Nonlinear Electrical Characteristics of Antimony and Copper Doped Tin Oxide Based Varistor Ceramics

The novel CuO-doped dense tin oxide varistor ceramics are investigated. The densification of tin oxide varistor ceramics could be greatly improved by doping copper oxide additives. The introduction of antimony additives into a SnO2. CuO ceramic system would make it possess excellent nonlinearity. The sample doped with 0.05 mol% Sb2 03 possesses the highest nonlinearity coefficient （α = 17.9） and the lowest leakage current density （ JL = 52μA cm^-2） among all the samples. A modified defect barrier model is introduced to explain the formation of the grainboundary barrier. The nonlinear behaviour of （Cu, Sb）-doped SnO2 varistor system could be explained by the barrier model.

Enhancement of room-temperature magnetoresistance in La_(0.5)Sm_(0.2)Sr_(0.3)MnO_3/(Ag_2O)_(x/2)

La0.5Sm0.2Sr0.3MnO3/（Ag2O）x/2 （x = 0.00, 0.04, 0.08, 0.25, 0.30） samples were prepared by the solid-state reaction method, and their transport behaviors, transport mechanism, and magnetoresistance effect were studied through the measurement and fitting of p-T curves. The results show that the element Ag takes part in reaction when the doping amount is small. Ag is mainly distributed at the grain boundary of the host material and is in metallic state when the doping amount is relatively large; then the system becomes a two-phase composite. A small amount of Ag doping can apparently increase grain-boundary magnetoresistance induced by the spin-dependent scattering. The resistivity of the sample doped with 30 mol% Ag is one order of magnitude smaller than that of low-doped samples, and its magnetoresistance in the magnetic field of 0.5 T and at 300 K is strengthened apparently reaching 9.4%, which is connected not only with the improvement of the grain-boundary structure of the host material but also with the decrease of material resistivity.

SIMULATION OF THE CHANGE OF SINTERING NECK BETWEEN TWO GRAINS IN TWO DIMENSIONS

A modified two-sphere model of sintering neck has been proposed, wherein three diffusion mechanisms including surface diffusion, grain-boundary diffusion and coupled surface and grain-boundary diffusion are assumed. Sintering neck is appropriately simulated using the modified model. The dynamic change of sintering neck is presented using the simulation. The variational shape of sintering neck in surface diffusion mechanism is continuous, whereas in grain-boundary diffusion mechanism, besides the variational shape of sintering neck being continuous, the center distance between the particles is also assumed to contract. However, the variational shape of sintering neck in coupling diffusion mechanism is integrated using the two diffusion mechanisms mentioned above.

The Role of Nd Solid-Solution and Grain-Boundary Segregation in Binary NiAl Intermetallic Compound

The role of Nd solid-solution and grain-boundary segregation in binary NiAl alloy was studied based on microhardness and compressive macrostrain. X-ray and Auger spectra studies indicate that Nd not only is soluble in grain interiors, but also segregates to the grain boundaries. Nd solid-solution induces an increase of the microhardness from 269 to 290 HV in grain interiors and segregation results in an enhancement of hardness from 252 to 342 HV on grain boundaries. Thus, the cohesion of grain boundaries is enhanced by Nd segregation, which make the alloy doped with 0.05 wt pct Nd exhibit more compressive microstrain, i.e. the higher the compressive ductility at room temperature, the better the final surface condition at elevated temperature. Finally, a discussion was made on the reason that Nd strengthens the grain boundaries in NiAl intermetallic alloy.

Reconsideration of Anelastic Relaxation Peaks in Pure Aluminium

The temperature dependence of internal friction （ T- tan φ curves） in pure aluminium （Al） is measured at sixteen different frequencies. Based on T- tan φ curves, the frequency dependence of internal friction （f -tan φ curves） is also obtained by the interpolation method. An internal friction peak is observed in both the T - tan φ curves and the f - tan φ curves. The activation energy of the peak in the f - tan φ curves is found to be 2.08 ± 0.02 eV and compared to the value of 1.60 ± 0.04 eV in the T - tan φ curves. It is suggested that the change of relaxation strength with temperature should be considered when one calculates the activation energy of the peak in T- tan φ curves.

Effect of graphene grain boundaries on MoS2/graphene heterostructures

The grain boundaries of graphene are disordered topological defects,which would strongly affect the physical and chemical properties of graphene.In this paper,the spectral characteristics and photoresponse of MoS2/graphene heterostructures are studied.It is found that the blueshift of the G and 2D peaks of graphene in Raman spectrum is due to doping.The lattice mismatch at the graphene boundaries results in a blueshift of MoS2 features in the photoluminescence spectra,comparing to the MoS2 grown on SiO2.In addition,the photocurrent signal in MoS2/hexagonal single-crystal graphene heterostructures is successfully captured without bias,but not in MoS2/polycrystalline graphene heterostructures.The electron scattering at graphene grain boundaries affects the optical response of MoS2/graphene heterostructures.The photoresponse of the device is attributed to the optical absorption and response of MoS2 and the high carrier mobility of graphene.These findings offer a new approach to develop optoelectronic devices based on two-dimensional material heterostructures.

Realization of Superplastic State of Polycrystals under Grain-boundary Diffusion

Creep behaviour of Mo and Fe polycrystals has been studied for a wide range of Strain rates in the presence of grain-boundary diffusion flux of Ni. The role of the impurity grain-boundary diffusion and grain-boundary migratin in the realization of the superplastic strain of metals under these conditions is examined

Creating and destroying vacancies in solids and nonequilibrium grain boundary segregation

How the vacancies in excess of equilibrium concentration are created and destroyed in solids is crucial for understanding many of their physical characteristics and processes. Grain boundaries are known as sources and sinks for bulk vacancies, but what exchange will occur between the grain boundary and the bulk under a stress is still obscure. In the present paper, we show that grain boundaries will work as sources to emit vacancies when a compressive stress is exerted on them and as sinks to absorb vacancies when a tensile stress is exerted. This is the sixth method of introducing and annihilating vacancies in solids in addition to the five methods mentioned in literature. At the same time, this physical process will produce solute nonequilibrium grain-boundary segregation/dilution. A set of kinetic equations is established to describe such a physical process. Additionally an attempt has been made to simulate Misra抯 experimental data with the kinetic equations to justify the physical process.

Theoretical progress in non-equilibrium grain-boundary segregation (Ⅰ): Thermally induced non-equilibrium grain-boundary segregation and intergranular embrittlement

The research progress of non-equilibrium grain-boundary segregation theories in the last 20 years is reviewed. Based on studies by the present authors, the critical time of non-equilibrium segregation and its impact on the development of non-equilibrium segregation theories are described. Quasi- thermo- dynamics and kinetics for thermal non-equilibrium grain boundary segregation are detailed along with a non-equilibrium grain boundary cosegregation model. The experimental validation of the theories and their application to the reversible temper embrittlement of steels and the intermediate temperature brittleness in metals and alloys are also addressed.

Supermodulus effect by grain-boundary wetting in nanostructured multilayers

The effect of thermal treatments on mechanical properties was systematically investigated in Ni/Mo multilayers with a constant modulation period(160 nm)prepared by magnetron sputtering deposition.A supermodulus effect was found in the annealed multilayers as compared to the as-deposited state.A large tensile stress development was observed in the multilayers.The evolution of grain-boundary(GB)wetting was observed at the interfaces of the multilayers,which results in an enhanced modulus based on the mechanism of GB-wetting-induced interfacial stress/strain.The GB wetting phenomenon was further supported by a thermodynamic calculation.The results not only bring clear evidence of the important role of interfacial structures in governing the elastic behavior of metallic multilayers,but also allow designing the multilayers with special properties through atomic diffusion and wetting at the interfaces based on the thermodynamic calculation.

Voltage Dependence of Resistivity in Semiconducting Ceramics

Voltage dependence of resistivity(VDR)is a widely existing phenomenon insemiconducting ceramics, but there is no quantitative study on this phenomenon. Inthis note, the quantitative results of voltage dependence of resistivity insemiconducting ceramics are obtained from the grain boundary barrier model, andthe methods to decrease the voltage dependence of resistivity in sendconductingceramics are proposed.

Piezoresistive effect in p-type polycrystalline diamond films

The piezoresistive effect of boron-doped polycrystalline diamond films was analyzed and discussed by the famous M-S polycrystalline model. It is found that the valence bands splitting-off and the grain-boundary scattering are the main factors responsible for the piezoresistive effect in the p-type polycrystalline diamond films. The gauge-factor calculation formula including the effect of both background scattering and grain-boundary scattering were obtained, and the calculation results are in accordance with the experimental results.

Structural and mechanical properties of nano-crystal TiN coatings

The structural and mechanical properties of TiN coatings prepared by ion beam assisted deposition (IBAD) were studied. The coatings have a polycrystal structure with grain size of ～10 nm or less. The hardness of the coatings increases with increasing grain size of TiN crystallites. The coating with grain size of 10.3 nm even has a superhardness of 44.7GPa. The relationship between the hardness and the grain size in the nano-crystalline coatings was discussed on the basis of grain-boundary triple junctions.

Enhanced mechanical performance of grain boundary precipitation-hardened high-entropy alloys via a phase transformation at grain boundaries

Grain-boundary(GB)precipitation has a significant adverse effect on plasticity of alloys,which easily leads to catastrophic intergranular failure in safety-critical applications under high external loading.Herein,we report a novel strategy that uses the local stress concentration induced by GB precipitates as a driving force to trigger phase transformation of preset non-equiatomic high-entropy solid-solution phase at GBs.This in situ deformation-induced phase transformation at GBs introduces a well-known effect:transformation-induced plasticity(TRIP),which enables an exceptional elongation to fracture(above 38%)at a high strength(above 1.5 GPa)in a GB precipitation-hardened high-entropy alloy(HEA).The present strategy in terms of"local stress concentration-induced phase transformations at GBs"may provide a fundamental approach by taking advantage of(rather than avoiding)the GB precipitation to gain a superior combination of high strength and high ductility in HEAs.

High-strain-rate superplasticity in oxide ceramics:a trial of microstructural design based on creep-cavitation mechanisms

From existing knowledge about high-temperature cavitation mechanisms, necessary conditions were discussed for the suppression of cavitation failure during superplastic deformation in ceramic materials. The discussion, where special attention was placed on the relaxation of stress concentrations during grain-boundary sliding and cavity nucleation and growth, leaded to a conclusion that cavitation failure could be retarded by the simultaneous controlling of the initial grain size, the number of residual defects, diffusivity, dynamic grain growth and the homogeneity of microstructure. On the basis of this conclusion, high-strain-rate superplasticity （defined as superplasticity at a strain rate higher than 0.01 s^-1） could be intentionally attained in some oxide ceramic materials. This was shown in tetragonal zirconia and composites consisting of zirconia, a-alumina and a spinel phase.