Igoshina S.E., Averin I.A., Vishnevskaya G.V., Pronin I.A., Karmanov A.A.

Penza State University, Penza, Russia

Multicomponent metal oxide nanomaterials with fractal structure for highly sensitive vacuum sensors

 

The attempts to create vacuum sensors based on nanomaterials that exhibit certain dimensional effects have begun relatively recently, but they show great prospects for the use of nanotechnology in this field [1]. The purpose of this research is the development of highly sensitive vacuum sensors, based on multicomponent metal oxide nanomaterials with fractal structure.

We used binary systems based on SiO2-SnO2 with various mass fractions of tin dioxide and ternary ones based on SiO2-SnO2-ZnO, synthesized by the sol-gel method [2, 3]. Film forming sols, based on tetraethoxysilane (TEOS) and hydrolyzed in an acidic medium, were applied. Binary metal oxide nanomaterials of SiO2-SnO2 composition were synthesized by adding modifying impurities SnCl2×2H2O to TEOS-based sol. Their hydrolysis products undergo simultaneous polycondensation according to the scheme:

In the reaction of hydrolytic polycondensation an ultrathin silica mesh (a matrix) of silicon dioxide with built-in clusters of tin dioxide is formed.

Ternary metal oxide nanomaterials of SiO2-SnO2-ZnO composition are synthesized by adding inorganic salts of tin (SnCl2×2H2O) and zinc (ZnCl2) to TEOS-based sol. A simplified scheme of simultaneous polycondensation of their hydrolysis products is as follows:

The mechanism of this reaction is quite complicated, it depends on many factors (e.g., the solvent type, pH of the medium, etc.), and results in the formation of nanomaterials with different structure, but preferably of "guest" (a semiconductor switch) – "host" (a dielectric matrix) type.

The sol was coated on the substrates of monocrystalline oxidized silicon of 5×5 mm2 by centrifugation at the table rotation speed of 4000 rev/min. The annealing was carried out at a temperature of 600 degrees Celsius for 30 minutes in the air. Planar silver contact pads were formed by thermal evaporation in the vacuum.

The main feature of the synthesized binary and ternary metal oxide nanomaterials is a fractal type of structural organization (Fig. 1).

 

Fig. 1. An AFM image of binary metal oxide nanomaterials of SiO2-SnO2 composition with fractal structure (a) and the model of sensitive elements of vacuum sensors on their base (b)

Figure 1 shows that the synthesized nanomaterials have a developed surface, formed by clusters of quasi-spherical form, to describe which it is appropriate to apply either the model of Witten–Sander fractal (box in Fig. 1a), or a three-dimensional Julien fractal (Fig. 1b). However, regardless of the used model, it is well established and confirmed by the experimental data that the multicomponent metal oxide nanomaterials with fractal structure have a high porosity due to a high concentration of macro, micro and mesopores [4]. In turn, the high porosity leads to active gases interaction with the surface and the volume of the investigated nanomaterials, and their desorption at pressure decrease lower than the atmospheric one forms the basis of the offered vacuum sensors work.

As the research shows, the sensitivity of vacuum gauges depends on the number of multicomponent metal oxide nanomaterials, i.e. on the mass fraction of the alloying components. Fig. 2 shows the relative change in resistance (R/R0) of the sensitive elements of vacuum sensors based on the binary system SiO2-SnO2 by varying the vacuum level. R0 stands for the initial resistance at the atmospheric pressure.

Fig. 2. The relative change in resistance of the sensitive elements of the vacuum sensor based on the binary system SiO2-SnO2 at different mass fraction of tin dioxide:

1 – 50%; 2 – 60%; 3 – 70%; 4 – 80%; 5 – 85%; 6 – 90%

Figure 2 shows that the resistance of the sensitive elements of the proposed vacuum gauges decreases monotonically with the decreasing pressure. Mass fraction of tin dioxide has almost no influence on the form of the submitted dependencies (curves 1-6 in Fig. 2), but it determines the value of the sensitivity, which increases along with the content of the semiconductor component, and reaches its maximum of 55.3% at 85% SnO2. A further increase in the mass fraction of tin dioxide has the opposite effect, which is likely due to the impossibility of forming an ultrathin silica matrix at a low content of SiO2. It is most likely, that the main contribution to the sensory feedback of the developed vacuum gauges is made by desorption of such gases as CO2, O2, H2O. Desorption of the water vapor increases the resistance of the sensitive elements of the vacuum sensors and desorption of carbon dioxide and oxygen causes its decrease. The content of tin dioxide in the binary system SiO2-SnO2 affects the sensitivity only indirectly. Increasing in the mass proportion of SnO2 leads to the increase in the total porosity of the sensitive elements, and hence, the efficiency of their interaction with the residual gas molecules.

The use of ternary metal oxide nanomaterials of SiO2-SnO2-ZnO composition as sensitive elements of the vacuum gauges can significantly increase their sensor feedback (Fig. 3).

Fig. 3. The relative change in resistance of the sensitive elements of the vacuum sensor based  on the multicomponent metal oxide nanomaterials of the following composition:  1 – SiO2-SnO2; 2 – SiO2-SnO2-ZnO (a) and an AFM image of their surface (b)

Figure 3a shows that the resistance of the sensitive elements of the proposed gauges decreases monotonically with the decreasing pressure, wherein the qualitative composition of the multicomponent metal oxide nanomaterials has almost no influence on the form of the submitted dependencies. However, the sensor feedback from the ternary system (mass fractions of SnO2 and ZnO are 50 and 10 ω, %, respectively) is significantly higher than that of the binary one (mass fraction of tin dioxide is 50 ω, %). The analysis of the AFM images presented in Fig. 3b shows that the ternary metal oxide nanomaterials of SiO2-SnO2-ZnO composition have three-dimensional porous structure with high concentration of macro and micropores. Probably, zinc oxide acts as an extra debonder of the inorganic polymer structure [5]. The increased feedback of the ternary metal oxide nanomaterials to pressure changes is associated with their higher overall porosity, which determines the concentration of adsorption/desorption centers.

Thus, the possibility of vacuum sensors creating based on the multicomponent metal oxide nanomaterials of SiO2-SnO2 and SiO2-SnO2-ZnO composition is demonstrated. It is concluded that the increase in the mass fraction of the modifying impurities leads to an increase in the total porosity of the sensitive elements, and consequently improves the sensor feedback of the proposed vacuum gauges. It is shown that the use of ternary nanomaterials instead of binary ones allows a greater control of their sensitivity.

This work was financially supported by the Foundation for Assistance to Small Innovative Enterprises in Science and Technology (program "UMNIK", No. 0015027), and scholarship of the President of the Russian Federation SP-4686.2013.1

References

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