Effects Of Microstructural Tuning On The Giant Dielectric Properties And Dielectric Relaxation Of TiO₂-based Materials

The search for colossal permittivity(CP,?'>103)materials has attracted an increasing amount of attention due to their potential applications in the microelectronic industry and high-energy-density storage.Frequency-and temperature-independent CP and a low dielectric loss(tan ?<0.1)of CP materials are required for their applications.However,many reports have shown that simultaneously achieving both of these properties is a challenge.A previously reported novel CP material,an In and Nb co-doped rutile TiO₂ ceramic,is one of the most promising candidates to overcome the problems faced by such materials.Frequency(20 Hz-2 MHz)and temperature(80 K-450 K)independent CP(?'>104)and a low dielectric loss(tan ?<0.05)of CP materials are obtained in this material.However,the origin mechanism of the CP for this material is still unclear.One view is that the CP behavior can be attributed to the electron-pinned defect-dipoles(EPDDs).Another view is that the CP behavior is caused by the interface polarization.At the same time,it is also believed that the CP properties of donor-acceptor co-doped rutile TiO₂-based CP material are ascribed to a variety of relaxation polarization mechanisms.How to reveal the polarization mechanism of TiO₂-based CP materials?How to overcome these kinds of ceramics are sensitive to the methods used to process them.These are important research topics of these kinds of materials.The main purpose of this thesis is to obtain excellent donor-acceptor co-doped rutile TiO₂-based ceramics.The relationship between the microstructure and electrical properties of Bi and Nb co-doped rutile TiO₂ ceramics was studied.The EPDDS mechanism was further discussed and analyzed in the La and Nb co-doped rutile TiO₂ ceramics.In order to improve the low breakdown field strength and reduce the high sintering temperature of this kind of TiO₂-based ceramics,the Bi_0.0025Nb_0.0025Ti_0.995O₂Bi₂Ti₄O₁₁ ceramics were prepared.In addition,the textured Al and Nb co-doped rutile TiO₂ ceramics were synthesized via strong magnetic field alignment at 6 T and sintered under a nitrogen atmosphere.Finally,the anisotropic CP properties and ferroelectric-like behavior were observed at TiO₂ single crystals after heating treatment.1.The(Bi_0.5Nb_0.5)_xTi_1-xO₂(x=0,0.005,0.01,0.025,0.05)ceramics were synthesized by a standard conventional solid-state reaction.(Bi_0.5Nb_0.5)_xTi_1-xO₂ ceramic with x=0.025 shows a high dielectric constant(155.1k)and low dielectric loss(0.042)at 1 kHz(room temperature).The secondary phase Bi_1.74Ti₂O_6.624 is detected with x increasing.Colossal permittivity is related to the EPDDs mechanism,electron hopping,and interfacial polarization.Bi_1.74Ti₂O_6.624 phase increases the activation energy of grain boundaries,reducing the dielectric loss.2.The stoichiometric La_0.0025Nb_0.0025Ti_0.995O₂ ceramics synthesized by a standard conventional solid-state reaction.The annealing treatment changed the content of Ti~3+ions and oxygen vacancies,resulting in that the cut-off frequency of colossal permittivity was increased from 1 MHz to 100 MHz.30 min annealed LNTO shows a low dielectric loss(tan ?<0.1)in a frequency range from 0.5 Hz to 10 MHz.The EPDDs mechanism was identified to be associated with the giant permittivity specifically measured at 100 MHz.Annealing in ambient conditions led to decreased relaxation times which gives the signature of decreased concentration of oxygen vacancies and Ti~3+.Dipoles that were related to oxygen vacancies and Ti~3+resulted in colossal permittivity up to 100 MHz.3.(1-x)Bi_0.0025Nb_0.0025Ti_0.995O₂-xBi₂Ti₄O₁₁(0? x?0.6)composite ceramics with colossal permittivity were fabricated by a conventional sintering method.The 0.8Bi_0.0025Nb_0.0025Ti_0.995O₂-0.2Bi₂Ti₄O₁₁ ceramic exhibited a low sintering temperature(1403 K),a decreased average grain size(1.5 ?m),an enhanced breakdown electric field strength(13 kV/cm),a colossal permittivity(?'?1100),and an excellent DC bias stability of dielectric constant(0-1050 V/cm).Microstructure,I-V behavior,and Impedance spectroscopy studies demonstrated that the enhanced breakdown strength and excellent dielectric properties were attributed to the small grain size and the highly resistant grain boundares.4.(Al_0.5Nb_0.5)_xTi_1-xO₂(x=0.05,0.2,0.3)ceramics were prepared via strong magnetic field alignment under a magnetic field of 6 T.The texture of Al and Nb co-doped TiO₂ ceramics became higher and higher with the increase of x value,which was attributed to the second phase .The parallel-6 T ceramic was found to present a CP value of ?r?8.2×104 and a low dielectric loss of tan ??0.019,at 1 kHz.The grains were elongated and aligned with the magnetic field,a brick-layered microstructure was observed in the parallel-6 T ceramic.The brick-layered microstructure increased the grain boundary resistance and activation energy,leading to low dielectric loss.5.The anisotropic colossal permittivity,hysteresis loops,and current density curves of the<001>and<100>oriented rutile TiO₂ single crystals were found after the single crystal TiO₂ was heated in the air at 1773 K for 10 h.The CP of the<001>oriented TiO₂ single crystal is up to 45000 at 1 kHz at room temperature.The saturated spontaneous polarization intensity,residual polarization intensity,and coercive field corresponding to the hysteresis loops are Pm=31 ?C/cm2,Pr=17 ?C/cm2,and Ec=3.5 kV/cm,respectively.Anisotropic domain structures have been observed in TiO₂ single crystals.The Ti~3+ ions and oxygen vacancies were formed after heating treatment,which only leads to the narrowing of the bandgap for the<001>oriented TiO₂ single crystal,and there was no significant effect on the<100>oriented TiO₂ single crystal.