Microstructure And Dielectric Properties Of LiTaO₃-based Ceramic Fabricated By Hot-pressing Sintering

Lithium tantalite single crystals have been widely used because of their excellent piezoelectric,dielectric and optical properties.Therefore,many researches have been focused on the single crystals and thin film of LiTaO₃ since 1950s,but a few of them have studied the fabrication,sinterability and dielectric properties of LiTaO₃ ceramics.In this work,the sinterability,microstructure,dielectric properties and ac impedance characteristics of LiTaO₃ ceramics with different MnO₂ content were systematic investigated by hot-pressing sintering at different temperature.The relative density and microhardness of LiTaO₃ ceramics increase with the increase of thepressure of hot-pressing which range from 25MPa to 35MPa,and the relative density and microhardness of the ceramics reach maximum?98.58%,792 HV,respectively?when the pressure is 30MPa.All specimens display LiTaO₃ phase and the diffraction peaks of the LiTaO₃ ceramics scarcely shift slightly.The surfaces and fractured surfaces of the scanning electron microscopy?SEM?shows the grain size of ceramics is more homogeneous,the pores are the least at 30MPa,It is more conduce to that Mn particles dissolve into the LiTaO₃ lattice.LiTaO₃ composite ceramics with added 3wt.%MnO₂ were obtained by hot-pressing sintering at different temperature which range from 1200 to 1350°C.The relative density of the ceramics reach maximum and the grain size of LiTaO₃ ceramics is homogeneous and the pores are the minimum at 1300°C.The XRD result reveals that all specimens are LiTaO₃ phase and the diffraction peaks shift slightly to the smaller angle.The XPS result shows that the Mn ions mainly existed in two forms of Mn²⁺and Mn³⁺,Moreover,there are a lot of oxygen vacancies in the samples.The EDS reveals that the first phase,gray circle particle,is LiTaO₃,while the second phase,black prismatic particles,are Mn₃O₄.In the range of frequency from 100 Hz to 1MHz,the relative dielectric constant of the LiTaO₃ composite ceramics at room temperature increased at first,and then decreased with the sintering temperature at the same frequency.It also achieved the maximum value of 78 at 1300°C.The maximum dielectric constant increased by up to 3500 at 1300°C.Only one dielectric constant peak appeared when sintered at different temperatures.The Curie temperature?T_c?of the specimens sintered at 1300°C and 1350°C moved to lower temperatures.LiTaO₃ ceramics have some relaxation characteristics.The relative density and microhardness of LiTaO₃ ceramics increased with moderating MnO₂ content,which enhances the sinterability.The XRD pattern indicates all samples display LiTaO₃ phase and the lattice expansion of the LiTaO₃ ceramics is caused by the substitution of the smaller Ta⁵⁺by Mn³⁺and Mn²⁺.The XPS result also confirmed that the Mn ions mainly existed in the forms of Mn²⁺and Mn³⁺.The grain size of LiTaO₃ ceramics is homogeneous and the pores are minimum at 3wt.%,and the second phase is Mn₃O₄ with XPS spectra analyzed.The relative dielectric constant of the LiTaO₃ composite ceramics at room temperature increased at first,and then decreased with the MnO₂ content at the same frequency.The relative dielectric constant achieved about 80and the dielectric loss were lower at 3wt.%.The maximum dielectric constant increased up to 3512 at 100k Hz.The Curie temperatures?T_c?of the samples move slightly to lower angle with the increase of MnO₂ content and ceramics have some relaxation characteristics.