Negative Thermal Expansion Materials Of A_1-xB_xMnO_3-δ: Synthesis And Properties

To meet the performance demands, most instruments should consist of various materials. The different thermal expansion coefficient of each component material may lead to thermal mismatch as temperature increasing. The small cracks in the device will cause performance failures or even instrument damages. NTE materials have attracted considerable research attention in production of composite bodies with precisely controllable positive, negative or zero coefficients of thermal expansion.Most A1-xBxMn O3 compounds due to its very interesting physical properties, such as electrical conductivity, the magnetic resistance effect, magnetocaloric effect and negative thermal expansion caused widespread concern. These compounds have great potential applications, including magnetic refrigeration compressor and solid fuel cells.This article mainly discusses the perovskite manganese oxide A1-xBxMn O3 materials for negative thermal expansion of the research, mainly includes two aspects.(1) the Sm3+ ions were doped by Sr2+ in vacancy which generated Sm1-xSrxMn O3-δ, and researching on its crystal structure and controllable thermal expansion performance;(2) Crystal structure and negative thermal expansion performance research in Nd Mn O3. Mechanism of the negative expansion materials were also discussed.First chapter is the introduction part, the paper introduces the definition, classification of negative thermal expansion, the negative thermal expansion mechanism, preparation methods, and the research content and purpose.The second chapter is the sample performance test, which mainly introduced the test instruments used.The third chapter materials preparation and performance characterization of Sm0.85Sr0.15 Mn O3-δ. The experimental results show that:(1) A novel negative thermal expansion material of Sm0.85Sr0.15 Mn O3-δ was synthesized by solid-state method with NTE coefficient of-10.08×10-6 /K from 360 to 873 K.(2) The particles are homogenous spherical or elliptic spherical particles with uniform particle size about 1-2μm. The material of Sm0.85Sr0.15 Mn O3-δ is relatively dense with the relative density of 95.5%.(3) The ceramic Sm0.85Sr0.15 Mn O3-δ crystallizes in orthorhombic structure with space group pbnm. When Sr2+ substitutes the Sm3+ in Sm Mn O3, Sr2+ will occupy the position of Sm3+. To maintain the valence balance, electronic transfer occurs in the Mn3+ converting into Mn4+ in Sm0.85Sr0.15 Mn O3-δ. The Mn3+-O2--Mn4+ structure forms in the process.(4) The thermal property of Sm0.85Sr0.15 Mn O3-δ is considered to related to the interaction of lattice vibration and electron transfer between Mn ions. As the temperature rises, the lattice vibrates dramatically and the more Mn3+ converting into Mn4+.Also, electron transfer rate will increase between Mn3+and Mn4+ ions as temperatures increasing. The number of Mn3+ that can cause Jahn-Teller effect increases. The oxygen ions in Mn3+O6 octahedron will get slant or even produce oxygen defects. The contributions of lattice vibrations and electron transfer between Mn3+and Mn4+ to the thermal expansion have a change with the increasing temperature.The fourth chapter is materials preparation and performance characterization of Sm0.85Sr0.15 Mn O3-δ. The experimental results show that:(1) The sample of Nd Mn O3 is composed with condensed matter of block structure. Average particle grain size is about 6 ~7 microns, high purity.(2) Nd Mn O3 in 1523 K. When the temperature is from 233 K to 269 K, the ceramic Nd Mn O3 shows negative thermal expansion of-0.8487×10-6 /K. When the temperature is from 293 to 759 K, the ceramic Nd Mn O3 shows negative thermal expansion of-4.7×10-6 /K. As temperature increases, the ceramic Nd Mn O3 presents NTE property range from 759 K to 1007 K. The average linear expansion coefficient is-18.88×10-6 /K; Nd Mn O3 in 1573 K, When the temperature is from 293 K to 690 K, the ceramic Nd Mn O3 shows positive thermal expansion of 1.26×10-6 /K. As temperature increases, the ceramic Nd Mn O3 presents NTE property range from 785 K to 1047 K. The average linear expansion coefficient is-23.75×10-6 /K;(3) The reasons that Nd Mn O3 material has negative thermal expansion performance are: the variable temperature XRD analysis shows that Nd Mn O3 crystal exists phase transition. With the increase of temperature, Nd Mn O3 samples transition to rhombohedral phase from orthorhombic phase in 493 K, continue to raise the temperature, Nd Mn O3 sample transition to the cubic phase from rhombohedral phase in 1023 K. this phase transitions cause Mn-O chemical bond shrink in Nd Mn O3, Nd-O chemical bond extension, and temperature changes in the whole process, the shrink of Mn-O chemical bond to a greater extent than Nd-O chemical bond extension extent, which shows volume shrinkage phenomenon on macro-scale...