Preparation Of Sr₂TiO₄ And Mn-Based Organic-inorganic Hybrid Materials By Wet Chemical Method And Investigation Of The Dielectric Properties

Microwave dielectric ceramic materials generally have high dielectric constant,high resonance frequency stability and low loss,and can be prepared into devices such as resonators,filters,dielectric antennas,dielectric waveguides,etc.They have a wide range of applications in portable mobile phones,military radars and GPS positioning systems,etc.In this paper,a sol-gel method was used to prepare Sr₂TiO₄ powders for Sr₂TiO₄ ceramic's excellent microwave dielectric properties.A low cost,low preparation temperature and controllable reaction process were achieved.The microwave dielectric properties and instability mechanism of Sr₂TiO₄ ceramics were studied.As another dielectric material,new organic-inorganic hybrid materials have gained widespread attention for their excellent ferroelectric,piezoelectric,electro-optical,acousto-optic effects,and are expected to be implemented in flexible piezoelectric sensors,photodetectors,and light-emitting diodes.In this paper,three organic-inorganic hybrid materials,[C₄H₁₂N]MnCl₃,[C₄H₁₂N]₂MnCl₄ and[C₄H₁₁NCl]₂MnCl₄,were prepared by wet chemical methods,and the relationship between their structure and properties was systematically studied.Using tetrabutyl titanate and strontium nitrate as raw materials and ammonia as a pH adjuster,precursor powders of Sr₂TiO₄ were prepared by a sol-gel method in a pH range of 9-11,and single phase Sr₂TiO₄ nano-ceramic powders were obtained by calcining at 1100?.Sr₂TiO₄ ceramics with a density of 95%were obtained by sintering at 1625?for 3 hours.This method overcomes the defects of difficult pH control and unstable precursor in the sol-gel method,and reduces the synthesis temperature.The microwave dielectric properties of Sr₂TiO₄ ceramics are Qf=13,600 GHz,?_f=148 ppm/?,?_r=38.The second phase Sr₃Ti₂O₇ appeared in Sr₂TiO₄ ceramics during sintering,and its content increased with the increase of sintering temperature.At room temperature,the SrO layer in the Sr₂TiO₄ layered perovskite structure reacted with H₂O and CO₂ in the air to form Sr?OH?₂·H₂O and SrCO₃,which caused the atomic stacks misaligned and resulted in more Sr₃Ti₂O₇ phases.Sr₂TiO₄ ceramics cracked and the density decreased.Single crystals and polycrystalline powders of[C₄H₁₂N]MnCl₃ were prepared by a slow evaporation solvent method.The X-ray diffraction datas of single crystals at-103?determine that[C₄H₁₂N]MnCl₃ belongs to the hexagonal system,space group is P6₃/m,and the unit cell parameters are a=9.0946?4??,b=9.0946?4??,and c=6.4763?2??.The DSC,dielectric temperature spectroscopy and temperature-variant Raman indicate that[C₄H₁₂N]MnCl₃ has a reversible structural phase transition at 116.9?.Microwave dielectric properties are Qf=9,530 GHz and dielectric constant?_r=4.3?resonant frequency f₀=14.790 GHz?.It emits red light under a 420nm laser with a fluorescence quantum yield of 39.67%and a fluorescence lifetime of 0.574 ms.Polycrystalline powders and single crystals of[C₄H₁₂N]₂MnCl₄ were prepared by a precipitation method and a slow evaporation solvent method,respectively.Single-crystal X-ray diffraction datas at-103?determine that[C₄H₁₂N]₂MnCl₄ belongs to the monoclinic system,the space group is P2₁/n,and the unit cell parameters are a=8.9966?6??,b=36.790?2??,c=15.3936?10??,?=90.736?2?°.The DSC,dielectric temperature spectrum and variable temperature Raman indicate that[C₄H₁₂N]₂MnCl₄has a reversible structural phase transition at-2.5?and 20.5?.Compared to[C₄H₁₂N]MnCl₃ with a simple perovskite structure,the microwave dielectric properties of[C₄H₁₂N]₂MnCl₄ with?-K₂SO₄ structure are significantly reduced,Qf=1,610 GHz?resonant frequency f₀=13.321 GHz,dielectric constant?_r=4.6?,the optical properties also changed,and the green light was emitted under a 420 nm laser with a fluorescence quantum yield of 1.3%and a fluorescence lifetime of 0.827 ms.Polycrystalline powders of[C₄H₁₁NCl]₂MnCl₄ were prepared by a slow evaporation solvent method.The similarity of single crystal X-ray diffraction data suggests that the phases of[C₄H₁₁NCl]₂ MnCl₄ and[C₄H₁₁NCl]₂ZnCl₄ are isomorphous each other.[C₄H₁₁NCl]₂MnCl₄ is a single phase.The DSC,dielectric temperature spectrum,variable temperature Raman and variable temperature XRD results indicate that the phase transition temperature is 6.7??heating?,-23.5??cooling?,and the thermal hysteresis is 30.2?.It is determined that the phase transition type is first-class phase change.Compared with[C₄H₁₂N]₂MnCl₄,the symmetry of[C₄H₁₁NCl]₂MnCl₄organic group is broken.Its microwave dielectric performance is Qf=290 GHz?resonant frequency f₀=13.408 GHz,dielectric constant?_r=5.2?,showing a large dielectric loss.And[C₄H₁₁NCl]₂MnCl₄ is also a ferroelectric,with a residual polarization P_r=0.65?C/cm² and a coercive field E_c=48 kV/cm under an electric field of 160 kV/cm.