Lanthanum aluminate belongs to ABO3 perovskite structure compounds, with a small dielectric constant, low dielectric loss, good lattice mismatch, thermal expansion coefficient, good chemical stability, wide energy gap, specific surface area is big, has a certain activity, good thermal stability, has been widely applied in the catalytic materials, substrate material, high temperature superconducting thin film substrate, synthesis of microwave dielectric ceramic materials, high temperature fuel cells, Microwave dielectric resonator, etc. At present, there are many methods used to prepare nanometer lanthanum aluminate, among which the more commonly used are: combustion synthesis method, co-precipitation method, sol-gel method, alcohol-brine hydrolysis method. Among these methods, combustion method has obvious advantages. It is the use of metal nitrate and combustion agent reaction, after reaching the ignition temperature of spontaneous combustion, to achieve in-situ oxidation, with the promotion of combustion wave, the reactants quickly transformed into final products, which greatly shortening the experiment cycle. And the product distribution is uniform, purity is high, product activity is high.
At room temperature, lanthanum aluminate belongs to the tripartite crystal system (symmetry group R3C), which is formed by the rotation Angle of Alo6 octahedron around a triple axis in the cubic phase. When the temperature increases, lanthanum aluminate enters the cubic crystal system, and the symmetry group is PM3m. The cell volume is doubled due to the rotation of the Alo6 octahedron. Thus, the zero phonons in the R3C structure are derived from the modules of Γ(0,0,0) and R(1/2, 1/2) points in the Brillouin region of the cubic cell. This phase transition can be represented by a soft mode in the Brillouin region. Figures 1.2 and 1.3 show the ball-and-stick model with lanthanum aluminate structure
The main preparation methods of lanthanum aluminate are: combustion synthesis, co-precipitation, homogeneous precipitation, sol-gel, alcohol-brine hydrolysis and liquid phase mixing. The source materials of lanthanum and aluminum are generally metal oxides, inorganic salts or alkyl salts.
Method 1: Weighed a certain amount of La (NO3) 3·6H2O, Al (NO3) 3·9H2O and C6H14O6 according to different molar ratios, put the reactants in a beaker, added 10mL of deionized water and dissolved them respectively, stirred them into a uniform solution with a magnetic agitator, and then transferred them to the same beaker and continued stirring until they were fully mixed. Slowly add nitric acid or ammonia to adjust the pH value of the mixed solution, the resulting solution is the precursor solution. Transfer liquid precursor to the crucible, crucible in the muffle furnace heating, when the temperature is high enough, the precursor of solvent evaporation, when light temperature is reached, self-propagating combustion reaction in the muffle furnace, violent reactions and is accompanied by a large amount of smoke generated, end of combustion process in a few seconds, continue roasting, full and fluffy powder reaction after the crucible.
Method 2: A method for preparing nanometer lanthanum aluminate powder with high crystallinity consists of steps in the following sequence:
(1) Aluminum nitrate and lanthanum nitrate with molar mass ratio of 1∶1 are fully dissolved in pure water at 50 ~ 70℃, then complexing agent and surfactant are added to dissolve, and the complexing is continuously stirred for 2 ~ 5 hours at the temperature of 70 ~ 100℃. Wherein, the molar mass ratio of the complexing agent to aluminum nitrate and lanthanum nitrate ranges from 0.5-2.0 to 0.5-1.0; The amount of surfactant is 5 ~ 15% of the total amount of aluminum nitrate and lanthanum nitrate oxide.
(2) The mixed solution with good complexation was explosively decomposed for 0.5 ~ 1.5 hours under the condition of 300 ~ 450℃ to make powder;
(3) Lanthanum aluminate nanometer powder was prepared by calcining the decomposed powder at 800 ~ 1000℃ for 2 ~ 4 hours.
Application of [3]
As a new rare earth material, lanthanum aluminate has been widely used in electronic devices, catalysis, high temperature fuel cell, ceramics, sewage treatment, substrate materials and so on. Lanthanum aluminate particles with perovskite structure, as a new type of rare earth particulate material, have been widely used in electronic devices, catalysis, high temperature fuel cells, ceramics, sewage treatment, substrate materials, etc.
Lanthanum aluminate belongs to ABO3 with perovskite structure. It has good microwave dielectric properties and good lattice matching with high temperature superconductivity and BST ferroelectric thin films, so it is widely used as substrate material.
Lanthanum aluminate single crystal is the most important single crystal material for large scale high temperature superconducting film substrate at present. It matches well with the lattice of high temperature superconducting materials such as Ybacuo, has low dielectric constant and low microwave loss, so it is suitable for making high temperature superconducting microwave electronic devices, such as high temperature superconducting microwave filter in remote communication, and has great practical and potential application prospects. At present, the spectral properties of lanthanum aluminate and lanthanum aluminate doped with Nd3+, Er3+, Eu3+ and Ce3+ have been reported. The growth of lanthanum aluminate thin films on silicon substrates is one of the research hotspots at home and abroad. Using the atomic layer controlled heteroepitaxy technique, the lanthanum aluminate thin films with high dielectric constant were fabricated on silicon substrates, and the leakage current of the thin thin films with high dielectric constant was reduced, which is one of the key technologies for the next generation of field effect devices.
