| In recent years,great progress has been made in the development of preparation,thermal shrinkage mechanism,and application research of the negative thermal expansion materials due to their unique thermal expansion property and broad application potential.And the performance of negative thermal expansion materials has been greatly improved.Even so,there are still some issues to be solved in both theory and application research of negative thermal expansion materials.Dielectric ceramicscan be applied widely,such as capacitor,communications equipment,random access memory.However,thermal mismatch would happenbecause of the difference in thermal expansion performance between dielectric ceramics and other materials,which may affect the valueof application and shortenthe life of the device.Toaddress this issue,negative thermal expansion materials are studied to adjustthe thermal expansion and dielectric properties of dielectric ceramics in this dissertation.The main work,results and innovation points arelisted below.1.Dielectric ceramic composites with controllable thermal expansion:SrTiO3/Zr2P2WO12Zr2P2WO12(ZWP)with excellent negative thermal expansion property was used to adjust thermal expansion property of dielectric ceramic SrTiO3(STO).SrTiO3/Zr2P2WO12 composites were prepared by using the solid-state method.Results showed that the thermal expansion coefficients could be adjusted in the range of(9.23-2.37)×10-6 K-1,Meanwhile,the dielectric constants changed between 17.15 and 39.56(1 MHz),and the dissipation factorcan be adjusted to a minimum of 0.002.After performance regulation,the thermal expansion property of the composites can match with Si and SiC when the composites are used in electron devices.2.The preparation and properties of high-frequency dielectric ceramics:SrTiO3/ZrMgMo3O12Given the high sintering temperature(1673 K)of Zr2P2WO12,and the uncertainty caused by hydroscopicity of the raw material P2O5,ZrMgMo3O12(ZMM),which is simpler and cheaper to produce and has excellent negative expansion performance,was considered to replace ZWP and combined with STO in this work.Dielectric ceramic compositesSrTiO3/ZrMgMo3O12with controllable thermal expansionhad been fabricated by solid-state method.The experimental results indicated that CTEs of the composites could be adjusted from 10.13×10-6 to 4.06×10-6 K-1.At 10 MHz,dielectric constants of composites were in the range of 40.70—21.17,while dissipation factors are between 1.17×10-4 and 79.7×10-4.The composite wherein the volume fraction of ZrMgMo3O12 was 40%showed the following excellent properties:low CTE(6.5×10-6 K-1),moderatedielectric constants(28.95)and low dissipation factor(tanδ,1.17×10-4)at 10 MHz.Therefore,this composite could not only be used as a high-frequency dielectric ceramic,but also match with the thermal expansion of SiC.3.The influence of ZrMgMo3O12 on the dielectric properties of BaTi4O9In the previous research,ZMM replaced ZWP to prepare composite materials,and the dielectric properties were improved.Furthermore,BaTi4O9(BTO)with medium dielectric constant was used to replace STO to recombine with ZMM.BaTi4O9/ZrMgMo3O12 composites were prepared by a solid state method.The results showed that the CTE of BaTi4O9/ZrMgMo3O12 composites decreased as the content of ZrMgMo3O12 increasing.And when the content of ZrMgMo3O12 reached 50%,theCTE would be 2.79×10-6 K-1,which was about1/3 of that of BaTi4O9.Besides,thetemperature coefficient of resonance frequency(τf)was 4.29×10-6 K-1.The changes in dielectric properties of composite samples can be mainly attributed to the changes of microstructure and complex polarization mechanism.And temperature coefficient of resonance frequency(τf)was regulated by the CTE owing toτf=(?),which is different from the method of doping and substitution.4.Effect of Sn on the dielectric properties of ZrMgMo3O12From the above research on the properties of composites SrTiO3/Zr2P2WO12,SrTiO3/ZrMgMo3O12 and BaTi4O9/ZrMgMo3O12,it was found that the dielectric properties of NTE materials Zr2P2WO12 and ZrMgMo3O12 need to be improved.Given that the application value of ZrMgMo3O12 is better than Zr2P2WO12,this work focused on improvement of the dielectric properties of ZrMgMo3O12:Sn,having the samevalence(+4)as Zr in ZrMgMo3O12,was used to partially replace Zr to adjust its dielectric properties.The results showed that not only the dielectric properties of Zr1-xSnxMgMo3O12(x=0.05,0.10,0.15,0.20,0.25)sampleswere improved significantly,but also thermal expansion performance was regulated compared to ZrMgMo3O12.Both the substitution amount of Sn and sintering temperature had a certain effect on the properties of Zr1-xSnxMgMo3O12.For the same sintering temperature,the CTEs became larger toward negative as the amount of Sn increased.The CTEs can be adjustedbetween near zero thermal expansion(ZTE)(-0.24×10-6 K-1)and NTE(-8.16×10-6 K-1).While the dielectric constant of ZrMgMo3O12 is about 5.5,the ones of Zr1-xSnxMgMo3O12 can be adjusted in therange of 130-155.The dissipation factors of Zr1-xSnxMgMo3O12 were adjustable between 4.3×10-4 and 5.1×10-4.Sn has significantly improved the dielectric properties of ZrMgMo3O12;meanwhile,the CTEs were adjusted in a large range.As a result,Zr1-xSnxMgMo3O12 materials have high value in the field of high-frequency dielectric ceramics. |
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