Research On Preparation And Infrared Emissivity Property Of Doped Lanthanum Manganites

Infrared emissivity is a physical property for material under certain temperature. According to Stefan-Boltzman law: E=σεT4, infrared emissivity is an important factor for infrared radiant energy. Higher emissivity can lead to higher radiant energy. Besides colossal magnetoresistance effects, metal-insulator and corresponding ferromagnetism-paramagnetism transition are found to occurr in alkaline earth doped lanthanum manganites. Emissivity of metal is low, while that of insulator is high. Thus, due to metal-insulator transition, infrared emissivity of doped lanthanum manganites can change significantly with temperature, which makes them attractive as thermal control material. In this work, we focus on the infrared emissivity and improvement of variable emissivity of ABO₃-type lanthanum manganites, investigating the structure, electromagnetic properties, infrared absorption and emissivity of La_1-xA_xMnO₃ (A=Sr and Ca), La_1-xA_xMnO₃ (A=Na and K), (La_0.8Sr_0.2)_1-xMnO₃ and La_0.8Sr_0.2Mn_1-xB_xO₃ (B=Cu and Ti) systems. Appropriate thermal control systems have been confirmed and variable emissivity property has been improved successfully. The relationship between ABO₃ doped structure and infrared emissivity has been discussed. On the basis of this, infrared emissivity of Sr doped lanthanum manganites film and coating has been primarily investigated.1. Divalent Sr and Ca doped lanthanum manganites have been prepared, respectively, and the relationship between emissivity and doping level, temperature, waveband, roughness, crystal lattice has been systematically investigated for the first time. The results indicate that Sr-doped samples are rhombohedral, but with increasing doping level, greatly reducing Mn³⁺ ions result in the transformation of crystal lattice from rhombohedral to cubic, and the structure of Ca-doped samples are cubic. Due to double-exchange interaction between Mn³⁺ and Mn⁴⁺, electrical resistivity and ferromagnetism of Sr doped samples are enhanced with increasing doping level, which leads to the decrease of emissivity. According to the close relationship between electric and magnetic properties, we conclude that emissivity of lanthanum manganites relates to, not only electrical resistivity, but also ferromagnetism, and intenser ferromagnetism can lead to lower emissivity value. For Ca doped samples, electrical resistivity was enhanced for doping level x=0.1⁰.3 samples, but weakened for x=0.4 sample. The ferromagnetism was enhanced for all the Ca doped samples. Ferromagnetic-insulated phase occurred in the sample due to orbital ordering. Emissivity of Ca doped samples decreased with increasing doping level. Thus, we concluded that the ferromagnetism has stronger effect on emissivity than electrical resistivity. Roughness has no drastic influence on emissivity for bulk lanthanum manganites, and emissivity of rhombohedral and orthorhombic samples, prepared by solid-state reaction and sol-gel method, respectively, was significantly different. The emissivity in the 8¹4μm waveband for x=0.2 Sr doped sample increases significantly with temperature, due to the metal-insulator phase transition, and the variable range is 0.13 between 288 K and 373 K. The calculated radiant energy arrives to 536.8 W/m2 at 328 K, and radiant property is improved about 18 %. However, in the 3–5μm waveband, no drastic emissivity change occurs. Emissivity of Ca-doped samples gradually decreases with increasing temperature. Therefore, x=0.2 Sr doped sample may have the potential for application as thermal control material.2. The monovalent-doped lanthanum manganites La_1-xA_xMnO₃ (A= Na and K) were prepared by standard solid-state reaction method. The solubility of Na and K ions in the system is low. The monovalent doped samples are in the paramagnetic state, and the intensity of electron spin resonance decreases with doping level, indicating decreasing concentration of single electron and orbital transition probabilities, which results in the decrease of emissivity. Due to increasing vibrating dipole moments induced by spontaneous polarization, new infrared absorption occurs in the 8¹4μm waveband. Due to the weakening of magnetic properties and infrared absorption, the emissivity of monovalent-doped lanthanum manganites is higher and K doped samples exhibit higher emissivity values than Na doped samples. The emissivity of monovalent-doped lanthanum manganites remains constant in the whole temperature range. Therefore, the monovalent-doped system is not suitable as thermal control material.3. The emissivity of non-stoichiometric (La_0.8Sr_0.2)_1-xMnO₃ system was systematically investigated for the first time and sample with bigger variable-emissivity range has been prepared successfully. The electrical resistivity and ferromagnetism of the samples decrease with A-site deficient levels, due to the increase of electron-phonon and magnetism scatter induced by the change of space structure and inner stress. And new infrared absorption occurs in the 8¹4μm waveband induced by distorted crystal lattice. The emissivity of the samples shows increase-decrease trend with deficient level, and highest value has been obtained when deficient level x is 0.2, which may be related to intense infrared absorption in the 8¹4μm waveband. Thus, we conclude that the influence of infrared absorption on the emissivity is stronger than that of electrical resistivity and ferromagnetism. The emissivity change of x=0.2 sample is 0.2 in the temperature range of 288³73 K, and even 0.15 near room temperature 288³13 K. The calculated radiant energy arrives to 440.8 W/m², and radiant property is improved about 24.6 %. Therefore, non-stoichiometric samples have greater potential for application as thermal control material.4. Due to the important effect of Mn-O-Mn network, the emissivity of Cu and Ti doped system was investigated, respectively. With increasing Cu doping level, ordered structure was formed, and then ordering lowers the symmetry, which enhances asymmetric vibration absorption and narrows electron spin resonance linewidth. Two new infrared absorption peaks occur in the 8¹4μm waveband for Ti doped samples. The emissivity of B-site doped lanthanum manganites is higher, due to magnetic transformation from ferromagnetic to paramagnetic and intense infrared absorption in the 8¹4μm waveband. No drastic change occurs in the whole temperature range for B-site doped samples. Therefore, the B-site doped system is not suitable as thermal control material.5. The relationship between infrared emissivity and crystal structure, electromagnetic properties, infrared absorption has been analysed on the basis of ABO₃ doping structure. We conclude that the emissivity property of doped lanthanum manganites is strongly related to the coupling effect between infrared radiation and crystal, electric, magnetic structure.6. On the basis of research on emissivity of bulk lanthanum manganites, Sr doped lanthanum manganites films and coatings have been prepared, and emissivity of the samples is primarily investigated. The films were prepared by sol-gel and reversed-dip-coating method. The emissivity of the films decreases with thickness, and remains constant when the thickness arrives to 1400 nm. Ultraviolet radiation results show that emissivity property is changeless after radiation. The lanthanum manganites coatings were prepared by Sr doped manganites and epoxy modified polyurethane. The emissivity of the coatings decreases with increasing pigment concentration.