Supposition of graphene stacks to estimate the contact resistance and conductivity of nanocomposites

In this study,the effects of stacked nanosheets and the surrounding interphase zone on the resistance of the contact region between nanosheets and the tunneling conductivity of samples are evaluated with developed equations superior to those previously reported.The contact resistance and nanocomposite conductivity are modeled by several influencing factors,including stack properties,interphase depth,tunneling size,and contact diameter.The developed model's accuracy is verified through numerous experimental measurements.To further validate the models and establish correlations between parameters,the effects of all the variables on contact resistance and nanocomposite conductivity are analyzed.Notably,the contact resistance is primarily dependent on the polymer tunnel resistivity,contact area,and tunneling size.The dimensions of the graphene nanosheets significantly influence the conductivity,which ranges from 0 S/m to90 S/m.An increased number of nanosheets in stacks and a larger gap between them enhance the nanocomposite's conductivity.Furthermore,the thicker interphase and smaller tunneling size can lead to higher sample conductivity due to their optimistic effects on the percolation threshold and network efficacy.

AHFO-based soil water content sensing technology considering soil-sensor thermal contact resistance

The actively heated fiber-optic(AHFO)technology has become emerged as a research focus due to its advantages of distributed,real-time measurement and good durability.These attributes have led to the gradual application of AHFO technology to the water content measurement of in situ soil.However,all existing in situ applications of AHFO technology fail to consider the effect of soilesensor contact quality on water content measurements,limiting potential for the wider application of AHFO technology.To address this issue,the authors propose a method for determining the soilesensor thermal contact resistance based on the principle of an infinite cylindrical heat source.This is then used to establish an AHFO water content measurement technology that considers the thermal contact resistance.The reliability and validity of the new measurement technology are explored through a laboratory test and a field case study,and the spatial-temporal evolution of the soil water content in the case is revealed.The results demonstrate that method for determining the soilesensor thermal contact resistance is highly effective and applicable to all types of soils.This method requires only the moisture content,dry density,and thermal response of the in situ soil to be obtained.In the field case,the measurement error of soil water content between the AHFO method,which takes into account the thermal contact resistance,and the neutron scattering method is only 0.011.The water content of in situ soil exhibits a seasonal variation,with an increase in spring and autumn and a decrease in summer and winter.Furthermore,the response of shallow soils to precipitation and evaporation is significant.These findings contribute to the enhancement of the accuracy of the AHFO technology in the measurement of the water content of in situ soils,thereby facilitating the dissemination and utilization of this technology.

Effect of La on arc erosion behaviors and oxidation resistance of Cu alloys

Cu with and without La addition was prepared and the effect of a trace amount of La on the arc erosion behaviors and oxidation resistance of Cu alloys was investigated. The results indicate that CuLa alloy exhibits superior oxidation resistance and arc erosion resistance. The contact resistance and temperature rise were obviously improved. The oxidation resistance of CuLa alloy mainly is due to the interface wrapping of La2O3 particles and CuLa alloy phase on Cu atoms. Thermodynamic calculation indicated that La2O3 could form preferentially in the CuLa alloy, which was beneficial for the protection of the Cu substrate. According to kinetics analysis, the activation energy of CuLa alloy was higher than that of pure Cu, indicating the better oxidation resistance of CuLa alloys.

Analysis of sliding electric contact characteristics in augmented railgun based on the combination of contact resistance and sliding friction coefficient

The contact resistance between the armature and rails is an important indicator of the contact characteristics in electromagnetic launches.As the contact resistance depends not only on the contact state but also on the contact stress and temperature,there are some limitations in analyzing the contact characteristics using only the contact resistance.In this paper,the contact characteristics of the augmented railgun are analyzed by the combination of contact resistance and sliding friction coefficient.Firstly,the theoretical calculation model of the contact resistance and friction coefficient of the augmented electromagnetic railgun is established.Then the contact resistance and friction coefficient are calculated by the measured values of the muzzle voltage,rail current and armature displacement.Finally,the contact characteristics are analyzed according to the features of the waveforms of the contact resistance and the friction coefficient,and the analysis conclusions are verified by experimental rail images.The results showed that:the aluminum melt film gradually formed on the contact surface reduces the contact resistance and the friction coefficient;the wear and erosion of the armature cause deterioration of the contact state;after the transition,the reliability of the sliding contact between the armature and rails decreases,resulting in an increase in contact resistance.

Ni/Pd-based ohmic contacts to p-GaN through p-InGaN/p^(+)-GaN contacting layers

Specific contact resistance to p-GaN was measured for various structures of Ni/Pd-based metals and thin(20-30 nm thick)p-InGaN/p^(+)-GaN contacting layers.The effects of surface chemical treatment and annealing temperature were ex-amined.The optimal annealing temperature was determined to be 550°C,above which the sheet resistance of the samples de-graded considerably,suggesting that undesirable alloying had occurred.Pd-containing metal showed~35%lower com-pared to that of single Ni.Very thin(2-3.5 nm thick)p-InGaN contacting layers grown on 20-25 nm thick p^(+)-GaN layers exhib-ited one to two orders of magnitude smaller values of compared to that of p^(+)-GaN without p-InGaN.The current density de-pendence of,which is indicative of nonlinearity in current-voltage relation,was also examined.The lowest achieved through this study was 4.9×10^(-5)Ω·cm^(2)@J=3.4 kA/cm^(2).

Modeling and application of thermal contact resistance of ball screws

Aiming at determining the thermal contact resistance of ball screws,a new analytical method combining the minimum excess principle with the MB fractal theory is proposed to estimate thermal contact resistance of ball screws considering microscopic fractal characteristics of contact surfaces.The minimum excess principle is employed for normal stress analysis.Moreover,the MB fractal theory is adopted for thermal contact resistance.The effectiveness of the proposed method is validated by self-designed experiment.The comparison between theoretical and experimental results demonstrates that thermal contact resistance of ball screws can be obtained by the proposed method.On this basis,effects of fractal parameters on thermal contact resistance of ball screws are discussed.Moreover,effects of the axial load on thermal contact resistance of ball screws are also analyzed.The conclusion can be drawn that the thermal contact resistance decreases along with the fractal dimension D increase and it increases along with the scale parameter G increase,and thermal contact resistance of ball screws is retained almost constant along with axial load increase before the preload of the right nut turns into zero in value.The application of the proposed method is also conducted and validated by the temperature measurement on a self-designed test bed.

Toward High Carrier Mobility and Low Contact Resistance:Laser Cleaning of PMMA Residues on Graphene Surfaces

Poly(methyl methacrylate)(PMMA) is widely used for graphene transfer and device fabrication.However,it inevitably leaves a thin layer of polymer residues after acetone rinsing and leads to dramatic degradation of device performance.How to eliminate contamination and restore clean surfaces of graphene is still highly demanded.In this paper,we present a reliable and position-controllable method to remove the polymer residues on graphene films by laser exposure.Under proper laser conditions,PMMA residues can be substantially reduced without introducing defects to the underlying graphene.Furthermore,by applying this laser cleaning technique to the channel and contacts of graphene fieldeffect transistors(GFETs),higher carrier mobility as well as lower contact resistance can be realized.This work opens a way for probing intrinsic properties of contaminant-free graphene and fabricating high-performance GFETs with both clean channel and intimate graphene/metal contact.

Influence of thermal contact resistance on dynamic response of bilayered saturated porous strata

Porous materials can be found in a variety of geophysical and engineering applications.The existence of thermal contact resistance at the interface between bilayered saturated porous strata would result in a significant temperature difference at the interface.An attempt is made to study the thermo-hydro-mechanical coupling dynamic response of bilayered saturated porous strata with thermal contact resistance and elastic wave impedance.The corresponding analytical solutions for the dynamic response of bilayered saturated porous strata under a harmonic thermal load are derived by the operator decomposition method,and their rationality is verified by comparing them with existing solutions.The influences of thermal contact resistance,thermal conductivity ratio,and porosity ratio on the dynamic response of bilayered saturated porous strata are systematically investigated.Outcomes disclose that with the increase of thermal contact resistance,the displacement,pore water pressure and stress decrease gradually,and the temperature jump at the interface between two saturated porous strata increases.

Effect of contact thermal resistance on temperature distributions of concrete-filled steel tubes in fire

To predicate the temperature distribution of concrete-filled steel tubes(CFSTs) being exposure to fire,a finite element analysis model was developed using a finite element package,ANSYS.A suggested value of contact thermal resistance was therefore proposed with the supporting of massive numbers of collected test data.Parametric analysis was conducted subsequently towards the cross-sectional temperature distribution of CFST columns in four-side fire,in which the exposure time,width of the cross section,steel ratio were taken into account with considering contact thermal resistance.It was found that contact thermal resistance has little effect on the overall temperature regulation with the exposure time,the width of cross-section or the change of steel ratio.However,great temperature dropping at the concrete adjacent to the contact interface,and gentle temperature increase at steel tube,exist if considering contact thermal resistance.The results of the study are expected to provide theoretical basis for the fire resistance behavior and design of the CFST columns being exposure to fire.

Experimental investigation of high temperature thermal contact resistance with interface material

Thermal contact resistance plays a very important role in heat transfer efficiency and thermomechanical coupling response between two materials,and a common method to reduce the thermal contact resistance is to fill a soft interface material between these two materials.A testing system of high temperature thermal contact resistance based on INSTRON 8874 is established in the present paper,which can achieve 600 C at the interface.Based on this system,the thermal contact resistance between superalloy GH600 material and three-dimensional braid C/C composite material is experimentally investigated,under different interface pressures,interface roughnesses and temperatures,respectively.At the same time,the mechanism of reducing the thermal contact resistance with carbon fiber sheet as interface material is experimentally investigated.Results show that the present testing system is feasible in the experimental research of high temperature thermal contact resistance.

Measurement of dynamic resistance in resistance spot welding

The conventional methods of determining the dynamic resistance were mostly done by measuring the voltage and current at secondary side of transformer in resistance welding machines, in which the measuring set-up normally interferes with the movement of electrode, and the measuring precision is influenced by inductive noise caused by the high welding current. In this study, the dynamic resistance is determined by measuring the voltage at primary side and current at secondary side. This increases the accuracy of measurement because of higher signal-noise ratio, and allows to apply to in-process system without any wires connected to electrodes.

Analysis of the Correlation between the Level of Contact Degradation and the Dynamic Resistance Curve in Circuit Breakers

The DRM （dynamic contact resistance measurement） in high voltage circuit breakers is a manner of evaluating the internal ageing condition of the chamber. DRM is similar to static contact resistance measurement testing, but instead of measuring a single value when the breaker contacts are closed （static value）, the ohmic resistance is measured at various contact positions, from the beginning of the contact opening until a complete separation of the contacts. The relationship between the contact resistances of the new circuit breaker and the ageing circuit breaker in operation provides subsidy for the evaluation of both the main and arcing contact conditions. This research aims to analyze the correlation between the various levels of degradation of the contacts and the configuration of the DRM curve. This work considers curve samples from new acceleration tests. breaker chamber contacts and different levels of degradation by

A Genetic Algorithm for Simultaneous Determination of Thin Films Thermal Transport Properties and Contact Resistance

A genetic algorithm （GA） was studied to simultaneously determine the thermal transport properties and the contact resistance of thin films deposited on a thick substrate. A pulsed photothermal reflectance （PPR） system was employed for the measurements. The GA was used to extract the thermal properties. Measurements were performed on SiO2 thin films of different thicknesses on silicon substrate. The results show that the GA accompanied with the PPR system is useful for the simultaneous determination of thermal properties of thin films on a substrate.

Nondestructive measurement of thermal contact resistance for the power vertical double-diffused metal-oxide-semiconductor

To obtain thermal contact resistance（TCR） between the vertical double-diffused metal-oxide-semiconductor（VDMOS） and the heat sink, we derived the relationship between the total thermal resistance and the contact force imposed on the VDMOS. The total thermal resistance from the chip to the heat sink is measured under different contact forces, and the TCR can be extracted nondestructively from the derived relationship. Finally, the experimental results are compared with the simulation results.

Characterization of low-resistance ohmic contacts to heavily carbon-doped n-type InGaAsBi films treated by rapid thermal annealing

Carbon-doped In Ga As Bi films on In P:Fe(100)substrates have been grown by gas source molecular beam epitaxy(GSMBE).The electrical properties and non-alloyed Ti/Pt/Au contact resistance of n-type carbon-doped In Ga As Bi films were characterized by Van der Pauw-Hall measurement and transmission line method(TLM)with and without rapid thermal annealing(RTA).It was found that the specific contact resistance decreases gradually with the increase of carrier concentration.The electron concentration exhibits a sharp increase,and the specific contact resistance shows a noticeable reduction after RTA.With RTA,the In Ga As Bi film grown under CBr4 supply pressure of 0.18 Torr exhibited a high electron concentration of 1.6×10^(21) cm^(-3) and achieved an ultra-low specific contact resistance of 1×10^(-8)Ω·cm^(2),revealing that contact resistance depends greatly on the tunneling effect.

Contact resistance asymmetry of amorphous indium–gallium–zinc–oxide thin-film transistors by scanning Kelvin probe microscopy

In this work, a method based on scanning Kelvin probe microscopy is proposed to separately extract source/drain（S/D） series resistance in operating amorphous indium–gallium–zinc–oxide（a-IGZO） thin-film transistors. The asymmetry behavior of S/D contact resistance is deduced and the underlying physics is discussed. The present results suggest that the asymmetry of S/D contact resistance is caused by the difference in bias conditions of the Schottky-like junction at the contact interface induced by the parasitic reaction between contact metal and a-IGZO. The overall contact resistance should be determined by both the bulk channel resistance of the contact region and the interface properties of the metalsemiconductor junction.

The Asymmetric Laplace Law for distribution of aluminum contact resistance in spot welding

This paper investigates the distribution of contact resistance of the aluminum alloy in the squeeze stage. A new method of measuring path＇ s resistance is proposed firstly. Contact resistances are calculated accurately by the systems of 5 linear equations and solutions show that three contact resistances are different. The probability density functions of contact resistance in workpiece/workpiece（ W/W） and upper electrode/workpiece（ E/W） show that the curve shape has steeper peak and heavier tail than that of the normal distribution. Non-parameter hypothesis test is performed and the result shows that R2 , R4 reject the normal distribution using chi-square and kolmogoroo statistic D method. Therefore, the Asymmetric Laplace distribution is fitted to empirical distributions and is applied to quantify the influence of random contact resistance. The result illustrates that AL distribution is very close to contact resistance of W/W and upper E/W and normal distribution has some deviation. The paper is helpful to research the initial nugget conditions, weldability and the transient multi-coupling field.

THE INVESTIGATION OF CONTACT RESISTANCE CHARACTERISTICS ON ELECTRICAL CONTACTS USED IN COAL MINES

Based on the electrical contact and arc theory, the experiments on contact resistance characteristics of electrical contacts are carried out, by analyzing the experimental results, some conclusions of contact resistance characteristics have been obtained in this paper.

Depositing aluminum as sacrificial metal to reduce metal–graphene contact resistance

Reducing the contact resistance without degrading the mobility property is crucial to achieve high-performance graphene field effect transistors. Also, the idea of modifying the graphene surface by etching away the deposited metal provides a new angle to achieve this goal. We exploit this idea by providing a new process method which reduces the contact resistance from 597Ω ·μm to sub 200 Ω ·μm while no degradation of mobility is observed in the devices. This simple process method avoids the drawbacks of uncontrollability, ineffectiveness, and trade-off with mobility which often exist in the previously proposed methods.

Contact Resistance and Arc Erosion of Tungsten-copper Contacts in Direct Currents

The arc erosion under medium direct currents in the argon flow was tested on tungsten-copper（W-Cu） contacts which were processed by hot extrusion and heat treatment. The scanning electron microscopy（SEM） and transmission electron microscopy（TEM） were used to study the microstructure of the W-Cu powders and compacts. The contact resistance, arcing energy, and arcing time were continuously measured by JF04C contact materials test system. Changes in tungsten-copper contact surface were observed by SEM. The test results showed that the arcing time and arcing energy all increase with current and voltage, but the changes of average contact resistance are more complicated. For a short arcing time, the average contact resistance decreases with increasing current due to the vaporization of Cu. However, for a longer arcing time, it slightly increases due to the formation of high resistant films, compound copper tungsten. The formation of compound copper tungsten was confirmed by the increased Rc kept in the range from 1.1 to 1.6 mΩ. The compound copper tungsten is first exposed with a tungsten and copper-rich surface, and then totally exposed due to evaporation of copper from the surface. At last a stabilized surface is created and the crystals decrease from 8 μm to 2 μm caused by the arc erosion.