Quartz fundamental properties

The literature concerning the quartz crystal microbalance (QCM) and its electrochemical analog, the electrochemical EQCM, is wide and diverse. Many reviews are available in the Uterature, discussing the fundamental properties of this device and its numerous appUcations, including its use in electrochemistry [1-7]. In this chapter we focus on the effect of interfacial properties on the QCM response, specifically when the device is immersed in a fiquid. 

When acting as a microbalance, it is sometimes stated that the change in resonant frequency is an absolute measure of the change in mass loading, namely that this device does not require calibration. This statement is only partially true, and one is well advised to calibrate the QCM. Admittedly, the constant C in Eq. (1) can be calculated from the fundamental properties of the quartz crystal, as given by the Sauerbrey equation [see Eq. (9)]. However, for this constant to be applicable for the determination of the added mass, several implicit assumptions must be made. Primarily, it must be assumed that the mass distribution on the surface is uniform. It must also be borne in mind that the Sauerbrey equation only applies to thin films, such that the thickness of the film is small compared to the thickness of the crystal itself. In addition, the EQCM can operate as a true microbalance only if, in the course of the process being studied, the nature of the interface—its roughness, the density and viscosity of the solution adjacent to it, and the structure of the solvent in contact with it—is kept constant. 

Quartz is a natural crystalline material which exhibits this form of behavior, although its relevant properties are highly temperature dependent and synthetic materials have been developed which, although fundamentally less accurate, are more stable under varying temperature conditions. 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

The most characteristic feature of any crystal is its symmetry. It not only serves to describe important aspects of a structure, but is also related to essential properties of a solid. For example, quartz crystals could not exhibit the piezoelectric effect if quartz did not have the appropriate symmetry this effect is the basis for the application of quartz in watches and electronic devices. Knowledge of the crystal symmetry is also of fundamental importance in crystal stmcture analysis. 

As the readers may see, quartz crystal resonator (QCR) sensors are out of the content of this chapter because their fundamentals are far from spectrometric aspects. These acoustic devices, especially applied in direct contact to an aqueous liquid, are commonly known as quartz crystal microbalance (QCM) [104] and used to convert a mass ora mass accumulation on the surface of the quartz crystal or, almost equivalent, the thickness or a thickness increase of a foreign layer on the crystal surface, into a frequency shift — a decrease in the ultrasonic frequency — then converted into an electrical signal. This unspecific response can be made selective, even specific, in the case of QCM immunosensors [105]. Despite non-gravimetric contributions have been attributed to the QCR response, such as the effect of single-film viscoelasticity [106], these contributions are also showed by a shift of the fixed US frequency applied to the resonator so, the spectrum of the system under study is never obtained and the methods developed with the help of these devices cannot be considered spectrometric. Recent studies on acoustic properties of living cells on the sub-second timescale have involved both a QCM and an impedance analyser thus susceptance and conductance spectra are obtained by the latter [107]. 

Any type of acoustic transducer, such as quartz crystal microbalance (QCM) or surface acoustic wave device (SAW), is fundamentally based on the piezoelectric effect. This was first described in 1880 by Jacques and Pierre Curie as a property of crystalline materials that do not have an inversion centre. When such a material is subjected to physical stress, a measurable voltage occurs on the crystal surfaces. Naturally, the opposite effect can also be observed, i.e. applying an electrical charge on a piezoelectric material leads to mechanical distortion, the so-called inverse piezo effect. These phenomena can be used to transfrom an electrical signal to a mechanical one and back, which actually happens in QCM and SAW. Different materials are ap-pHed for device fabrication, such as quartz, Hthium tantalate, lithium titanate... 

Although the Factory Mutual Apparatus [101] has never been proposed as a standard test method, it is briefly mentioned here since it has been used for basic research work by several organizations and contains a number of novel features. A 100 mm. square specimen is mounted in a vertical chimney and heated (up to 65 kW m") by banks of quartz lamps mounted outside the chimney. Preheated air or oxygen nitrogen mixtures are pa.ssed up the column at a constant rate. The apparatus has been mainly used to determine the fundamental fire properties of materials, although products such as cables have also been tested. 

The fundamental silicon-oxygen compound is silica, which has the empirical formula SiOi. Knowing the properties of the apparently similar compound carbon dioxide, one might expect silica to be a gas that contains discrete SiOi molecules. In fact, nothing could be further from the truth— quartz and some types of sand are typical of the materials composed of silica. What accounts for this difference The answer lies in the bonding. 

A new optical approach was demonstrated by Kuwana and coworkers [235] in 1964, that is, the use of TCO electrode in which the product of an electrochemical reaction is monitored spectroscopically. There has been considerable interest in the fundamental electrochemical properties of Sn02 semiconducting electrodes deposited on glass or quartz substrate [236-241]. In this form, electrodes have been prepared that can be used to observe electroactive species near the electrodesolution interface by internal reflectance spectroscopy in the visible region of the spectrum [237-239]. The electrochemical and surface characteristics of doped Sn02 and In203 film electrodes [242] were discussed and compared to those of Pt and Au film electrodes [242, 243]. 

It is particularly important in this case, the fact that fused silica (or quartz) presents excellent transmittance properties in the NIR region (with much poorer transmittance properties in the MIR, VIS, and UV regions) and is insensitive against water. As a matter of fact, this explains why NIR-based technologies developed much faster in real industrial applications, when compared to MIR, VIS, and UV spectroscopic methods. Efficient transmission of fundamental IR, VIS, and UV radiations through long distances with the help of... 

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