Conducting Ceramics Based On ZnO Co-doped By?Al₂O₃?TiO₂,MgO?:Microstructure,Electronic Active Defects And Electrical Properties

ZnO based ceramics with appropriate doping elements show excellent electrical and optical properties such as high exciton binding energies,a modulated optical transparency and electrical conductivities.Therefore,ZnO based conducting ceramics have been extensively investigated in the aim of their application in the fields of resistor,varistors,gas sensors,conductive film target,transparent electrodes,solar cells,liquid crystal displays,piezoelectric,electro-optical and thermoelectric devices.The high conductivity of ZnO ceramics is closely related to the doping elements along with the ceramic microstructure and the processing conditions.Thus,grain boundaries,crystalline structure,and structural disorder within a polycrystalline ZnO ceramic depend critically on the doping elements,sintering method,and sintering atmosphere during the preparation process.The involved conductivity in doped ZnO ceramics is highly dependent on the above mentioned parameters.Thus,the present thesis is devoted to the investigations and the analysis of related properties of ZnO-based ceramics co-doped by Al₂O₃,Ti O₂,and MgO.The conductivity is adjusted by the nature of the structure influenced by the doping elements,the sintering atmosphere,and the sintering method.The correlation"Preparation-Structure-Conductivity"has been established to pave the way for highly conducting ZnO-based ceramics.The main developed research tasks,their results and achievements are hereafter outlined:1.The reducing nature of the sintering atmosphere is an important parameter affecting the structure and performance of doped ZnO ceramics.The conductivity increases up to?=1.5×10⁵S·m^-1by sintering under CO+N₂atmosphere and the positive temperature coefficient is reaching 749 ppm/°C.The improvement by the reducing atmosphere is caused by the low oxygen partial pressure which increase the solubility of Al and Ti in ZnO,thereby increasing donor defects and then the charge carrier concentration.Meanwhile,the process reduces the acceptor defects at the grain boundaries enhancing the carrier mobility.In addition,the Al _Zn-Zn_icomplex has been identified as the main shallow donor defect that enhances the conductivity of ZnO ceramics.A consistent analysis has been carried out based on experimental investigations by Raman,EPR,and NMR techniques.2.As Al doping improves highly the electrical conductivity of ZnO-based ceramics,the role of Al₂O₃doping ratios has been investigated within the range of 0.1-0.55mol.%and for a processing technique with CO+N₂as reducing atmosphere.Structural and electronic active defects analysis by XRD,SEM,Raman,EPR and NMR methods,shows that the Al₂O₃content modulates the shallow donor Al _Zn-Zn_icomplexes,grain boundaries and the fraction of the secondary Zn Al₂O₄spinel phase in ZnO ceramics and thereby altering the electrical conductivity.3.The effect of the spark plasma sintering?SPS?method on the structure and electrical properties of ZnO ceramics doped with 0.25 mol%Al₂O₃has been studied.This method improves the solubility of Al in ZnO,reduces the spinel phase content,and limits the grain size growth.The comprehensive analysis of Raman spectra,UV-visible absorption spectra and photoluminescence spectra show the role of the SPS method to increase the structural disorder and the concentration of shallow donor Al _Zn-Zn_icomplexes enhancing the electrical conductivity of ZnO ceramics.4.Although the combined process of co-doping ZnO ceramics leads to high conductivities,the role of each doping element such as Al,Ti and Mg must be clarified on how it may affect the key features of the host ceramics.Different doping combinations with different contents of Al,Ti and Mg ions have been realized and altered differently the phase composition,microstructure and defects in ZnO-based ceramics,and then affect the conductivity.The similar characteristics?valence,size?of Mg ions and Zn ions essentially contribute to the formation of defects.Doping by Ti ions,leads to deep donors(Ti³⁺)and induces a secondary insulating spinel phase which limits the conductivity.As for doping with Al ions,it has a significant effect on the conductivity owing to the shallow Al _Zn-Zn_idonor defects,which contribute to a high concentration of charge carriers.5.Studies have been devoted to the role of microstructure and grain morphology on the conductive properties of ZnO-doped ceramics.Thus,the electrical behavior was compared between particles with large dimensions?20?m?as in massive ceramics and particles of smaller sizes?<100?m?.The correlation between the microstructure and the electrical properties of doped ceramics was established by analyzing the coordination of doping ions Al in ZnO,the nature of electronic defects and the local disorder of ZnO materials.