Contact Potential Measurements Research Articles

Wear induced changes in surface topography during running-in of rolling-sliding contacts

Running-in occurs in rolling-sliding contacts under mixed-friction conditions and typically involves asperity smoothing through mild wear and plastic deformation. To improve the prediction of service life or friction of rolling-element bearings under mixed-friction conditions, knowledge of surface topography changes during running-in and their dependence on the operating condition is an important prerequisite. Therefore, this study aims to describe the surface mechanisms due to wear, during running-in and their dependence on one of the variables, slip.AISI 52100 steel specimens were tested in a mini traction machine (MTM) instrumented with friction and contact potential measurement in the presence of a PAO base oil, operating in the mixed lubrication regime. A novel method of pre and post-test surface relocation with 3-D optical profilometry and scanning electron microscopy (SEM) was implemented. Rolling-sliding tests were performed to study the effect of slide-roll ratio on the surface topography changes during running-in. Additional tests were conducted to track the changes to surface topography during this period.The results exhibit the rapid nature of running-in and how most of the surface topography changes occur in the first few load cycles. Surface topography transitions such as asperity removal due to wear, increase in the load-bearing area of the asperities and in tests with slip, plastic flow of material from peaks into adjacent valleys are shown at high magnification. Surface profilometry measurements show that the reduction in depth of the valleys due to plastic flow adds to the reduction in roughness, thus accurately identifying the surface mechanism, and redressing the consensus that the reduction in roughness during running-in is mainly due to wear and plastic deformation diminishing the height of the peaks, which is true for ideal running-in alone.

Contact potential and scanning Kelvin force microscopy measurements on sulphate-anion intercalated graphite

In the current interpretative model of graphite anion intercalation, the characteristic blisters, formed after the incipient surface oxidation, have the same structural behaviour of pristine graphite. However, this result is deduced from scanning tunneling microscopy data, which cannot give a chemical fingerprint of the electrode surface. Conversely, the contact potential difference between a reference tip and a sample depends on the chemical as well as on the structural conditions of the surface. As a consequence, in this paper, we combine a contact potential measurement with a scanning Kelvin force microscopy analysis, in order to extract information on electronic changes occurring to the electrode with a sub-micrometre investigation of the surface morphology after the incipient graphite oxidation process in diluted sulphuric acid. The results suggest that, as well as the structural properties, no significant electronic differences occur between blisters and the graphite basal plane after the intercalation process.

Photoluminescence in Spray Pyrolysis Deposited β-In2S3 Thin Films

Spray pyrolysis deposited In2S3 thin films exhibit two prominent photoluminescent emissions. One of the emissions is green in color and centered at around ∼ 540 nm and the other is centered at around ∼ 690 nm and is red in color. The intensity of the green emission decreases when the films are subjected to annealing in air or vacuum. The intensity of red emission increases when films are air annealed and decreases when vacuum annealed. Vacuum annealing leads to an increase in work function whereas air annealing leads to a decrease in work function for this thin film system relative to the as deposited films indicating changes in space charge regions. Surface photovoltage analysis using a Kelvin probe leads to the conclusion that inversion of band bending occurs as a result of annealing. Correlating surface contact potential measurements using a Kelvin probe, x-ray photoelectron spectroscopy and photoluminescence, we conclude that the surface passivation plays a critical role in controlling the photoluminescence from the spray pyrolysis deposited for In2S3 thin films.

Construction of the Energy Band Diagram of Hydrogen Terminated Diamond and Silicon Nanowires

We demonstrate the use of a combined ambient pressure and UHV photoemission system for absolute work function characterization. We describe measurements of the valence band position of Hydrogen Terminated Synthetic Diamond (HTSD) under different pressures. The diamond exhibits a nanoscale surface conductivity with a 2D hole gas of ≈10 nm depth. The same system also allows contact potential difference Kelvin probe measurements for determination of relative Fermi level position from 1 to 1.1 × 10−11 Bar. Combined with UV‐Surface Photovoltage determination of the band gap, this allows a complete reconstruction of the surface energy band diagram of this material. The valence band edge was found to be 4.9 eV under ambient conditions and 4.4 eV under UHV, relative to the vacuum level. The ambient pressure photoemission technique was also used to reconstruct the energy band diagram of bottom‐up grown silicon nanowires. They had a valence band maximum at 5.04 ± 0.05 eV. Both the HTSD and silicon nanowires had high photoemission yield characteristics which suggests that these materials could be used as an electron source. This illustrates the suitability of photoemission and Kelvin probe techniques for investigating the macroscopic work function characteristics of surfaces with nanostructures and nanoscale surface conductivity.

Influence of atmospheric oxygen on hydrogen detection on Pd using Kelvin probe technique

Influence of oxygen concentration in the measurement atmosphere on detection of hydrogen using Kelvin probe was studied. The studied material was a 100-μm-thick palladium foil, which was mounted in a 3D printed electrochemical flow cell. The used setup enables hydrogen loading with in-situ contact potential measurement of the hydrogen exit side of the Pd electrode. The hydrogen loading and unloading procedure, including insertion of different amounts of hydrogen into the Pd membrane and recording resulting values of contact potential difference, was performed at distinct oxygen concentrations ranging between 1 and 80 vol%. An increasing amount of oxygen in the atmosphere surrounding the hydrogen-loaded Pd electrode resulted in an accelerated removal of hydrogen from the Pd. The kinetics of this reaction was studied based on Kelvin probe measurements, and a reaction mechanism is discussed.

Using contact potential measurements to analyse future intercity links made possible by the Tours–Bordeaux High-Speed Rail line

New high-speed rail transport infrastructures produce profound time-space changes that make new activities possible. Our aim is to measure the impact of such changes on potential business or leisure travel between cities.We develop a contact potential indicator (Tornqvist, 1970) that measures the possibility of making a trip to a distant location within the desirable time-space prism. The indicator is developed in the context of the high-speed rail line between Tours and Bordeaux.The results demonstrate the possibility of contact from a series of key cities affected by new high-speed rail infrastructure, both before and after project completion. Our analysis shows that Bordeaux benefits greatly from the extension of the rail line: the preferred method of contact to Paris will switch to rail from the current air preference, and several cities that cannot be reached in the current pre-project conditions will become 'contactable'. Other cities that benefit are Angouleme, Tours and Poitiers.

Challenges in hydrogen quantification using Kelvin probe technique at different levels of relative humidity

Kelvin probe-based contact potential measurements were conducted with in situ electrochemical hydrogen loading of a Pd membrane with a thickness of 100 μm. The theoretical basis of diffusion coefficient calculation based on measured response time of potential drop, arising from hydrogen arriving at the detection side of the Pd membrane, was discussed. In situ hydrogen loading was also utilized for insertion of different amounts of hydrogen into the Pd sample while monitoring the resulting contact potential difference changes. Measurement of contact potential difference at different final values of hydrogen concentration in the Pd membrane was performed at various atmospheric conditions, focusing in this work on increasing relative humidity (4 to 85% rH). Moreover, the effect of humidity changes on hydrogen effusion kinetics at room temperature and low oxygen content (<1%) was studied.Graphical abstractᅟ

Determination of Contact Potential Difference by the Kelvin Probe (Part II) 2. Measurement System by Involving the Composite Bucking Voltage

Abstract The present research is devoted to creation of a new low-cost miniaturised measurement system for determination of potential difference in real time and with high measurement resolution. Furthermore, using the electrode of the reference probe, Kelvin method leads to both an indirect measurement of electronic work function or contact potential of the sample and measurement of a surface potential for insulator type samples. The bucking voltage in this system is composite and comprises a periodically variable component. The necessary steps for development of signal processing and tracking are described in detail.

Meaning and Measurability of Single-Ion Activities, the Thermodynamic Foundations of pH, and the Gibbs Free Energy for the Transfer of Ions between Dissimilar Materials.

Considering the relationship between concentration and vapor pressure (or the relationship between concentration and fugacity) single-ion activity coefficients are definable in purely thermodynamic terms. The measurement process involves measuring a contact potential between a solution and an external electrode. Contact potentials are measurable by using thermodynamically reversible processes. Extrapolation of an equation to zero concentration and ionic strength enables determination of single-ion activity coefficients. Single-ion activities can be defined and measured without using any extra-thermodynamic assumptions, concepts, or measurements. This method could serve as a gold standard for the validation of extra-thermodynamic methods for determining single-ion activities. Furthermore, it places the concept of pH on a thermodynamically solid foundation. Contact potential measurements can also be used to determine the Gibbs free energy for the transfer of ions between dissimilar materials.

Effects of hydroxylation and silanization on the surface properties of ZnO nanowires.

Silanization is commonly used to form bonds between inorganic materials and biomolecules as a step in the surface preparation of solid-state biosensors. This work investigates the effects of silanization with amino-propyldiethoxymethylsilane on hydroxylated sidewalls of zinc oxide (ZnO) nanowires (NWs). The surface properties and electrical characteristics of NWs are analyzed by different techniques after their hydroxylation and later silanization. Contact angle measurements reveal a stronger hydrophobic behavior after silanization, and X-ray photoelectron spectroscopy (XPS) results show a reduction of the surface dipole induced by the replacement of the hydroxyl group with the amine terminal group. The lower work function obtained after silanization in contact potential measurements corroborates the attenuation of the surface dipole observed in XPS. Furthermore, the surface band bending of NWs is determined from surface photovoltage measurements upon irradiation with UV light, yielding a 0.5 eV energy in hydroxylated NWs, and 0.18 eV, after silanization. From those results, a reduction in the surface state density of 3.1 × 10(11) cm(-2) is estimated after silanization. The current-voltage (I-V) characteristics measured in a silanized single NW device show a reduction of the resistance, due to the enhancement of the conductive volume inside the NW, which also improves the linearity of the I-V characteristic.

Contact potential measurement using a heated atomic force microscope tip

This letter reports contact potential measurement between the tip of a heated atomic force microscope cantilever and a biased gold film. Force-distance experiments were performed with tip temperature, tip potential, and substrate potential independently controlled. Experiments were conducted for probe temperatures of 23 to 200 °C and tip potentials of −1 V to 1 V. The measured contact potential was a function of temperature, due to the thermoelectric properties of the tip and substrate. The Seebeck coefficient for the combined system was close to −4.30mV∕K, consistent with the tip and substrate materials. The technique is scalable to arrays suitable for large area imaging.

Analytical Solutions of the Frequency Shifts of Several Modes in Dynamic Force Microscopy Subjected to AC Electrostatic Force

Kelvin probe force microscopy is currently used to measure contact potential difference and topography of sample on a nanometer scale, based on the behaviors of the first two resonant frequencies. In this study, an analytical method to determine the frequency shifts of several modes in dynamic force microscopy subjected to AC electrostatic force is proposed. The numerical results determined by the proposed method are close to the experimental ones. If the amplitude and the frequency of the AC voltage are suitable, a large frequency shift and a linear relation between the frequency shift and the tip-sample distance will be obtained. In other words, properly controlling the AC voltage will increase the accuracy of measuring the sample's topography. Moreover, it is found that even without an optimized compensation dc voltage, increasing the first frequency shift and decreasing the tip radius and the half open angle of the tip can increase the accuracy of measuring a step height. Finally, it is found that when the oscillation amplitude and the tip-sample distance are constant, increasing the negative dc voltage will increase the accuracy of the measurement of contact potential by using the frequency shift. In addition, an assessment of the conventional perturbation method is presented and the two methods are compared.

Spontaneous Emergence of Cl- Anions from NaCl(100) at Low Relative Humidity

The emergence of Cl- ions from the surface of alkali halide salts at low relative humidity (RH) is predicted by density functional theory (DFT) calculations and supported by contact potential measurements. We find from DFT that, in the presence of water at the regime of one monolayer coverage on the (100) cleavage plane of NaCl, Cl- ions are displaced at very low energetic cost from their crystal lattice positions toward the plane of water molecules. Starting from plausible low-temperature layered structures, we use total energy DFT to calculate the energy cost to bring Cl- ions to the height of the water layer. We show the importance of the screening of the electrostatic interactions by the water layer in order to explain our findings and to determine the value of the surface dipole as a function of the chloride position. The theoretical surface dipole is used to estimate the concentration of raised Cl- ions at the surface. As dissolved chloride ions are the reactive component of the sea salt aerosols at high RH, we propose that the facile destabilization of Cl- is one of the mechanisms behind the catalytic activity of NaCl at low RH on reactions involving atmospheric gases.

Optical and electrochromic properties of oxygen sputtered tungsten oxide (WO 3) thin films

Thin films of tungsten oxide (WO 3) were deposited onto glass, ITO coated glass and silicon substrates by pulsed DC magnetron sputtering (in active arc suppression mode) of tungsten metal with pure oxygen as sputter gas. The films were deposited at various oxygen pressures in the range 1.5×10 −2−5.2×10 −2 mbar. The influence of oxygen sputters gas pressure on the structural, optical and electrochromic properties of the WO 3 thin films has been investigated. All the films grown at various oxygen pressures were found to be amorphous and near stoichiometric. A high refractive index of 2.1 (at λ=550 nm) was obtained for the film deposited at a sputtering pressure of 5.2×10 −2 mbar and it decreases at lower oxygen sputter pressure. The maximum optical band gap of 3.14 eV was obtained for the film deposited at 3.1×10 −2 mbar, and it decreases with increasing sputter pressure. The decrease in band gap and increase in refractive index for the films deposited at 5.2×10 −2 mbar is attributed to the densification of films due to ‘negative ion effects’ in sputter deposition of highly oxygenated targets. The electrochromic studies were performed by protonic intercalation/de-intercalation in the films using 0.5 M HCl dissolved in distilled water as electrolyte. The films deposited at high oxygen pressure are found to exhibit better electrochromic properties with high optical modulation (75%), high coloration efficiency (CE) (141.0 cm 2/C) and less switching time at λ=550 nm; the enhanced electrochromism in these films is attributed to their low film density, smaller particle size and larger thickness. However, the faster color/bleach dynamics is these films is ascribed to the large insertion/removal of protons, as evident from the contact potential measurements (CPD) using Kelvin probe. The work function of the films deposited at 1.5 and 5.2×10 −2 mbar are 4.41 and 4.30 eV, respectively.

Effect of substrate atomic structure on intersolubility and alloying in Pd/Nb adsorption system

In order to investigate to what extent the crystallographic structure of the substrate affects the process of interdiffusion and surface alloy formation in the case of the same components, adsorption systems of Pd on Nb(0 1 1), Nb(0 0 1) and Nb(1 1 1) have been studied by means of low-energy electron diffraction, Auger electron spectroscopy, and contact potential measurement. For all the systems, Pd dissolves in the substrate, when its population on the surface exceeds 1 ML. Kinetics of the mutual solution depends on the amount of deposited Pd, and increases with increasing amount of the adsorbate. Independent of the substrate structure, there was no evidence for a mixing of Pd and Nb in the top-most layer. For the Pd/Nb(0 1 1) system, the lateral interactions inside the Pd adlayer surpass the adsorbate–substrate interaction. The adsorbate–substrate interaction is dominant for the Pd/Nb(0 0 1) and Pd/Nb(1 1 1) systems, and results in the formation of a sandwich-like laminar surface alloy with a Pd layer on top.

Measurement of surface contact potential of AlGaN/GaN heterostructure and n‐GaN by Kelvin probe force microscopy

The contact potential images of the AlGaN/GaN heterostructure and n-GaN before and after thermal treatment have been successfully obtained by Kelvin probe force microscopy (KFM). It was found that the removal of the surface potential shielding effect of the adsorbed water-related layer by means of thermal treatment was effective in the observation of low potential regions at around the dislocations. The low potential regions at dislocations indicated that the dislocations of AlGaN/GaN heterostructure and n-GaN were negatively charged. The contact potential variations around the dislocations in AlGaN/GaN heterostructure was much larger than that of n-GaN in both magnitude and diameter. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and compositional changes of the ultrathin Pd layers on Nb(0 0 1) caused by annealing

Pd layers of thickness from a fraction to several monolayers (ML), deposited at room temperature (RT) on Nb(001) surface, are annealed from 400 to 2150K. Changes of composition, atomic and electronic structure, caused by annealing, have been studied using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and contact potential measurement (CPD). Up to 2.8ML the growth of Pd films is isomorphous with the substrate. Above this coverage, hcp palladium domains are formed with the orientation {112̄0}Pd||{001}Nb and 〈0001〉Pd||〈011〉Nb, and the stacking sequence AB|AB|, etc. along the [0001] direction. The work function (WF) of the (112̄0)Pd film amounts to 5.2eV. For the annealing temperature from 600 to 900K, beginning from coverage 6.8ML, the Pd adlayer undergoes phase transition to another hcp structure with the stacking sequence ABAC|ABAC|, etc. For coverage higher than 1ML, intermixing becomes noticeable above 700K. After annealing at higher temperatures, surface alloy formation is observed in the entire coverage range studied, 0.5–9.5ML. The alloy has a (1×1) lateral structure with reference to Nb(001). The topmost layer of the alloy consists of Pd atoms and the work function amounts to 4.8±0.1eV.

Direct Observation of Photo Switching in Tethered Spiropyrans Using the Interfacial Force Microscope

The interfacial force microscope has been used to establish the changes in the surface chemistry that accompany photoactivated opening and closing of rings in tethered spiropyran monolayers. Contact potential measurements show that ring opening via ultraviolet light creates a more polar surface, while exposure to visible light closes the rings and resets the surface. Normal force measurements show that open ring structures are protonated in electrolyte solutions, amplifying the switching of electrical double-layer forces.

Effect of substrate atomic structure on intersolubility and alloying in Pd/Nb adsorption system

In order to investigate to what extent the crystallographic structure of the substrate affects the process of interdiffusion and surface alloy formation in the case of the same components, adsorption systems of Pd on Nb(0 1 1), Nb(0 0 1) and Nb(1 1 1) have been studied by means of low-energy electron diffraction, Auger electron spectroscopy, and contact potential measurement. For all the systems, Pd dissolves in the substrate, when its population on the surface exceeds 1 ML. Kinetics of the mutual solution depends on the amount of deposited Pd, and increases with increasing amount of the adsorbate. Independent of the substrate structure, there was no evidence for a mixing of Pd and Nb in the top-most layer. For the Pd/Nb(0 1 1) system, the lateral interactions inside the Pd adlayer surpass the adsorbate–substrate interaction. The adsorbate–substrate interaction is dominant for the Pd/Nb(0 0 1) and Pd/Nb(1 1 1) systems, and results in the formation of a sandwich-like laminar surface alloy with a Pd layer on top.

Contact potential differences measurement: short history and experimental setup for classroom demonstration

Methods for the measurement of contact potential differences (CPDs) are discussed from a historical point of view. In addition, a simple experimental setup using common laboratory equipment is presented. Experimental results show, for instance, that the measured CPDs between platinum and nickel can be very close to those obtained using sophisticated equipment. Moreover, it is demonstrated that the implemented method, though very simple, is suitable for surface mapping.