Pressureless Sintering And Properties Of Si₃N₄ Ceramics Fabricated At Low Temperature Using Rare Earth Fluoride Additives

Silicon nitride is considered as a promising inorganic material due to its excellent mechanical properties and dielectric properties.Traditional rare earth oxide sintering additives obtain high sintering temperature and exploring the technology of low temperature sintering and microstructure regulation of silicon nitride ceramics becomes an important development direction.In this study,the influence of types and content of rare earth fluoride sintering additives on the microstructure,mechanical properties and dielectric properties of silicon nitride ceramics fabricated by low temperature（1550°C）pressureless sintering is investigated.Influence on microstructure and properties using different kinds of rare earth fluoride Re F₃（Re=La,Gd,Dy,Tm,Yb）sintering silicon nitride ceramics is investigated.When the sintering additives Gd F₃、Dy F₃、Tm F₃、YbF₃ are added,theα-βphase conversion rate increases with the decreasing of the rare earth cation radius,that is because rare earth atoms can be adsorped on silicon nitride crystal and the phase conversion activation energy decreases.Theα-βphase conversion rate can reach 100%when adding YbF₃,and silicon nitride ceramics obtain good flexural strength and fracture toughness.To optimize YbF₃ system,influence of YbF₃-Yb₂O₃ compound sintering additives on the microstructure and properties of silicon nitride ceramics is investigated.The results show that in the YbF₃-Yb₂O₃system,the increasing of YbF₃ content improves theα-βphase conversion rate obviously.And rod-like crystals in morphology are obtained,the aspect ratios of which are over 8.Theα-βphase conversion rate is 77%,the aspect ratio is 8.36 and the flexural strength is 269 MPa when adding 5 wt%YbF₃and 2.5 wt%Yb₂O₃ as sintering additives.In order to obtain higher conversion rate,higher aspect ratio silicon nitride ceramic,influence of YbF₃-Mg F₂ compound sintering additives on the microstructure and mechanical properties of silicon nitride ceramics is investigated.Mg F₂ shows a positive effect on the elongating ofβ-Si₃N₄ crystal grains along the c axis growth in morphology.The aspect ratio improves from 8.60 to 11.11when the addition of Mg F₂ increasing from 2.5 wt%to 7.5wt%.Compared with YbF₃system,YbF₃-Mg F₂ system demonstrates unparalleled superiority in mechanical properties.In addition,when 7.5 wt%YbF₃and 2.5 wt%Mg F₂ are added,the flexural strength sharply increases to 298 MPa,an increase of～49%,while the porosity is increased to 37%,an increase of～15%compared with 10 wt%YbF₃.Meanwhile,the structure factor reaches a minimum of 3.23.Dielectric properties of silicon nitride ceramics using YbF₃-TiO₂-CaO as sintering additives are investigated.The dielectric constant temperature change rate decreases from 5.2%to 2.5%from 25°C to 500°C under 1 MHz when the addition of TiO₂-CaO increasing from 9 wt%to 12 wt%,and the dielectric temperature stability of silicon nitride is improved.Besides,the dielectric constant temperature change rate from 25°C to 500°C is 2.7%,the dielectric constant at room temperature is 9.8 and the dielectric loss is 4×10^-4 when the addition of TiO₂-CaO is 11 wt%.