Vibrating quartz crystal

A different type of extremely sensitive gravimetric technique is based on the effe change of mass on the resonance frequency of a vibrating quartz crystal (see Fi 3.12). In this case, the adsorbent must be firmly attached to the crystal. Its area be as small as a few square centimetres and mass changes as low as 10-2 pg ca detected from the frequency shift (Krim and Watts, 1991). 

In the vertical ultrasonic nebulizer the sample solution is injected continuously at a constant rate directly onto the vertical face of the vibrating quartz crystal (Figure 39). The nebulization efficiency is about 50 and 25% at sample flow rates of 0.1 and 3 mlmin respectively. 

The liquid interface sensors tested included a gamma ray radiation gage, a capacitance gage," two float switches, a vibrating quartz crystal level gage, and a thermistor level switch. These devices were all specified as capable of gaging a quiet level to i 1/16 inch. 

So we understand that as the emf changes with temperature, so the quartz crystal vibrates at a different frequency - all because... 

We first experimented with the Quartz Crystal Microbalance (QCM) in order to measure the ablation rate in 1987 (12). The only technique used before was the stylus profilometer which revealed enough accuracy for etch rate of the order of 0.1 pm, but was unable to probe the region of the ablation threshold where the etch rate is expressed in a few A/pulse. Polymer surfaces are easily damaged by the probe tip and the meaning of these measurements are often questionable. Scanning electron microscopy (21) and more recently interferometry (22) were also used. The principle of the QCM was demonstrated in 1957 by Sauerbrey (22) and the technique was developed in thin film chemistiy. analytical and physical chemistry (24). The equipment used in this work is described in previous publications (25). When connected to an appropriate oscillating circuit, the basic vibration frequency (FQ) of the crystal is 5 MHz. When a film covers one of the electrodes, a negative shift <5F, proportional to its mass, is induced ... 

In addition to silica (silicon dioxide SiO ), the crystal form of silicon is found in several semiprecious gemstones, including amethyst, opal, agate, and jasper, as well as quartz of varying colors. A characteristic of quartz is its piezoelectric effect. This effect occurs when the quartz crystal is compressed, producing a weak electrical charge. Just the opposite occurs when electric vibrations are fed to the crystal. These vibrations are then duphcated in the crystal. Quartz crystals are excellent timekeeping devices because of this particular characteristic. 

Use of a Quartz Crystal Vibrator in Vacuum Destination Invstigations... 

New method of measuring vibration amplitudes of quartz crystals. 

Piezoelectric microbalance The piezoelectric microbalance is a resonant frequency device. The piezoelectric effect is the development of a charge on some crystals such as quartz when a stress is applied the stress may be mechanical (e.g., added weight) or electrical. Such crystals may be used as part of a resonance circuit to provide very stable, narrow-band frequencies the quartz crystal is plated on two sides with a thin conducting layer and leads are connected to the resonance circuit so the crystal replaces an LC network. The obtained frequency of vibration (pu) depends on a number of parameters of the crystal but is usually 5-10 MHz. However, if a mass (Am) becomes attached to one side of the crystal, it changes the resonant frequency by an amount At , such that... 

Because of its piezoelectric properties, synthetic CC-quartz is used for frequency control in electrical oscillators and filters and in electromechanical transducers. When mechanically stressed in the correct direction, CC-quartz develops an electric polarization. The opposite is also tme an applied electric field gives rise to a mechanical distortion in the crystal. Thin sections of quartz are cut to dimensions that produce the desired resonance frequency when subjected to an alternating electric field the vibrating crystal then reacts with the driving circuit to produce an oscillation that can be narrowly controlled. Quartz is ideal for this application because it is hard, durable, readily synthesized, and can be tuned to high accuracy, for example, quartz crystal clocks can be made that are stable to one part in 109. 

This is the correct name for most popular mass sensors, although they are better known as Quartz Crystal Microbalances (QCMs). A piezoelectric crystal vibrating in its resonance mode is a harmonic oscillator. For microgravimetric applications, it is necessary to develop quantitative relationships between the relative shift of the resonant frequency and the added mass. In the following derivation, the added mass is treated as added thickness of the oscillator, which makes the derivation more intuitively accessible. 

Figure 4.3 shows the shear-mode vibration of a quartz crystal of mass M and thickness t. At resonance, the wavelength X is... 

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... 

The transducers most commonly employed in biosensors are (a) Electrochemical amperometric, potentiometric and impedimetric (b) Optical vibrational (IR, Raman), luminescence (fluorescence, chemiluminescence) (c) Integrated optics (surface plasmon resonance (SPR), interferometery) and (d) Mechanical surface acoustic wave (SAW) and quartz crystal microbalance (QCM) [4,12]. 

The strong dependence of the layer structure on the nature of the contacting electrolyte has been further investigated by using the electrochemical quartz crystal microbalance (EQCM). As discussed above in Chapter 3, this technique is based on the measurement of the frequency with which a coated quartz crystal vibrates, and this frequency can then be related to the mass of this crystal provided that the material attached to the surface is rigid. In this way, the changes that occur in thin films as a result of redox processes can be monitored. 

A truly radical improvement was the development of quartz crystals, used in high-Q resonators. The vibrations of a quartz timing force cantilever can be transformed into a voltage across the faces of the crystal, by the piezoelectric effect The space group of a-quartz is P3221, and a voltage... 

The crystal cut determines the mode of oscillations. Shear vibrations are generated if one large crystal face moves parallel with respect to the underlying planes as in QCMs with AT-cut a-quartz crystals. This crystal wafer is prepared by cutting the quartz at approximately 35.17° from its Z-axis. A typical crystal plate is a cylindrical disk of a diameter 10 mm and thickness about 0.7 to 0.1 mm for resonant operation in the 2 to 15 MHz frequency range. This type of crystals shows weak dependence of the resonant frequency on the temperature and stress for room temperature operation. 

The addition of mass provides the means of transduction for many chemical sensors, including surface acoustic wave (SAW) devices, quartz crystal microbalances (QCM), and microcantilevers. In all these devices, the mass addition either perturbs the vibration, oscillations, or deflection within the transducer. The mode of transduction in an optical interferometer can also be linked to mass addition the sensor s response is altered by refractive index changes in the material being monitored. It is possible that this change can be elicited solely from refractive index changes without the addition of mass, although in sensing a particular... 

When an atom makes a transition from a high-energy quantum state to a lower energy state, electromagnetic radiation with a definite frequency and a definite period is emitted. When properly detected, this frequency, or period, becomes the ticking of an atomic clock, just as the crystal vibration frequency and the swinging frequency are the inaudible ticks of a quartz clock and a pendulum clock. The frequency emanating from the atom, however, is much less influenced by environmental factors such as temperature, pressure, humidity, and acceleration than are the frequencies from quartz crystals or pendula. Thus, atomic clocks hold inherently the potential for reproducibility, stability, and accuracy. 

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