Rare Earth Doping Modification Of Zn-Pr Based Pressure Sensitive Ceramics

The piezoelectric ceramic materials with non-linear voltage characteristics in a specific voltage range are called piezoelectric ceramics.By utilizing their non-linear voltage characteristics,various piezoresistive resistors can be produced.Currently,ZnO piezoresistive resistors can be divided into three major series:Zn-Bi series,Zn-Pr series,and Zn-V series.The research on ZnO piezoresistive resistors is roughly divided into two directions:one is to improve the piezoresistive performance by doping different oxides in ZnO piezoresistive resistors,and the other is to adjust the manufacturing process of ZnO piezoresistive resistors to achieve the goal of improving the performance.In this thesis,the relevant characteristics of rare earth oxide-doped Zn-Pr series piezoresistive resistors were analyzed,and the electronic structure of the corresponding ZnO crystal cells was calculated by first-principles method.The following conclusions were drawn:(1)This thesis presents an analysis of the effects of adding rare earth oxide Dy2O3 to ZnO-based piezoresistive resistors.It was found that when Dy2O3 was doped into the ZnO piezoresistive resistor,the added Dy2O3 and La2O3 would form a substance of DyCoO3 at the grain boundaries.However,the addition of Dy2O3 did not significantly affect the grain size.Excess addition of Dy2O3 actually inhibited grain growth.The dielectric constant of the sample decreased as the frequency increased from 200 Hz to 2 MHz due to polarization not being able to occur in sync with the electric field,thereby causing a relaxation phenomenon that reduced the dielectric constant.At low frequencies,the tanδvalue decreased gradually with increasing doping concentration.Increasing the doping concentration effectively reduced the dielectric loss of the sample.The highest nonlinearity was achieved when the doping concentration was 1.5 mol%,with a value of 21.421 and a breakdown electric field of 581 V·mm-1.(2)In this thesis,the effects of adding rare earth oxide Er2O3 to ZnO-based piezoresistive resistors were analyzed.It was found that the addition of Er2O3 led to the formation of a new substance of LaErO3.Most of the Er2O3 existed in the grain boundaries of the ZnO piezoresistive resistor in elemental form.Moderate doping improved the breakdown electric field of the resistor,but excessive doping led to increased grain size and leakage current.When the doping concentration was 1.5 mol%,the nonlinearity coefficient of the piezoresistive resistor was 19.652,the breakdown electric field was 983 V·mm-1,the leakage current was as low as 0.042 A/cm2,the grain size was a minimum of 1.701 μm,and the dielectric loss was the lowest at 0.067.The dielectric constant decreased rapidly to the lowest value at low frequencies,due to the inability of some parts of the sample to synchronize polarization changes with electric field.At high frequencies,electron and ion displacement in polarization were the main reasons for changes in dielectric constant.(3)In this study,we investigated the effects of La and Co single doping,as well as the co-doping of La and Co on the structure of ZnO crystal through firstprinciples calculations.Three models,including Co-doped ZnO,La-doped ZnO,and La and Co co-doped ZnO,were designed.After multiple attempts,we determined that the U value for Zn’s 3d and O’s 2p was 10.200 and 7.200,respectively.We found that the hypercell volume of ZnO increased under La single doping,while the change was small under Co single doping.Pure ZnO is a direct bandgap semiconductor with a bandgap width of 3.378 eV.After Co doping,substitutional ZnO became a direct bandgap semiconductor,with a reduced bandgap width of 2.541 eV and asymmetric upper and lower spin levels.This result was caused by the effect of Co’s d-orbital electrons.La-doped ZnO is a direct bandgap semiconductor with a bandgap width of 3.221 eV and no impurity levels.Co and La co-doping led to a reduced bandgap width of 2.468 eV and impurity levels in the forbidden band.Moreover,the Fermi level was found to be located near the bottom of the conduction band,which primarily resulted from the coupling between Co’s 3d and O’s 2p orbitals.This study suggests that Co2O3 and La2O3 doping in experiments can enhance the electrical properties of ZnO pressure sensitive resistance.La doping can cause the Fermi level to move through the conduction band,while Co doping can produce impurity levels near the Fermi level.(4)In this study,we investigated the electronic structure and lattice parameters of ZnO crystal doped with Dy elements using first-principles calculations.As Dy concentration increases,the volume and lattice constants of the crystal increase significantly,but the c/a ratio deviation remains within an acceptable range.In the energy level structure,the introduction of Dy causes a narrowing of the band gap and spin asymmetry,and impurity levels appear due to the contribution of Dy’s f states and d orbital electrons.In the Dy-doped density of states,the Fermi level enters the conduction band and generates impurity levels.In the high-spin state,the lower conduction band between 2 eV and 4 eV is mainly affected by Dy’s d orbital electrons,while in the low-spin state,it is mainly from f orbitals.In the Dy2O3 doped experiment,the addition of Dy2O3 can enhance the nonlinearity of ZnO pressure-sensitive resistance and reduce dielectric loss.Combined with the calculated results,this is because Dy doping causes the Fermi level of ZnO to pass through the bottom of the conduction band and generate impurity levels,which enhances ZnO’s conductivity.(5)In non-ideal ZnO crystals,defects are present.This article analyzes the effects of zinc and oxygen atoms around intrinsic ZnO point defects on band structures and density of states through calculation and analysis.The computational results indicate that zinc interstitials and oxygen vacancies produce donor levels and impurity levels,attracting excess negative electrons and causing the Fermi level to lean towards the bottom of the conduction band,forming an n-type semiconductor.Zinc vacancies and oxygen interstitials introduce acceptor levels,attracting excess holes,causing the Fermi level to lean towards the top of the valence band,forming a p-type semiconductor.