Method of producing thermoelectric gas-sensitive material

FIELD: chemistry.

SUBSTANCE: method includes forming a film with thickness of not more than 200 nm from semiconductor nanoparticles of SnO2 with size of not more than 50 nm. The film of SnO2 nanoparticles is then annealed at temperature of 330±20 K or 500±20 K for at least 15 min in an oxygen-containing atmosphere, followed by cooling to room temperature at a rate of at least 10 K/s.

EFFECT: broader functional capabilities of the material.

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The invention relates to electronics, and is intended to create a material based on semiconductor nanoparticles or otherwise nanomaterial with gas thermoelectric effect, i.e. the magnitude of thermo-EMF of a nanomaterial can be sensitive to various gases into the external atmosphere. The invention can be used in thermoelectric devices that convert heat energy into electrical energy. Can also be used in various fields of science and technology for the development of gas sensors.

For the prototype selected nanomaterial-based nanocrystalline semiconductor films SnO2consisting of particles with a typical size of 10-100 nm [1]. Such materials are widely used as gas sensitive layers of sensors and can be obtained by different methods of deposition (e.g., thermal, sputtering, ion-beam) followed by annealing or the Sol-gel method [1, 3]. The conductivity of such films depends strongly on the concentration of various detectable gases. It is known that an important role in the mechanism of sensitivity of such sensors to various detektivami gases plays chemisorption of oxygen, as detected gases, as a rule, actively interact with hammarbyhamnen on the surface of semiconductor particles with oxygen [1-3]. When is hemosorption molecules of oxygen, playing the role of the acceptor on the surface of semiconductor particles with a conductivity of n-type are formed negatively charged oxygen ions, and in the near-surface space charge region is formed depleted electrons charged layer and a corresponding bending of the energy bands near the surface [2]. As a result, between the individual particles are formed of potential barriers and the conductivity of such a system can approximately be described by the following equation:

G=Gvexp(-eVs/kT)(1)

where Gv- the factor describing the bulk conductivity of the semiconductor, Vs- the height of the potential barrier. Increasing the height of the potential barriers Vsbetween nanoparticles during chemisorption of oxygen will lead to a reduction in conductivity. If chemisorption of oxygen occurs in a certain range of temperatures, at these temperatures the value of Vswill be maximum, and the temperature dependence of the conductivity will receive a minimum of [2, 3]. For thermopower S and the coefficient peltie P in a semiconductor is known to follow what her expression (up to a negligible here the constant term) [4]:

P=ST=-(1/e)(Ec-E)(2)

or with the height of the potential barrier Vs:

P=-(1/e)(E0+γT+eVs)(3)

where S is thermopower, S0- the energy difference between the bottom of the conduction band and the Fermi level at zero temperature, γ is the coefficient for the temperature dependence of the Fermi level position, Vssurface potential barrier between the nanoparticles. Thus, increasing the height of the potential barrier between the semiconductor nanoparticles, due to the increase in bending energy zones near their surface, can lead to increased thermoelectric properties of semiconductor nanomaterials. It is known that the efficiency of termoelectrica the fir materials is determined by the quality factor, equal to the product ZT. Here

ZT=S2σT/k(4)

where k is thermal conductivity [W/(MK)], σ is the electrical conductivity, S - thermo-EMF [V/K]. Currently the best value of the quality factor reaches ZT≈2 for some thermoelectric materials, for example, Bi2Te3, PbSe, but these materials have certain disadvantages - high working temperature, contain toxic, rare or expensive items [5-7]. As an alternative promising thermoelectric materials recently proposed oxides of metals, such as stable at high temperatures, more environmentally friendly and cheap. For example, available materials based on doped ZnO (ZT=0,47 at 1000 K) and layered cobalt oxide Ca3Co4O9(ZT=0,22 at 1000 K) [5, 8, 9]. In [10] proposed a material based on a mixture of tin oxide SnO2with the addition of ZnO and Ta2O5or Nb2O5. A powder mixture of oxides is then pressed into tablets, which are sintered at a temperature of from 1000 to 1400°C. the General formula of the obtained material can be written in the form of Sn1-x-yZnxMy 2where 0,76≤1-x-y≤0.99, and with inclusions of phase ZnSn2O4from 1 to 25 wt%. The particle size of the obtained polycrystalline porous material is in the range from 100 nm to 100 μm, and the preferred amount is from 5 to 70 micrometers. The disadvantage of this material is not sufficiently high values of thermo-EMF and the quality factor, which are 100-200 μv/K and 0.06 to 0.13, respectively, at 1000 K.

The technical result of the invention is

• functional enhancement of thermoelectric materials due to the possibility of changes in thermo-EMF of nanomaterial depending on the concentration of oxygen or other gases (H2, NH3, CO, CH4, NO2H2S) in the air;

• simplify and reduce the cost of thermoelectric material due to its production of nanoparticles SnO2without the use of toxic, rare or expensive materials such as lead, silver, bismuth, tellurium, or rare earth elements;

• increase thermo-EMF to 1.3 mV/K at the operating temperature of 330 K and up to 1.1 mV/K at the operating temperature of 500 K;

• increase the quality factor ZT of thermoelectric material to 1 at the operating temperature of 330 or 500 K.

To achieve the specified result, we propose a way to obtain

thermoelectric gatchev twitterlogo material, which is manufacturing film thickness of not more than 200 nm from the semiconductor nanoparticles SnO2with a size of not more than 50 nm, while after the production of the film of nanoparticles SnO2annealed at a temperature of 330±20 500±20 K for at least 15 minutes in an oxygen-containing atmosphere, followed by cooling to room temperature at a rate not less than 10 K/S.

This annealing is carried out in the air.

The figure 1 shows the temperature dependence of thermopower of the material.

The figure 2 shows the temperature dependence of the coefficient peltie, which reflects the temperature dependence of the Fermi level position according to equation (2).

The figure 3 shows the temperature dependence of the conductivity of the material.

The figure 4 shows the temperature dependence of the quality factor of the material.

The measurements were carried out on nanocrystalline film SnO2thickness of 200 nm, obtained by magnetron sputtering. The size of the individual nanoparticles in the obtained film are defined by electron microscope, was about 50 nm. Structurally experimental samples was paligorova substrate with dimensions of 5×to 0.5×0.2 mm, one side of which was the semiconductor film SnO2and on the other sputtered film of platinum, with Ujamaa heater. The heater was both resistance, the magnitude of which is controlled by the temperature of the sample. The sample temperature could be changed and stabilized at a predetermined value by using a specially designed electronic power supply unit with an accuracy of 0.1°C. To obtain a temperature gradient on the sample platinum heater was located only at one end of the sample. The temperature difference was measured using two thermocouples Au-Ni, placed at opposite ends of the sample. Differential thermo-EMF was measured in the temperature range of 300 to 550 K (Figure 1). The corresponding coefficient peltie, which reflects the temperature dependence of the Fermi level position according to equation (2)shown in figure 2. Figure 3 shows the temperature dependence of the conductivity. The obtained dependencies are clearly observed two extremum at temperatures of about 330 and 500 K or, respectively, 60 and 230°C. These extremes can be explained by the chemisorption of charged forms of oxygen O2-and O-at these temperatures. The maximum depth of the Fermi level in dependence on the temperature is determined by the change in height of the potential barrier at the chemisorption of oxygen and reaches around to 0.55 eV in the region of temperature of 500 K (Figure 2). If after heating to such temperature made the STI rapid cooling to room temperature at a rate not less than 10 K/s, increased the amount of the potential barrier remains as hammarbyhamnen oxygen molecules remain on the surface. Thus, thermo-EMF metal oxide semiconductor nanomaterials type SnO2, ZnO, can be substantially increased by the corresponding temperature material processing. Assessment of the quality factor ZT according to equation (4) based on the measured thermopower (Fig 1) and conductivity for the proposed nanomaterial (Figure 3) shows that its value reaches the value 1 when the two optimal temperatures of 330 and 500 K (Figure 4), which is comparable with the best thermoelectric materials. The value of the coefficient K for SnO2relied equal to 0.5 W/(m K) in the whole temperature range [11]. Because of the strong phonon scattering at the boundaries of the particles, and various defects and impurities of the conductivity of polycrystalline porous materials can be much less than that of single crystals, therefore reducing the size of the nanoparticles and the film thickness can lead to a reduction in thermal conductivity [12]. Thus, there is a possibility to further reduce thermal conductivity for the proposed nanomaterial and increase the quality factor ZT. Also in the proposed nanomaterial you can control and adjust the amount of potentialenergy between nanoparticles, to optimize the transport properties for maximum thermoelectric effect.

The resulting nanomaterial can be used in thermoelectric generators, as well as for the manufacture of various gas sensors to determine the content of oxygen or other gases (H2, NH3, CO, CH4, NO2H2S) in the air, and on the contacts of the gas sensor is generated EMF, which depends on the concentration of the detected gas.

LITERATURE

1. S. Song, J. Cho, W. Choi et al, Sensors and Actuators 46 (1998) 42-19.

2. Morrison S.R. Chemical physics of solid surfaces. -M.: Mir, 1980. S.

3. A. Varfolomeev, A.V., Ariskin, V.V. Malyshev, ALEXANDER Razumov, S. Yakimov, Journal of analytical chemistry, vol 52, No. 1 (1997) p.66-68.

4. V.L. Bonch-Bruevich, YEAR Kalashnikov, Physics of semiconductors, -M.: Nauka, 1990.

5. MRS BULLETIN, vol.31, March 2006, p.193.

6. X.H. Ji, X.B. Zhao, Y.H. Zhang, B.H. Lu, H.L. Ni, J. Alloys Compd. 387 (2005) 282.

7. J. Seo, C. Lee, K. Park, J. Mater. Sci. 35 (2000) 1549

8. M. Ohtaki, T. Tsubota, K. Eguchi, H. Arai, J. Appl. Phys. 79 (1996) 1816.

9. Y. Zhang and J. Zhang, J. Of Materials and Processing technology, 208 (2008) 70-74.

10. Patent EP 2447233 A1, Tin oxide-based thermoelectric materials, 2012.

11. P.R. Bueno, J.A. Varela et al, J. American Ceram. Soc., 88 (9) (2005) 2629-2631

12. C. Poulier, D. Smith, J. Absi, Journal of the European Ceramic Society 27 (2007) 475-478.

1. Method for manufacturing thermoelectric gas sensitive material, which consists in the manufacture of film thickness h is more than 200 nm from the semiconductor nanoparticles SnO 2with a size of not more than 50 nm, wherein after the production of the film of nanoparticles SnO2annealed at a temperature of 330±20 500±20 K for at least 15 minutes in an oxygen-containing atmosphere, followed by cooling to room temperature at a rate not less than 10 K/S.

2. The method according to claim 1, characterized in that the annealing is carried out in air.



 

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