高导热氮化硅晶粒原位自形生长调控及致密化堆积研究

Silicon nitride ceramics as a newly developed insulating and thermal conductive material has many excellent properties that other materials can not match. To raise the thermal conductivity of silicon nitride ceramics can further enlarge the applicable area of the ceramics. The thickness of grain boundary film between β-Si3N4 grains is much large than that of the equilibrium one, owing to the nature of random packed faceted grains in the ceramics. A theoretical calculation and some experiment results reveal that the thick grain boundary film is extremely harmful to the thermal conductivity of silicon nitride ceramics. To minimize the thickness of the film, the random packed faceted grains must be adjusted. This project aims at studying the morphological change of the β-Si3N4 grains on Ostwald ripening during sintering and a heat treatment of the β-Si3N4 raw powders; and to reduce the aspect ratio of the grains by changing sintering additives and the processing parameters of both the sintering and the heat treatment. Besides that, the powders of different sizes will be blended to study the grains size distribution after Ostwald ripening, in order to moderate the grain size of the ceramics, and to minimize the thickness of the grain boundary film. Based on these work, the impact of the grain boundary film on the thermal conductivity of silicon nitride will be studied.

氮化硅陶瓷在导热绝缘封装材料领域也有其不可替代的地位。并且随着氮化硅陶瓷热导率的提高，其应用领域有望进一步拓展。由于氮化硅晶粒各向异性生长形成杂乱堆积的棱柱状晶体，氮化硅陶瓷晶粒间的晶界相薄膜的厚度远远超过其平衡晶界层厚度，而已有理论计算和实验表明该晶界相薄膜厚度对氮化硅陶瓷的热导率有着巨大的影响。减小晶界相厚度需要控制原位自生氮化硅晶粒的形状以及晶粒尺寸级配才能实现。本项目拟对β氮化硅原料粉在奥氏熟化过程中发生的晶粒形貌演变及其动力学进行系统研究，通过改变烧结助剂种类及配比、改善烧结及热处理工艺等，控制氮化硅晶粒的各项异性生长，获得长径比较小的氮化硅晶粒。另外通过调整原料粉颗粒级配研究其对烧结及热处理后晶粒分布的影响，通过原料粉的合理级配控制原位奥氏熟化后的晶粒的级配，改变晶界相厚度。在此基础上开展晶界相厚度对氮化硅陶瓷热导率的影响影响研究。

采用放电等离子烧结后高温热处理的方法研究了氮化硅原料粉晶型、烧结助剂添加量、稀土烧结助剂种类对氮化硅晶粒原位自形生长的影响，并探讨了氮化硅陶瓷热导率的影响因素，得到以下主要结果：.1 氮化硅可以看作复合材料，应用复合材料的串并联模型进行计算。热导率λ取决于氮化硅晶粒的体积分数V，以及氮化硅晶粒自身的热导率λ_f和晶界相的热导率λ_m,即：.lgλ=〖(C〗_f lgλ_f-lgλ_m)V+lgλ_m.或者.λ=(〖λ_m)〗^(1-V) (〖λ_f〗^(C_f ) )^V.其中Cf为表征柱状晶之间导热的参数，可由实验确定。.2 α→β相变可加快粉体的溶解-析出烧结，促进β-Si3N4晶粒的原位自形生长，形成高长径比的异常生长柱状晶。该柱状晶在异常长大的的过程中推开相邻的晶粒，导致晶粒间间隙增大，降低晶粒的堆垛密度，对导热产生不良影响。而采用β-Si3N4原料粉可以抑制异常生长，提高堆垛密度，显著改善热导率。然而，由于原位自形生长的β-Si3N4柱状晶在氮化硅陶瓷中起着自增韧作用，失去异常长大的柱状晶会减小柱状晶的平均长径比，降低抗弯强度。.3、稀土元素的化合物负离子的电负性不同，则与溶于液相中的Si3N4元素的反应不同。反应会延迟Si3N4元素的扩散，从而影响氮化硅晶粒的原位自形生长以及晶粒的堆垛密度。与Si3N4的反应趋势按照La2O3、LaF3、LaCl3的顺序依次增强，柱状晶的尺寸和长径比则按照该顺序依次减小，而氮化硅晶粒的堆垛密度则按照该顺序依次增加。相应地，其热导率也按照该顺序依次增加。.该研究阐明了氮化硅陶瓷的微观组织和热导率的关系。明确了氮化硅陶瓷可以看作氮化硅晶粒和晶界相的复合材料，并推导出了氮化硅陶瓷热导率计算的本构方程。还阐明了柱状晶异常生长和氮化硅堆垛致密度的关系，证明了可以通过选择原料粉和稀土化合物种类调控氮化硅晶粒原位自形生长，提高堆垛密度。研究成果为进一步开展氮化硅陶瓷热导率研究提供了新思路。