Silver sensor

This technique has been recently applied in different studies. One of them evaluated the environmental conditions in the treasure rooms of Reims Cathedral, where silver sensors were exposed during a five-year period [306]. The reduction curves point out that the results strongly depend on whether the coupons were displayed inside or outside a case. Regarding the nature of the products developed on the surface during the exposure, only silver sulphide has been found on the coupons kept inside the showcase, while the additional peak found with coupons exposed outside the case suggests the additional presence of silver chloride (see Fig. 6.4a). Moreover,... 

Johnston, B. Goldenberg. L.M. Bryce. M.R. Kataky, R. A tetrathiafulvalene derivative with an acyclic S-4 domain as a voltammetric silver sensor. J. Chem. Soc. Perkin Trans. 2 2000. 189-190. 

Titration Cell, containing a reference electrode, a working electrode, and a silver sensor electrode, as well as a magnetic stirrer. An inlet from the pyrolysis tube is also required. 

Sequence-specific biosensor, 183, 185 Selectivity, 92, 143, 147, 155 Selectivity coefficient, 143 Self-assembled monolayers, 39, 118 Selenium, 85 Sensor, 171 Silver halide, 159 Simulation, 35... 

After each series of experiments with beams of various intensity the section plate would be removed from the cell and disassembled, with radioactive silver washed out by nitric acid. Radioactivity of the solutions obtained was measured by a multichannel spectrometric scintillation y-counter with sensitivity of up to 10 G, i. e. around 10 of atoms which, according to calculations, is 10 times lower than sensitivity of ZnO sensor 10 G or 10 of Ag atoms respectively [28]. This difference in sensitivity lead to great inconveniences when exposing of targets was used in above methods. Only a few seconds were sufficient to expose the sensor compared to several hours of exposure of the scintillation counter in order to let it accumulate the overall radioactivity. It is quite evident that due to insufficient stability during a long period of exposure time an error piled up. 

Table 3.3 shows the data obtained while measuring intensities of a Ag beam by isotope and semiconductor sensor method using a ZnO oxide film as a sensitive element. The data reveals that in this case there is also a satisfactory linear dependence between the number of silver... 

Atoms of metals are more interesting tiian hydrogen atoms, because they can form not only dimers Ag2, but also particles with larger number of atoms. What are the electric properties of these particles on surfaces of solids The answer to this question can be most easily obtained by using a semiconductor sensor which plays simultaneously the role of a sorbent target and is used as a detector of silver adatoms. The initial concentration of silver adatoms must be sufficiently small, so that growth of multiatomic aggregates of silver particles (clusters) could be traced by variation of an electric conductivity in time (after atomic beam was terminated), provided the assumption of small electric activity of clusters on a semiconductor surface [42] compared to that of atomic particles is true. 

Fig. 4.22. Reaction cell 1 - ZnO sensor 2 - evaporator of silver atoms 3 shutter used to terminate the beam of Ag atoms 4 - collimating apertures 5 an aperture used for pumping the cell out 6 - magnet 7 - magnetic drive for a shutter 8 - getter 9 — vacuum-measuring tube iO, 11 - electrodes 12 - thermocouple.

Fig. 4.23. Distribution of concentration of oxygen in upper atmosphere / -obtained with a semiconductor sensor on December 27, 1979 near observation site Volgograd (present investigation) 2, 3 the data of mass-spectrometric measurements 4, 5 - the data of a resonance spectroscopy 6 — silver films 7, 8 - model calculations...

Fig. 6.8. Emission of silver atoms 1 - the change in resistivity of a sensor-substrate 2 - the change in resistivity of a sensor-detector...

The procedures of experiments were the following [15, 26]. After deposition of a specific quantity of silver on substrate the heating of a tray with silver was turned off, the shutter 7 was opened and the sensor was positioned opposite to the substrate in such a manner that the surface of the sensor was parallel to the surface of substrate. In these experiments we detected an irreversible donor signal of the sensor which can be related to adsorption silver atoms on the sensor made of a zinc oxide film. It is known [27] that silver atoms are donors of electrons. Note that the signals of the sensor were observed only when the sensor was positioned in front of a substrate. There were no signals detected in any other arrangement between sensor and substrate. 

This conclusion is proved by the element analysis of sensors which have registered more than 30 portions of silver emitted from the surface of substrate after completion of deposition of silver on its surface. The experiments on local analysis of tiie sensor indicated that such sensor has areas containing up to 0.3 wt.% of silver in 1 pm. The total amount of silver atoms incident on sensor was 7-10. This means that silver atoms get deposited on the surface of the sensor inhomogeneously, being localized close to defect allocation area which made it possible to detect... 

Therefore these experiments showed a very interesting phenomenon, namely the emission of adsorbed silver atoms from the surface of a substrate after accomplishing the deposition process. In these experiments the semiconductor sensors were used in two ways sensor-substrate onto which the silver was deposited from, the tray, which made it possible to monitor the behaviour of silver atoms on the surface of adsorbent and sensor-detector of emitted silver atoms. 

We heated the substrate of zinc oxide containing 10 cm 2 of silver atoms (in this case there was already no emission after completion of deposition) at 300 C. Such thermal treatment results in formation of microcrystals, rather than evaporation adatoms on the surface of the substrate made of zinc oxide. In paper [34] it was shown that microcrystals with diameter 100 A deposited on the zinc oxide surface are acceptors of electrons, therefore the formation of microcrystals results in increase of resistivity of a sensor substrate above the initial value (prior to silver deposition). In this case the initial value of the resistance of sensor-substrate was 2.1 MOhm, after adsorption of silver atoms it became 700 kOhm, and as a result of heating at 300°C and formation of microcrystals - acceptors of electrons it in increased up to 12 MOhm. If such a substrate is subject to deposition of 3-10 5 cjjj-2 silver again, then emission of silver atoms gets detected. From the change of resistivity of sensor-detector due to deposition of silver atoms one can conclude that in this case the emission of atoms is 4 times as low than in experiment with pure substrate made of zinc oxide, which confirms the supposition made on the mechanism of emission of adatoms. 

Finally, if we heat the sensor-substrate with deposited silver atoms using internal heater (platinum film attached to the back side of the sensor-substrate) up to 700 C then the surface of zinc oxide gets completely cleaned of silver. This can be confirmed by the value of resistivity of sensor-substrate which comes back to the initial value of 2.1 MOhm (the silver during such treatment partially evaporates, partially migrates to the contacts). The experiment showed that as a result... 

Therefore, the use of a combination of sensors (sensor-substrate and sensor-detector of emitted atoms) enabled us to obtain unique information concerning the mechanism of emission of adsorbed atoms of silver in the course of their surface aggregation. The data obtained makes it possible to provide an insight of mechanisms and energetics of condensation-stimulated phenomena [11]. 

These studies were carried out on industrially manufactured piezoquartz resonators with an AT-cut featuring silver thin-film electrodes. Oscillations with a frequency of 10 MHz (resonant frequency) were generated by generator of the TKG-3 type. The sensor of silver atoms (films of zinc oxide) were positioned in the same vial with resonator, the sensor was positioned parallel to the resonator plane the distance between them was about 5 mm. Prior to the experiment the vial containing resonator and sensor was heated up to 473 K and kept at above... 

The evaluation of amount of silver atoms desorbed from resonator during 80 h operation was made applying the known relations [36] for silver atoms adsorbed on the sensor surface in charged form accounting for the fraction of atoms transferred from the source (resonator) to the target (sensor). The estimates indicate that intensity of the flux of silver atoms from the surface of resonator in this experiment was about 1.5-10 3 m 2-s S i.e. during 75 h operations approximately 10% of the surface silver atoms left resonator (under condition that the amount of surface silver atoms is about 4-10 m ) explaining the shift of resonant frequency from its nominal value 10 MHz by 11 Hz. 

Fig. 6.11. The schematics of experimental set-up to study emission of atomic oxygen. 1 — sensor of oxygen atoms 2 samples of reduced silver 3 shutter 4 weights to brake membranes 5 platinum filament to calibrate sensor against the concentration of oxygen atoms.

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