Relationship between nonlinear resistivity and the varistor forming mechanism in ZnO ceramics

The use of a number of perovskite phases M� M�O3-x, as the only forming additive in ZnO ceramics, produces a high nonlinearity index, ?(up to 45), where M� is a multivalent transition?metal ion and M� is an alkaline earth or a rare?earth ion. From this study, the formation parameters crucial to high nonlinearity, such as nonstoichiometry in the as?received ZnO powder, low x values of the additives and fast cooling rate after the sintering, are explainable on the basis of a depletion layer formation at the presintering stage. This is because of the surface states arising out of the chemisorbed oxygen. The depletion layer is retained during sintering as a result of the higher valence state of M� ions, preferentially present at the grain?boundary regions. The fast cooling freezes in the high?temperature concentration of donor?type defects, thereby decreasing the depletion layer width.

Microstructural stability and nanoindentation of an electrodeposited nanocrystalline Ni-1.5wt.%P alloy

Microstructural stability of nanocrystalline Ni-1.5wt.%P alloy with an initial grain size of 3 nm processed by pulsed electrodeposition was studied using differential scanning calorimetry (DSC) and annealing. Microstructural characterization suggests that the observed exothermic peak during heating in DSC is related to both concurrent grain growth and Ni3P formation. Nanoindentation on samples with grain sizes from 3 to 50 nm revealed a breakdown in Hall-Petch strengthening in nano Ni-P alloy at grain sizes <= 10 nm, consistent with some previous observations. It is concluded that there is a grain boundary weakening regime for grain sizes < 10 nm, based on analysis which show that the data cannot be rationalized in terms of microstrain relaxation, variation in elastic modulus, texture evolution and duplex structure formation.

Surface studies on uranium monocarbide using XPS and SIMS

The air-exposed surfaces of sintered and are-melted UC samples were examined by XPS and SIMS. XPS results indicate that the surface is covered with a very thin layer of UO2 mixed with free carbon, which would have formed along with the oxide during the reaction between UC and oxygen or moisture. From the SIMS depth profile of oxygen, the thickness of the oxide layer is found to be approximately 10 nm. The SIMS oxygen images of the surface as a function of etching time reveal that the surface of UC consists of a top layer of adsorbed moisture/oxygen; this contamination layer is followed by a layer containing uranium oxide, uranium hydroxide and free carbon and then grain boundary oxide and finally bulk UC. The behaviour of sintered and are-melted samples is similar.

Influence of Alkali Ions in Enhancing the Nonlinearity of ZnO-Bi2O3-Co3O4 Varistor Ceramics

Zinc oxide ceramic varistors with simplified compositions of ZnO+Bi2O3+Co3O4+M(2)O (M=K or Na) show nonlinearity coefficients (alpha) of 40-75. The electron paramagnetic resonance spectra and optical reflectance spectra show that there is a direct interdependence between the oxidation state of transition metals and the alkali ions. The X-ray photoelectron spectra indicate that the alkali ions preserve a higher oxidation state of cobalt, Co(III), in the grain boundary regions than in the grain interiors having more Co(II). Admittance spectroscopy shows that, while the nature of traps remains unaltered, the trap density increases with the concentration of alkali ions near the interface. The observed defect states are associated with the grain bulk than with the grain boundary interfaces, as indicated by the isothermal capacitance transient signals

Influence of carbide particles on the dynamic recrystallization of as-cast 304 stainless steel: a study using secondary ion mass spectrometric (SIMS) analysis

The domain of dynamic recrystallization (DRX) in as-cast 304 stainless steel material occurs at higher temperatures (1250 degrees C) and lower strain rates (0.001 s(-1)) than in wrought 304 stainless steel (1100 degrees C and 0.01 s(-1)). The above result has been explained earlier on the basis of a simple theoretical DRX model involving the rate of nucleation versus rate of grain boundary migration. The present investigation is aimed at examining experimentally the influence of carbide particles on the DRX of ascast 304 using secondary ion mass spectrometric (SIMS) analysis. Isothermal compression tests at a constant true strain rate have been performed on wrought 304 and as-cast 304 materials in the temperature and strain rate ranges of 1000 to 1250 degrees C and 0.001 to 1 s(-1) respectively. The SIMS analysis carried out on the deformed samples revealed that the large carbides present in the as-cast 304 material strongly influence the DRX process. In as-cast 304 material, the presence of large carbide particles in the microstructure shifts the DRX domain to higher temperature and lower strain rate in comparison with wrought 304 material.

Microstructural and dielectric properties of KCl-added bismuth vanadate ceramics

The effect of KCI addition on the microstructural, structural and dielectric properties of bismuth vanadate, Bi2VO5.5 (BiV) has been examined. The average grain size of BN ceramics increases with increase in KCl content (from an average grain size of TO to 80 mu m) as a result of the increased liquid-phase formation of KCI, at the grain boundaries. Differential scanning calorimetry (DSC) carried out on the KCl-added samples indicates an upward shift in the transition temperature (T-c), from 723 K (for BN) to 734 K (for 5 mol% KCl-added BiV). On further increase in the KCI content, T-c shifts down to about 722 K for 10 mol%. This trend is consistent with that of the lattice strain data. The relative permittivity as well as the dielectric loss decrease by more than half of the original values upon the addition of KCI. The relative permittivities of the KCl-added ceramics are comparable with the values predicted by the logarithmic mixture rule. Impedance analyses suggest that the grain boundary resistance of the KCl-added BiV ceramics is higher by two orders of magnitude than that of BN ceramics. The KCl-added BN ceramics exhibit ferroelectric domains and the domain density decreases as the grain boundary region is approached.

Nucleation and growth characteristics of cavities during the early stages of tensile creep deformation in a superplastic zirconia-20 wt% alumina composite

Constant-stress tensile creep experiments on a superplastic 3-mol%-yttria-stabilized tetragonal zirconia composite with 20 wt% alumina revealed that cavities nucleate relatively early during tensile deformation. The number of cavities nucleated increases with increasing imposed stress. The cavities nucleate at triple points associated largely with an alumina grain, and then grow rapidly in a cracklike manner to attain dimensions on the order of the grain facet size. It is suggested that coarser-grained superplastic ceramics exhibit lower ductility due to the ease in formation of such grain boundary facet-cracks and their interlinkage to form a macroscopic crack of critical dimensions.

Microstructural stability and superplasticity in an electrodeposited nanocrystalline Ni-P alloy

Stabilization of nanocrystalline grain sizes by second phase particles can facilitate superplasticity at high strain rates and/or low temperatures. A metastable single phase nano-Ni-P alloy prepared by electrodeposition, with a grain size of similar to 6 nm, transforms to a nanoduplex structure at T> 673 K, with similar to 4 vol.% Ni3P particles at triple junctions and within Ni grains. The nanoduplex microstructure is reasonably stable up to 777 K, and the growth of Ni grains occurs in a coupled manner with the growth of Ni3P particles such that the ratio of the two mean sizes (Z) is essentially constant. High temperature tests for a grain size of 290 nm reveal superplastic behavior with an optimum elongation to failure of 810% at a strain rate of 7 x 10(-4) s(-1) and a relatively low temperature of 777 K. Superplastic deformation enhances both grain growth and the ratio Z, implying that grain boundary sliding (GBS) significantly influences the microstructural dynamics. Analysis of the deformation processes suggests that superplasticity is associated with GBS controlled by the overcoming of intragranular particles by dislocations, so that deformation is independent of the grain size. The nano-Ni-P alloy exhibits lower ductility than nano-Ni due to concurrent cavitation caused by higher stresses. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The effect of aluminosilicate on the delayed onset of upturn voltages of zinc oxide varistor ceramics

The effect of aluminosilicate (Al2SiO5) on the upturn characteristics of ZnO varistor ceramics has been investigated. Addition of Al2SiO5 shifts the point of upturn above 10(4) A cm(-2). The extended nonlinearity in the high current density region is better correlatable to the presence of higher density of trap stales and changing pattern of trap depths at the grain boundary interface as much as the grain interior conductivity. Microstructure studies show the formation and involvement of a liquid phase during sintering. The secondary phases, predominantly are antimony spinel, Zn7Sb2O12, zinc silicate, Zn2SiO4 and magnesium aluminium silicate. MgAl2Si3O10. Energy dispersive X-ray analyses (EDAX) show that Al and Si are distributed more in the grain boundaries and within the secondary phases than in the grain interiors. Capacitance-voltage analyses and dielectric dispersion studies indicate the presence of negative capacitance and associated resonance, indicative of the oscillatory charge redistribution involving increased trapping at the interface states. The admittance spectroscopy data show that the type of trap slates remains unaltered whereas the addition of Al2SiO5 increases the density of low energy traps. (C) 1997 Published by Elsevier Science S.A.

Alternating current conduction and impedance spectroscopy analysis on pulsed excimer laser ablated (Pb, La)TiO3 thin films

Lanthanum doped lead titanate (PLT) thin films were identified as the most potential candidates for the pyroelectric and memory applications. PLT thin films were deposited on Pt coated Si by excimer laser ablation technique. The polarization behavior of PLT thin films has been studied over a temperature range of 300 K to 550 K. A universal power law relation was brought into picture to explain the frequency dependence of ac conductivity. At higher frequency region ac conductivity of PLT thin films become temperature independent. The temperature dependence of ac conductivity and the relaxation time is analyzed in detail. The activation energy obtained from the ac conductivity was attributed to the shallow trap controlled space charge conduction in the bulk of the sample. The impedance analysis for PLT thin films were also performed to get insight of the microscopic parameters, like grain, grain boundary, and film-electrode interface etc. The imaginary component of impedance Z" exhibited different peak maxima at different temperatures. Different types of mechanisms were analyzed in detail to explain the dielectric relaxation behavior in the PLT thin films.

Magnetically tunable nonlinear current-voltage characteristics in oxygen deficient manganite perovskites

(La0.667Ca0.333Mn1-xMO3-delta)-O-x (M = Mg, Li or Re) exhibit insulating behaviour and nonlinear current-voltage (J-E) relationship with voltage-limiting characteristics at temperatures below the ferromagnetic transition (T-c). The high current region is set in at field strengths <60 V/cm. Nonlinearity exponent, alpha in the relation J = kE(alpha) increases inversely with temperature. In presence of an external magnetic field, the J-E curves show higher current density at lower field strengths. Microstructural studies indicate that there is no segregation of secondary phases in the grain boundary regions. There is remarkable changes in p(T) as well as J-E curves with the grain size. Annealing studies in lower p(O2) atmospheres indicate that there is significant out-diffusion of oxygen ions through the grain boundary layer (GBL) regions creating oxygen vacancies in the GBL regions. The concentration of Mn4+ ions is lowered at the GBL due to oxygen vacancies, reducing the probability of hopping and resulting in insulating behaviour. Therefore an insulating barrier is introduced between two conducting grains and the carrier motion between the grains is inhibited. Thus below T-c, where sufficient increase in resistivity is observed the conduction may be arising as a result of spin dependent tunneling across the barrier. External electric field lowers the barrier height and establishes carrier transport across the barrier. Above certain field strength, barrier height diminishes significantly and thereby allowing large number of carriers for conduction, giving rise to highly nonlinear conductivity. (C) 2002 Elsevier Science B.V. All rights reserved.

Analysis of deformation mechanisms in superplastic yttria-stabilized tetragonal zirconia

There have been extensive experimental observations of changes in the apparent rate controlling creep parameters in studies on superplastic materials. The three most common explanations associated with these changes in the stress exponent, n, the activation energy Q and the inverse grain size exponent, p involve the effect of concurrent grain growth, the operation of a threshold stress or transitions in creep mechanisms. Each of these factors may influence experimental creep data in a similar manner. Therefore, a careful analysis of the consequences of all three factors must involve the development of a consistent set of experimental observations in order to adequately distinguish the effects of each. This paper discusses the role of concurrent grain growth, a threshold stress and transitions in creep mechanisms in superplastic materials. Specific attention is given to the analysis of data on superplastic yttria-stabilized zirconia ceramics for which an increase in n has been observed at low applied stresses. It is demonstrated that neither concurrent grain growth nor a threshold stress can account for all the relevant experimental observations in this material. It is concluded that the changes in rate controlling creep parameters are associated with the operation of two distinct sequential mechanisms as part of a grain boundary sliding process.

Interfaces In Quasicrystalline Systems

Structural relations between quasicrystalline and related crystalline rational approximant phases have been of interest for some time now. Such relations are now being used to understand interface structures. Interfaces between structural motif - wise related, but dissimilarly periodic phases are expected to show a degree of lattice match in certain directions. Our earlier studies in the Al-Cu-Fe system using the HREM technique has shown this to be true. The structural difference leads to well defined structural ledges in the interface between the icosahedral Al-Cu-Fe phase and the monoclinic Al13Fe4 type phase. In the present paper we report our results on the HREM study of interfaces in Al-Cu-Fe and Al-Pd-Mn systems. The emphasis will be on heterophase interfaces between quasiperiodic and periodic phases, where the two are structurally related. An attempt will be made to correlate the results with calculated lattice projections of the two structures on the grain boundary plane.

Interfaces In Quasicrystalline Systems

Structural relations between quasicrystalline and related crystalline rational approximant phases have been of interest for some time now. Such relations are now being used to understand interface structures. Interfaces between structural motif - wise related, but dissimilarly periodic phases are expected to show a degree of lattice match in certain directions. Our earlier studies in the Al-Cu-Fe system using the HREM technique has shown this to be true. The structural difference leads to well defined structural ledges in the interface between the icosahedral Al-Cu-Fe phase and the monoclinic Al13Fe4 type phase. In the present paper we report our results on the HREM study of interfaces in Al-Cu-Fe and Al-Pd-Mn systems. The emphasis will be on heterophase interfaces between quasiperiodic and periodic phases, where the two are structurally related. An attempt will be made to correlate the results with calculated lattice projections of the two structures on the grain boundary plane.

Influence of homogenization on the processing map for hot working of as-cast Mg-2Zn-1Mn alloy

Processing maps have been developed for hot deformation of Mg-2Zn-1Mn alloy in as-cast condition and after homogenization with a view to evaluate the influence of homogenization. Hot compression data in the temperature range 300-500degreesC and strain rate range 0.001-100 s(-1) were used for generating the processing map. In the map for the as-cast alloy the domain of dynamic recrystallization occurring, at 450degreesC and 0.1 s(-1) has merged with another domain occurring at 500degreesC and 0.001 s(-1) representing grain boundary cracking. The latter domain is eliminated by homogenization and the dynamic recrystallization domain expanded with a higher peak efficiency occurring at 500 degreesC and 0.05 s(-1). The flow localization occurring at strain rates higher than 5 s(-1) is unaffected by homogenization.