Quartz microbalances

The hydrogen gas was later" shown to originate from the formaldehyde, not the water. Instead of cumbersome and slow ex situ weighing, Wiese and Weil measured the plating rate in situ with a quartz microbalance. They connected the electrode to a potentiostat to keep it at the plating... 

Despite H/D kinetic isotope studies, application of modern techniques such as atomic force microscopy (AFM), electrochemical mass spectrometry (EMS) [60], and electrochemical quartz microbalance (EQCM), the mechanism of electroless nickel and cobalt, whatever reducing agent is involved, continues to be the subject of much discussion and varying opinions. 

Figure 4 Quartz microbalance electrode with a protein A-HRP conjugate immobilized on the gold surface, (a) Transmitted light image (b) chemiluminescent signal after addition of CL substrate (c) 3-D display of the light signal spatial distribution.

The redox electrochemistry of thin polymer films is a particularly useful field of application for the quartz microbalance. As an example, we review experiments on poly(xylylviologen) films [15]. The viologen groups can be reversibly reduced in two discrete one-electron steps. 

While the mass is not a specific quantity for a chemical species, the quartz microbalance is a useful device in cases where the chemical identity of the participating species is known, or where there are only a few candidates, which can be distinguished by their molecular masses. 

Varieties of polymers are also employed as sensitive material for electronic nose applications, and the operating temperature may reach about 100 ° C. In the case of quartz microbalance-based sensors a large role is played by the chemically interactive material (CIM) on which it is deposited. A rather efficient room temperature operating CIM is the metal-porphirin, by which it is possible to construct varieties of nostrils, just changing the type of coordinated metal. Interesting metals success-... 

Figure 2. Examples of data characterized by strong concentration effects. Data are related to a quartz microbalance array exposed to six volatile compounds each measured three times at different concentrations. Only triethylamine (TEA in the plot) data emerge (experimental details in [10]).

Figure 3.1. Multichamber deposition system for organic light emitting diodes (S sample, RF 02 plasma generator, P vacuum pump, Sh shutter, Q quartz microbalance, C crucibles, M mask for electrode patterning, T tungsten wires for metal deposition).

Deposition was performed using a vacuum system [1]. Before deposition of the alkoxlde, the sample was evacuated at 673 K for 2 hr, and the temperature was then lowered to 593 - 293 K for the deposition of silicon methoxlde. S1(0CH3)4 vapor was then admitted to the dried zeolite at a vapor pressure of 2.5 Torr. The resultant Increase of weight was measured by the quartz microbalance. After the deposition, the decatlonlzed zeolite was calcined Iji situ by oxygen at 673 K to remove the coke residue, while the Na-type mordenlte was treated with water vapor at 593 K. The amount of SI deposited was... 

Direct measurement of oxygen uptake can be performed with O2 sensors in the headspace of a closed system and organic deposition by measuring the weight changes with a quartz microbalance. This method was applied to investigate the dependence of jet fuel autoxidation on temperature and the presence of antioxidants . 

Daujotis et al. [32] have described the use of electrochemical quartz microbalance for the quantitative studies on monolayer adsorption on working mercury electrodes. Mercury was deposited on Pt at negative potentials (—0.4 to —0.5 V versus AgjAgCljKClgat)- In order to avoid undesirable transformation of mercury into, for example, larger droplets, the thickness of mercury film could not exceed 20 nm. Then, the linear dependence of the frequency change on the added mass was achieved. Applicability of such an electrode for EQCM measurements has been demonstrated by performing electroreduction of Pb(II) and T1(I), as an example. 

It has also been reported recently that metalloporphyrins have been introduced as coating materials of quartz microbalances to... 

Figure 15 Modified Pourbaix diagram for Ti02 illustrating the origins of pH-dependent band energetics and the pH-independent back-ET kinetics for covalently anchored dye species. The open circles are experimentally determined values of Ecb (combined electrochemical quartz microbalance and reflectance measurements). The driving force for the overall back reaction [coupled electron and proton transfer cf. Eqs. (10) and (11) for analogous reactions at Sn02] is pH dependent, but the driving force for the back ET in isolation [cf. Eq. (10)] is pH independent. (Data from Ref. 78.)...

Film electrodes have been essential components of quartz microbalance studies of stoichiometry of many electrodeposition and dissolution experiments as well as polymer electrode characterization [25]. In the quartz microbalance, changes in mass are detected by measurement of changes in the resonant frequency of a quartz crystal oscillator as the mass adhering to the surface changes. The oscillation is feasible because thin-film metal electrodes (typically gold) applied to opposing faces of a piezoelectric quartz crystal serve both to induce the oscillation and to provide a site for electrochemical reaction. 

Experimentally, adsorption isotherms are determined by gravimetric or volumetric measurements for powders or porous adsorbents. For isolated flat surface a quartz microbalance or an ellipsometer can be applied to measure adsorption isotherms. 

In spite of the strong economic importance of friction and wear and the resulting scientific effort, our understanding of the fundamental processes is still rudimentary. This results from the complexity of these topics. In addition, this complexity demands a multi-disciplinary approach to tribology. In recent years the development of new experimental methods such as the surface forces apparatus, the atomic force microscope, and the quartz microbalance made it possible to study friction and lubrication at the molecular scale. However, this new wealth of information does not alter the fact, that there are no fundamental equations to describe wear or calculate friction coefficients. Engineers still have to rely largely on their empirical knowledge and their extensive experience. 

Other factors, such as mechanical clamping, damping in the electrical circuit, and temperature also affect the absolute accuracy. For this reason it is necessary to use calibration curves for quantitative work. In spite of these limitations, the quartz microbalance is an extremely sensitive and versatile sensor. 

Mercury binding leads to an increase of mass of the gold layer which can be detected by electro-acoustic transducers based on quartz microbalance (QMB the abbreviation QCM = quartz crystal microbalance is also widely used), surface acoustic waves (SAW)—devices [20] or microcantilevers [21,22], Adsorption of mercury vapour increases resonance frequency of shear vibrations of piezoelectric quartz crystals (Fig. 12.2). This process can be described by Sauerbrey equation [23]. For typical AT-cut quartz, this equation is... 

This experimental assembly is much more complex than the preceding one. The oxide surfaces are ultrathin alumina films grown on NiAl(l 1 0) single crystals, in the preparation chamber following a standard procedure [16]. The alumina films are characterized in situ by AES and LEED. The metal clusters are prepared by vacuum condensation at RT of a metal atoms beam generated by an electron bombardment evaporator calibrated by a quartz microbalance. Metal atoms condense only on the sample through an aperture placed closed to it. After preparation the sample is transferred in the reaction chamber. The characterization of the metal clusters is based on STM observations of deposits performed in the same conditions in another UHV chamber [16]. 

Two different adsorbents, activated carbon Norit R 0.8 Extra (Norit N.V., The Netherlands) and molecular sieve (type 4A, Merck), were used to study tert-butylbenzene, cyclohexane, and water vapour breakthrough dynamics. Structural parameters of the carbon adsorbent were calculated from benzene vapour adsorption-desorption isotherms measured gravimetrically at 293 K using a McBain-Bakr quartz microbalance, and nitrogen adsorption-desorption isotherms recorded at 77.4 K using a Micromeritics ASAP 2405N analyzer described in detail elsewhere.22,24 Activated carbon Norit has a cylindrical... 

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