Vibrating capacitor

Historically, the first and most important capacitance method is the vibrating capacitor approach implemented by Lord Kelvin in 1897. In this technique (now called the Kelvin probe), the reference plate moves relative to the sample surface at some constant frequency and tlie capacitance changes as tlie interelectrode separation changes. An AC current thus flows in the external circuit. Upon reduction of the electric field to zero, the AC current is also reduced to zero. Originally, Kelvin detected the zero point manually using his quadrant electrometer. Nowadays, there are many elegant and sensitive versions of this technique. A piezoceramic foil can be used to vibrate the reference plate. To minimize noise and maximize sensitivity, a phase-locked... 

The relative work function and the relative electrode potential of electrodes in aqueous solutions and in inactive gases can be measured by a vibrating capacitor technique called Kelvin s method [Samec-Johnson-Doblhofer, 1992]. The Kelvin method estimates the difference in the work function between a test electrode and a Kelvin probe (KF) by measuring the applied voltage V at which the difference in the outer potential ij s- l KP between the test electrode and the Kelvin probe becomes zero (V = liJs - i Kp) as shown in Pig. 4—28. 

Fig. 4-28. Schematic layout of Kelvin s vibrating capacitor method to measure relative electrode potential of (a) electrode immersed in aqueous solution and of (b) electrode emersed from solution KP = Kelvin s probe 4 s = outer potential of aqueous solution 4>kp = outer potential of Kelvin s probe V and = applied voltages to cancel out a difference in the outer potential.

Experimentally, the work function itself can be measured with, among other methods, photoemission, since the work function appears as a clearly distinguishable threshold energy there. Changes in work function are often measured by the Kelvin method, which uses a vibrating capacitor. 

The Kelvin probe (a vibrating capacitor) is used to measure the difference in work function of two electronic materials. It is often incorrectly called a Contact Potential Difference (CPD) measurement. Explain why this term is incorrect. 

Mode 2 devices which rely on a different detection principle are the Kelvin probe sensor and the CHEMFET. In the first case, a vibrating capacitor measures the change of the work function (see Figure 2), while in the second case the interaction is detected in the field-effect transistor mode.29 31... 

These important equations have been established using both the Kelvin probe (vibrating capacitor) technique [11,140] and UPS [89] (electron cutoff energy). At the molecular level the variation of <5 with catalyst-electrode potential Uwr is due to the spillover-backspillover of (or Na ) from the solid electrolyte onto the catalyst-gas interface. 

An important step in the understanding of the origin of NEMCA was the realization that solid electrolyte cells with metal electrodes are both work function probes and work function controllers for the gas-exposed surfaces of their electrodes [7,27]. Both theory [9,14] and experiments via the Kelvin probe (vibrating capacitor) technique [7,27] and more recently via UPS [35] have shown that ... 

Figure 4. The ac current in the vibrating capacitor as a function of applied...

The principles of the measurements of the surface potential in general, and the AV -potential in particular, based either on a vibrating capacitor plate or the use of radioactive probes are... 

The Kelvin probe is a noncontact, nondestructive, vibrating capacitor technique for measuring work functions, or more precisely the difference between the work function of sample and probe. It was first used by Thomson, later Lord Kelvin, in 1862 [93]. This method has been further improved throughout the following decades [94] and is now a well-established method for measuring work functions, or, from a more electrochemical point of view, Volta potentials. Whereas in traditional Kelvin probes, the probe is a small gold plate or mesh of several square millimeters or centimeters, in Scanning Kelvin Probes (SKP), the probe is a small metal tip with a diameter of typically several tens of micrometers, which can be scaimed across the surface of the sample. 

There are various experimental methods that can lead us to the value of Vs Physico-chemists consider the vibrating capacitor method to be one of the most efficient because it allows us to work under gas and at different temperatirre. 

The vibrating capacitor method known as the Kelvin-Zisman method was developed by Zisman based on the previous method. Instead of simply distancing the electrodes from each other, Zisman periodically vibrates an electrode, thus creating an alternating current that is easier to detect. 

Figure 6.34 presents the working principle of the vibrating capacitor method U is an opposite electromotive force that is inserted in the measurement circuit, Vdpc is the change in contact potential between the solid that forms the electrodes of capacitor C (previously denoted by Vrs), and R is an electrical resistance. 

The vibrating capacitor method is a zero-determining method, where measuring the contact potential difference Vdpc amoimts to measuring the opposite voltage U that leads to a zero current. Compared to other methods, this one has some interesting characteristics which we will develop throughout this study. 

The vibrating capacitor method, or Kelvin-Zisman method, is a non-destructive method that does not disturb the surface using photon or electron beams. It can be used for a large number of materials, and in a wide temperatnre and pressure range. It is therefore highly appropriate for the stndy of snrface properties of polyciystalline metal oxides. 

The vibrating capacitor method does not require high vacuums it measures work function values over a wide range of pressures and in varions atmosphere types. 

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