared by molar ratio (x=0~0.12). The powder is best wet-mixed with plastic balls in a plastic ball mill for 48 hours, dried and then added with a binder, and then pressed into shape.
The sintering of lanthanum chromate ceramics is carried out in an oxidizing atmosphere, and the sintering temperature is about 2000°C.
lanthanum chromate ceramics
3. The properties and uses of lanthanum chromate conductive ceramics
The melting point of lanthanum chromate is relatively high, about 2500℃, density is 6.5g/cm3, and electrical conductivity is relatively high, at 200~300℃, it is 0.1Ω-1·cm-1; at 1000℃, it is about 10^( -3) Below Ω-1·cm-1, similar to metal conductivity, it is a very good pure electronic conductive ceramic. Lanthanum chromate ceramics are mainly used for heating elements and high-temperature electrode materials, as well as composite electrode materials with ZrO2. The lanthanum chromate ceramic heating element can be directly energized at room temperature, and its surface temperature can reach 1900℃, which can maintain stable performance under high-temperature oxidizing atmosphere.