Preparation And Investigation Of Mg₂Al₄Si₅O₁₈ Based Microwave Dielectric Ceramics

Microwave dielectric ceramics is one kind of functional ceramic materials.It refers to the ceramic materials used in the circuit of microwave frequency band?300MHz³00GHz?as the dielectric materials to perform one or several functions.The Microwave dielectric properties of ceramics rely on three key functional parameters,namely relative permittivity??_r?,quality factor?Q×f?and temperature coefficient of resonant??_f?,which are closely related to the application direction,loss and stability of microwave device and equipment,respectively.Through research and study of a great number of literatures,it is found that cordierite(Mg₂Al₄Si₅O₁₈)is based on a six-membered ring composed of[?Si₄Al₂?O₁₈],which is connected sequentially by[SiO₄]and[AlO₄]tetrahedra,and the six-membered ring is connected by[MgO₆]octahedral which lead to a hexagon channel formed in the centre of the ring.The microwave dielectric properties of Mg₂Al₄Si₅O₁₈ ceramics change when the six-membered ring structure is changed because of the change in the shape of the ion replacement or the filling of atoms in the channel.In this paper,Mg₂Al₄Si₅O₁₈8 system ceramics were systematically studied,and the effects on the crystal structure and microwave dielectric properties of the ceramic materials were obtained by researching the excess of raw materials,two-phase composite and ion substitution.In this paper,the effect of non-stoichiometric ratio of Mg²⁺on Mg_?2+2x?Al₄Si₅O₁₈?x=0.00,0.025,0.05,0.075,0.10?microwave dielectric ceramics was obtained by solid phase reaction method.With the increase of excess MgO,the dielectric constant of the ceramics are between 3.25-4.8,the quality factor increases firstily then decreases after the first increases,and the frequency temperature coefficient is between-32 ppm/?to-34ppm/?,little change about the sintering temperature.When x=0.075,the microwave dielectric ceramics of Mg_?2+2×0.075?Al₄Si₅O₁₈ sintered at 1430?temperature after three hours obtained the optimal microwave dielectric properties:?_r=4.75,Q×f=76,000GHz,?_f=-34 ppm/?.The paper studied Mg₂Al₄Si₅O₁₈+x wt.%Mg₂SiO₄?x=0-80?microwave dielectric ceramics.The experiments have improved that Mg₂Al₄Si₅O₁₈ and Mg₂SiO₄ can coexist,and no other phases emerged.Then the effect of the crystal structure of Mg₂Al₄Si₅O₁₈+x wt.%Mg₂SiO₄?x=0-80?ceramics and the microstructure observed by Scanning Electron Microscope on the microwave properties of the composite ceramics was analyzed in detail.The microwave dielectric performance of composite phase ceramics changed with the value of parameter x,resulting rising firstly and then falling.When x=50,the sintering temperature is 1340?,and the samples obtained the optimal microwave dielectric properties:?_r=5.73,Q×f=76374GHz,?_f=-24 ppm/?.The effects of partial substitutions of Mg²⁺on the microstructure of(Mg_0.8R_0.2)₂Al₄Si₅O₁₈?R=Ca,Zn,Co,Mn,Sr,Ba,Nd?ceramics and microwave dielectric properties of(Mg_0.8R_0.2)₂Al₄Si₅O₁₈ were investigated.For(Mg_0.8R_0.2)₂Al₄Si₅O₄ ceramics,when R=Ca,Zn,Co,Mn,Sr,Ba,Nd,the main crystal phase stay the same(it is Mg₂Al₄Si₅O₁₈),and the second phases are all generated,which means partial ions replacement is generated.Through testing,it was found that the?_f of the ceramics was not changed with the ion substitution.Partial substitutions Mg²⁺position by Co and Zn could improve the quality factor of the Mg₂Al₄Si₅O₁₈ ceramics.When R=Co,sintering Mg₂Al₄Si₅O₁₈ ceramics at 1400 degree Celsius,the ceramic samples got the best microwave dielectric properties:?_r=4.86,Q×f=63,828GHz,?_f=-32ppm/?.To adjust the?_f of Mg₂Al₄Si₅O₁₈ ceramics,this paper studied the?1-x wt.%?Mg₂Al₄Si₅O₁₈+x wt.%CaTiO₃?x=5-25?microwave dielectric ceramic.Through the test of microwave dielectric performances of Mg₂Al₄Si₅O₁₈+xwt.%CaTiO₃?x=5-25?,finding that as CaTiO₃ increase,the dielectric constant of the compound ceramics decreases after the first increases,quality factor increased with the increase of x,and the frequency temperature coefficient rises.The addition of CaTiO₃ makes the system of Mg₂Al₄Si₅O₁₈ ceramics lower sintering temperature.When x=25,the 75 wt.%Mg₂Al₄Si₅O₁₈+25 wt.%CaTiO₃ ceramics sintered at 1370?sintering temperatures three hours obtained excellent microwave dielectric properties:?_r=5.9,Q×f=7,822GHz,?_f=-19ppm/?.