以MgO-Al_2O_3-SiO_2(MAS)体系作为烧结助剂,采用无压烧结,通过控制烧结工艺,制备出具有不同晶相组成的Si_3N_4陶瓷.研究了晶相组成对氮化硅陶瓷微波介电性能的影响.借助XRD、SEM对Si_3N_4陶瓷微观组织进行了研究.结果表明:在烧结过程中,有中间相Si_2N_2O产生;经1850 ℃、0.5 h烧结,β-Si_3N_4全部转变为具有较大长径比,显微结构均匀的长柱状β-Si_3N_4晶粒;Al~(3+)和O~(2-)能够进入β-Si_3N_4晶体内形成β-Si_(6-x)Al_xO_xN_(8-x)固溶体,使晶体内部产生较大的空隙或晶格畸变,在外电场作用下,易于产生离子位移极化,导致介电常数升高;同时,随着烧结温度的提高,存在于晶界的玻璃相含量增加,试样的介电常数随之升高.
The influences of crystal phase on dielectric properties of silicon nitride ceramics produced by pressureless sintering with MgO-Al_2O_3-SiO_2(MAS) as sintering additive were investigated. The samples with different crystal phase were obtained at different sintering temperatures. The microstructure of the materials sintered at 1850 ℃ consists of elongated grains, with almost identical size and aspect ratio, which distribute uniformly throughout the body. The difference in dielectric properties of Si_3N ceramics was mainly due to the difference of the relative content of α-Si_3N_4, β-Si_3N_4, the intermediate product (Si_2N_2O) and glass phase in the samples. Compared with α-Si_3N_4 and Si_2N_2O, β-Si_3N_4 is believed to be a major factor influencing the dielectric constant. Al and O atoms can exist in β-Si_3N_4 and form a solid solution. Because of the different valence between Si, N and Al, O, there will occur dangling bonds and unpaired electrons. The free charges would move in response to the electric field, and diffusion current results from the field propagation. The high-dielectric constant of β-Si_3N_4 could be attributed to the ionic relaxation polarization.
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