| With the rapid development towards miniaturization,intellectualization and integration of electronic information technology and microelectronics industry,functional ceramics have become the most active area in the field of the inorganic nonmetallic materials as the basic key materials of the various passive electronic devices.Seeking for high dielectric performance materials is an effective approach to accomplish electronic component miniaturization.Perovskite-type structured CaCu3Ti4O12(CCTO)-based non-ferroelectric ceramics with giant dielectric constant(～104-105)in the kilohertz range and good thermal stability(100-400 K),in recent years,have been attracted wide attention.However,the complex polycrystalline,multiphase,multilayered structure,high dielectric loss and the complicated intrinsic physical mechanisms underling the functional effects,have restricting the development of the CCTO-based colossal dielectric functional ceramics and devices.A series of various Ni,Co,Nd-doped CCTO-based ceramics with relatively few researches were prepared by solid-state reactions.For the sake of improving dielectric properties and voltage nonlinearity,and further more,in-depth understanding of the related physical mechanisms,the phases,microstructures and micro-area compositions,elemental valence states,giant dielectric properties,and voltage-current nonlinearity of were investigated.The main resulting conclusions are as follows:1)、The novel effects of Cu-deficient on the microstructures,dielectric properties,and voltage-current nonlinearity of CaCu3-xTi4O12-x(x=0,0.025,0.05,0.10,0.125,0.15,0.20,0.30,0.40)ceramics with Cu-deficiency were investigated systematically.With Cu content decreasing,the CuO secondary phase disappeared starting from CaCu2.95Ti4O11.95(Cu-05),and then the TiO2 and CaCO3 phases were gradually observed in the XRD patterns after Cu-05.The abnormal enhancement of the εhigh’ value up to 4.96 × 105 in Cu-10 should be attributed to the novel enhanced Maxwell-Wagner-type relaxation related to the heterogeneity increase between the big and small grains(the increase content of rutile TiO2 in the small grains from the energy-dispersive X-ray spectroscopy(EDX))and the surface roughness reduction before Cu-10.By comparing the capacitance changes in the samples with different thickness of 0.7,1.0,1.4 mm,the variation of x=0-0.15 indicates the voltage-current nonlinearity of CaCu3-xTi4O12-x was mainly attributed to the internal barrier layer capacitor(IBLC)model and surface effect,respectively,with Cu-10 as the critical composition,that was related to the mechanism of the dielectric properties.Besides,an interesting backward phenomenon in the J-E curves was also found,which might be associated with the TiO2 related phase.2)、Compared to CCTO+wNiO and CaCu3Ti4-vNivO12,the improved dielectric properties and voltage-current nonlinearity of nickel-doped CaCu3-xNixTi4O12(CCNTO,x=0.00,0.10,0.20,0.25,0.35,0.40)ceramics were obtained.The approach of A’-site Ni doping resulted in optimal dielectric properties in the CCTO system,with a dielectric constant ε’≈1.51 × 105 and dielectric loss tanδ≈0.051 found for the sample with a Ni doping of 20%(CCNTO20)at room temperature and 1 kHz.It was found that the Ni dopants initially preferred to displace Cu site and then partial to displace Ti site with x=0.25(CCNTO25)as the critical composition from EDX.The X-ray photoelectron spectroscopy(XPS)analysis verified the co-existence of Cu+/Cu2+and Ti3+/Ti4+mix-valence structure.A steady increase inε’(f)and a slight increase in α observed upon initial Ni doping were ascribed to a more Cu-rich phase in the intergranular phase that increased grain boundary resistance Rgb and promoted the grains growth.The low-frequency relaxation leading to a distinct enhancement in ε’(f)beginning with CCNTO25 was confirmed to be a Maxwell-Wagner-type relaxation strongly affected by the Ni-related phase with the formation of a first established(Cu+,Ti)co-doped NiO core-shell structure.The reduction of the dielectric loss was mainly associated with the promoted densification of CCNTO and the increase of negative cation vacancies,due to Ni doping on the Cu site.3)、Compared to CCTO+zCoO and CaCu3Ti4-yCoyO12 those with obvious performance degradation,the improved dielectric properties with ε’≈7.42×104 and tanδ≈0.034 were observed in the sample with 5%Co-doped CCTO(CCTO05)at room temperature and 1 kHz.XPS analysis verified the co-existence of Cu+/Cu2+and Ti3+/Ti4+mix-valence structure in all samples,and Co3+/Co2+starting from 10%Co-doped CCTO(CCTO10).It was found that Co2+and Co3+preferred to substitute Cu and Ti sites,respectively.The enhanced dielectric properties in the Co-doped CCTO were related to the multi-relaxations.Relaxation RI(<0℃)was the combined effects of the IBLC and mix-valence structure.RⅡ(-20-40℃)and RⅢ(100-150℃)were demonstrated to be a Debye-like relaxation and a Maxwell-Wagner relaxation related to the single-and double-ionization of oxygen vacancies.The low dielectric loss of CCTO05 was associated with the high grain boundary resistance Rgb related to the increase of the negative accepter defects V"Cu,Cu’Cu and Ti’Ti.Besides,the improved nonlinear electrical properties(CCTO05,with nonlinear coefficients α≈5.22 and breakdown electric field Eb≈300.46 V/cm)and the ferromagnetism induced by the Co3+displacement for Ti site were also discussed.4)、Enhanced dielectric properties with the high dielectric constant ε’～104 in the frequency range of 40-106 Hz,low dielectric loss tanδ<0.05(2)in the frequency range of 103-105 Hz)and tanδ(T)<0.06(1)below～350 K were achieved in Ca1-xNdxCu3Ti4O12+0.5x(CNCTO,x=0.0,0.04,0.08,0.12)ceramics with x=0.08.With Nd doping,the increase of the CuO and TiO2 phases in XRD and enhancement of Cu+/Cu2+and Ti3+/Ti4+mix-valence structure from XPS indicated that the surplus Nd3+ions doped in Ca-site could also substitute the Cu and Ti ions.The distinct increase of Rgb and cation vacancies caused by Nd doping were conducive to depressing dielectric loss.With the temperature increasing,two relaxations including R1 and a barely found R2(1.53 eV)and an anomaly A were observed,were attributed to oxygen vacancy ionization,long-range migration of carriers and a relaxor-like phase caused by oxygen-vacancy ordering,respectively.The amphoteric dopant that the Ca-site doped Nd would serve as a similar role of acceptor(substitute for Ti)-donor(substitute for Cu)co-doping,which could enhance temperature stability of ε’(T)and reduce tanδ(T),was also proposed.The results above suggest that the manipulations of microstructures and electrical properties in CCTO-based ceramics modified by properly doping to achieve enhanced overall performance,and thus promoting applications in high performance capacitors,sensors,and energy storage. |
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