Preparation And Electrical Properties Of Strontium And Hafnium Oxide Materials

With the development of science and technology and the progress of the times,ferroelectric materials have always been the focus of international research.Its outstanding performance and related applications have attracted much attention,and its performance and application are described in detail below.This article described preparation and study of strontium and hafnium-related oxide materials,including Ba_1-xSr_x TiO₃?BST?ceramics,Pb_x Sr_1-xTiO₃?PST?ceramics,PbHf_1-xSn_x O₃?PHS?ceramics,(Pb_1-1.5x-1.5x La_x)HfO₃?PLH?ceramics,Hf_1-xZr_xO₂?HZO?thin films,Sr(Ti_0.9Fe_0.1)O₃/SrTiO₃?STFO/STO?heterojunction films.We studied the properties of the above materials from five aspects:ceramic defects,energy storage,electricaloric effect,resistive switching property and photovoltaic effect,which includes five sections as follow:First,BST?x=0.35,0.50,abbreviated as BST35 and BST50,respectively?ceramics were prepared by sol-gel route.We researched their dielectric relaxation and tunability in a large temperature range of 150 to 950 K.The dielectric tunabilities and figure of merit of BST35 and BST50 ceramics at 288 K were 68.58%and 54.6,19.4%and 114,respectively,which indicated the ceramics are promising candidates for tunable capacitor applications.Sr²⁺doping can reduce the phase transition temperature of ferroelectric to paraelectric.The dielectric diffuse in the dielectric relaxation was observed in the high temperature region.A broad dielectric maximum shifted to higher temperature with increasing frequency,signifying the relaxor-type behavior of these ceramics.Impedance and dielectric measurements were studied to analyze their temperature dependence of dielectric properties.A single frequency are observed in Cole-Cole plots of BST35 ceramics suggested the relaxation mainly correspond to a grain boundaries response;while two frequency arcs indicated both grain and grain boundaries contribute to the dielectric relaxation of BST50ceramics.The activation energy for relaxation and conduction was calculated,which suggested the oxygen vacancies?OVs?play a critical role in the dielectric relaxation process of Ba_xSr_1-xTiO₃ ceramics at high temperatures.In addition,we studied the energy storage properties and electrocaloric effect of BST35 ceramics near room temperature.The diffusion phase transition?DPT?characteristic was found in the temperature range from 150 to-35°C.The value of the DPT parameter?values are 1.53 and 1.75,respectively applied dc field of 0and 16.7 kV/cm.It is implied that the phase transition of BST ceramics become more diffuse under the applied electric field.Temperature dependent polarization-electric field hysteresis loops were studied.It was found that the energy storage properties and electrocaloric effect near room temperature were the function of electric fields.The maximum value of recoverable energy density was 0.2812 J/cm³ with energy storage efficiency???of 78.67%obtained at room temperature under an electric field of 75 kV/cm.The largest?can achieve98%under low electric field of 50 kV/cm.A large electrocaloric temperature change(?T_max=0.49 K)near room temperature was obtained by indirect method.Taken together,the ceramics have a promising candidate for energy-storage applications and cooling systems in the room temperature.Second,PST?x=0.30,0.35,0.40,0.45,0.50 and 0.55?ceramics were fabricated by a solid-state reaction route.X–ray diffraction data at room temperature shows PST samples shift from cubic to tetragonal phase with the increase of Pb²⁺content.The microstructures were observed by scanning electron microscopy.Dielectric measurement was employed to investigate the ferroelectric–paraelectric phase transition behavior.Temperature dependent polarization–electric field hysteresis loops were conducted to study the electrocaloric effect?ECE?of the ferroelectric ceramics by indirect methods over a wide temperature range.Direct measurement of temperature change??T?at room temperature for all samples can achieve 0.79¹.86 K.What's more,a giant ECE??T=2.05 K,EC strength??T/?E?=0.51×10^-66 K m/V,under 40 kV/cm?was obtained in the sample of x=0.35 near phase transition temperature.Our results suggest that the ceramics are a promising cooling materials with excellent EC properties for energy related applications.Third,PHS?x=0,0.025,0.05,0.075,0.1,0.15,0.2,0.25 and 0.3?and PLH?x=0,0.04,0.08,0.12 and 0.16?ceramics were prepared by a solid-state reaction route.X–ray diffraction data at room temperature shows the two ceramics indexed in orthorhombic structure.Dielectric properties of two samples were investigated from room temperature to 650 ^oC.Sn⁴⁺doping decreases the phase transition temperature of an antiferroelectric?AFE1?to another intermediate antiferroelectric?AFE2?phase and AFE2 to the paraelectric?PE?phase in PHS ceramics.Also,a ferroelectric?FE?phase occurs in the composition of x?0.075 in PHS ceramics and the temperature of FE to PE increases with the Sn⁴⁺doping.Impedance spectroscopic data of two samples presented both grain and grain boundary responses are responsible for the dielectric relaxation.Activation energy calculated from the impedance data suggests the OVs play a critical role in the dielectric relaxation process at high temperature in two samples.Activation energy calculated from the dielectric data suggests the relaxation is governed by the dipolar conduction process in PHS ceramics,the dominate species of oxygen vacancies for conduction in PLH ceramics are singly and doubly ionized OVs for grain and grain boundaries,respectively.The ferroelectric properties of PHS ceramics were investigated in the temperature range from 30 to 200 ^oC.A pinning phenomenon was observed in x=0.075 and 0.1,resulting from the variation of structure as theSn⁴⁺increases.Fourth,Hf_0.5Zr_0.5O₂ thin films were prepared by sol-gel method.The ferroelectric and resistivity characteristics of HZO films from amorphous to polycrystalline were studied using different annealing temperatures on different substrate?FTO/Glass,Si,LaNiO₃?LNO?/Mica?.For Au/HZO/FTO/Glass devices,samples annealed at 450?C exhibited a good resistance characteristics with the transmission mechanism of space charge limited current?SCLC?conduction,the characteristic of bipolar switching devices can be well explained by the conductive filament model,the migration of oxygen vacancies under positive and negative electric field played a key role in the formation and breakage process of conductive filament.It is found that the film has a certain photoelectric effect by testing its optical properties.For Au/HZO/Si device annealed at 400°C to 850°C??T=50°C?,all samples are showing good ferroelectric hysteresis loops,of which 800°C annealed samples,its ferroelectricity is best.Schottky conduction mechanism was found by measuring the I-V curves of samples annealed at 450°C and 500°C.For Au/HZO/LNO/Mica devices,the fatigue resistance was found to be good.Finally,STFO/STO thin films were grown on LNO/Si?100?substrates by sol-gol method.The resistance switching characteristics of heterojunctions films were studied by annealing at 600?650?700 and 750^oC,respectively.XRD patterns indicated both STO and STFO characteristic peaks existed.The microstructures were observed by SEM.The sample annealed at 650^oC had a good surface structure.The current-voltage?I-V?characteristics of films were measured by keithley 2400.The surface of top electrode Au and STFO is ohmic contact.The I-V characteristic curve of sample annealed at 600 ^oC presents rectifying characteristic,while the others present bipolar resistance behavior.The current conduction mechanism of the four samples was analyzed,and it was found that the samples were all conducted by space charge limited current?SCLC?.The cyclic measurement of the I-V characteristic curves of samples shows that the heterojunction films have good repeatability and stability,indicating the STFO/STO are promising candidate for future memory applications.