Synthesis Of Boron Nitride Nanotubes And The Strengthening And Toughening On Alumina And Silicon Nitride Ceramics

Carbon nanotubes (CNTs) attract worldwide attention due to their unique physical, chemical, electronic, and mechanical properties. A lot of research has been proved that CNTs are very promising strengthening agency for composites. High quality composites have been fabricated via incorporating CNTs into polymer and ceramic matrices. Boron nitride nanotubes (BNNTs), which can be imaged as rolled up hexagonal BN layers, are structure analogue of CNTs. Similar to CNTs, BNNTs possess high tensile modulus and tensile strength, which are also regarded as the proper strengthening agency for composites. BNNTs are more chemically and thermally stable than CNTs, and the oxidizing temperature is as high as900℃. Therefore, BNNTs are more promising strengthening agency for high-temperature structural ceramics.Up to now, the methods available have the disadvantages of complicated apparatus, high reaction temperatures, high cost, and catalyst impurities encapsulated, which restrict investigations on BNNTs in both fundamental research and technological applications. Therefore, synthesis of BNNTs in large-scale is still a great challenge. To develop large-scale technique, simplify process and reduce the cost has great theoretical significance and potential value. In this work, owing to structural similarity between BNNTs and CNTs, we proposed a simple and effective method to prepare BNNTs in large scale using CNTs as template, and investigated the synthesis mechanism. Based on large scale synthesis of BNNTs, they were incorporated into Al2O3and Si3N4. The properties and micro structures of the composites were characterized, and the influence of BNNTs on ceramics was discussed in detail. The main contents are summarized as follows:(1) The BNNTs synthesized in different conditions (reactants, time, temperature and CNTs template) were investigated, and the synthesis process was optimized. The BN-coated CNTs were synthesized by the reaction between NaBH4and NH4Cl at500-600℃for10-18h. Finally, pure BNNTs were obtained by oxidizing the products at800℃in air. The proposed method had the advantages of simple apparatus, low temperature and catalyst-free growth. Meanwhile, CNTs did not take part in the reaction during synthsis, avoiding emergence of BxCyNz nanotubes.(2) The BNNTS/Al2O3composites were fabricated by hot pressing using BNNTs as strengthening agency and micro-sized γ-Al2O3as raw materials. The ambient mechanical properties and thermal shock resistance were investigated and reported for the first time. The experimental results showed that BNNTs could fine grain size, and produce obvious strengthening and toughening effect. Compared to the properties of monolithic Al2O3ceramic, the composite containing2.0wt%of BNNTs exhibited bending strength of523MPa, increased by67%, and that containing1.0wt%of BNNTs had fracture toughness of6.4MPa·m1/2, increased by31%. However, BNNTs hindered even restrained thermal conductivity in the sintered body due to the low content. Therefore, the thermal shock resistance was not improved obviously.(3) The BNNTs/Al2O3composites were fabricated by hot pressing using BNNTs as strengthening agency and submicro-sized α-Al2O3as raw materials. The related properties and interfacial bonding were investigated and reported for the first time. The experimental results showed that the dispersing agent--SDS could improve the dispersion of BNNTs in matrix, and the chemical bonding between BNNTs and Al2O3could improve the interfacial bonding, leading to the increase of room and high temperature mechanical properties of Al2O3effectively. In comparison to monolithic Al2O3ceramic, the composite containing1.5wt%of BNNTs exhibited bending strength and fracture toughness of580.9MPa and6.1MPa-m1/2, increased by59%and21%, respectively. Meanwhile, the composites possessed higher bending strength than the monolith in the test range up to700℃. However, thermal shock resistance was not improved sufficiently due to the low content.(4) The BNNTS/Si3N4composites were fabricated by hot pressing using BNNTs as strengthening agency and submicro-sized Si3N4as raw materials. The related properties of the composites were investigated for the first time. The experimental results showed that for the composites sintered at1700℃, bending strength was increased obviously. Compared to the monolithic Si3N4, bending strength of1.5wt% BNNTs/Si3N4composite was838.9MPa, increased by33%. For the composites sintered at1750℃, BNNTs can promote growth of β-Si3N4, resulting in improvement of aspect ratio. Therefore, bending strength decreased a little, but fracture toughness was increased drastically. The composite containing0.5wt%and1.5wt%BNNTs was increased by40%, reaching9.7and9.8MPa·m1/2, respectively.(5) The addition of BNNTs in Al2O3and Si3N4ceramics produce obvious strengthening and toughening effect. The strengthening and toughening mechanisms of BNNTs for Al2O3and Si3N4ceramics included crack deflection, debonding, bridging, pulling out and breaking of BNNTs, especially the coupling effect of grain bridging and BNNTs bridging and special fracture mode of BNNTs in the experiment. The matrix bore tensile stress along the axis direction and compress stress along radial direction due to the mismatch between BNNTs and Al2O3/Si3N4. The compress stress along radial direction can improve interfacial bonding, and restrain crack propagation, which is an important strengthening and toughening method.This work was supported by the National Natural Science Foundation of China (Nos.50972076,50872072and51042005), Science and Technology Development Project of Shandong Province (Nos.2011GGX10205and2009GG10003003), Independent Innovation Foundation of Shandong University (Nos.2009TS001and2012TS213) and Graduate Independent Innovation Foundation of Shandong University (No. yzc10115).