Скачати 34.58 Kb.
semiconductOR chemiluminescent SENSOR FOR the DETERMINATION of the partial PRESSURE of ATOMS In the GASES
V.P. Grankin, S.А. Voloschuk
Priazovsky State Technical University,
Ukrain, 87500, town. Mariupol, Donetskaya obl., st. Universitetskaya, 7
e-mail: email@example.com, firstname.lastname@example.org
The principles of the creating of the semiconductor sensors for the determination of the atom concentration (H, O) in gases (Н2, О2, О2+Ar etc.) are described. The principles of the creating of the sensors on the basis of the semiconductor surface excited by heterogeneous chemiluminescence (HCL) owing to energy of heterogeneous atoms recombination. Conditions of work and semiconductor for nonequilibrium chemiluminescent sensors on solid are determined. It’s showed, that the HCL excitation of semiconductor sensor takes place as a result of the impact atoms recombination by the Rideal-Eley’s mechanism only. It’s retrieved, that to this wide band gap semiconductors AIIBVI (ZnS, CdS) satisfy. The processes taking place during the HCL excitation were modeled numerically using the developed excitation mechanism of heterogeneous chemiluminescence.
The interaction of atomic particles from a gas phase with the semiconductor’s surface is accompanied by the adsorptive - desorptive and the reactionary processes on a surface, in the course of which there is an energy release in some eV per elementary act. The main channel of the energy accommodation is the phonon channel resulting in the heating of the semiconductor. Apart from this there is an electronic channel competing with the phonon one , manifested in the excitation of nonequilibrium electronic effects on the semiconductor’s surface: radical-recombination luminescence (RRL), adsorboluminescence, nonequilibrium adsorbconductivity etc. These phenomena are connected to electronic excitation of the semiconductor’s surface (1-2 monolayers) and consequently bear the information purely about a solid surface and gas surrounding. The luminescenct centres on a surface and in a near-surface stratum ZnS are picked out and investigated on the basis of measurings RRL . Also offered the optical methods of the reseaching of the plasmachemical reactions and determination of the concentration of the atomic particles on the semiconductor’s surface. Thus, the principles of the usage of the semiconductors as the sensors for the determination of the fragment concentration in gases (control for the concentration in an environment of one or another gas) are determined.
There are two basic approaches in the construction of the semiconductor chemical sensors. The principle of the former is based on the transformation of magnitude of an adsorption in an electric cue corresponding to an amount of fragments, adsorbing from an environment or appearing on the semiconductor’s surface due to heterogeneous chemical reactions. These are equilibrium sensors. The second approach is based on purely nonequilibrium effects of electronic excitation of the semiconductor’s surface in the course of heterogeneous chemical reactions of an adsorption and recombination of atomic particles from a gas phase. The relaxation electronic chemo-excitation can be accompanied by a luminescence of the semiconductor’s surface (heterogeneous chemiluminescence), emission of the charged particles in a vacuum (chemoemission - CE) and other nonequilibrium chemoeffects. The intensity of the chemiluminescence is determined by the speed of heterogeneous chemical reaction, which, in its turn, depends on the concentration of the atomic particles in the environment, what defines a possibility of construction of the luminescent semiconductor sensor.
Radical-recombination luminescence is that of the semiconductor excited by heterogeneous reaction of the recombination of the atoms (radicals) on the semiconductor’s surface with atoms (radicals) from a gas phase. There are two mechanisms of excitation RRL: "ionizating" and "direct".
Based on the ideas of the theory of the chemosorption and the catalysis on the semiconductors, the "ionizating" mechanism was offered by V.А. Sokolov and A.N. Gorban  and refers to a case of the RRL excitation when the energy of recombination of atoms exceeded the forbidden band gap of the semiconductor. According to this mechanism the free atom or radical, being adsorbed on the semiconductor’s surface, will derivate with a grating a common quantum-mechanical system and can be considered as a structural defect. Thus in the forbidden band gap of the semiconductor there is a local electronic level having donor or acceptant properties depending on the nature of the semiconductor and of the chemosorping atom. The ionization of the centre of luminescence in this mechanism occurs by means of the electronic excitation of the crystal grating the acts of the recombination of the radicals on the semiconductor’s surface including the charged form of the chemosorption.
From the power reasons for the semiconductors, which forbidden band gap is much more than the energy of recombination of the atoms, the “ionizating” mechanism becomes hardly probable, however RRL on these samples is supervised. On a surface of such wide band gap semiconductors the "direct" mechanism of the excitation of RRL, consisting in the consequent power transmission on oscillatory - electronic or gas-kinetic mechanism after the act of recombination takes place just on the centre of luminescence, or near to centre.
The recombination of the atoms on the semiconductor’s surface is possible both by the shock mechanism of Rideal-Eley, and by the mechanism of Langmuire-Hinshelwood, when the atoms recombine previously adsorbed on the semiconductor’s surface . By the consideration of the HCL it was supposed, that the recombination of the atoms in a molecule occurs predominantly by the mechanism Rideal-Eley. The given supposition was based on the discontinuous magnification of intensity HCL on some order of magnitudes, caused by the turning on the atomic stream in a gas phase.
The kinetic mechanism of RRL:
1. The adsorption and the desorption of atoms on the regular centres of the semiconductor’s surface:
2. The adsorption and the desorption of the molecules on the regular centres of the semiconductor’s surface:
3. The recombination of the radical from a gas phase with the atom on the semiconductor’s surface by the mechanism of Rideal-Eley:
4. The recombination of the adatoms during diffusion on the semiconductor’s surface by the mechanism of Langmuire-Hinshelwood: (2)
Above the arrows are marked, accordingly the adsorption rate of the radical from a gas phase on a node of the crystal grating and the rate of the reverse process - desorption; - probability of the recombination of the radical from a gas phase and adatom; - accordingly adsorption rate of a molecule from a gas phase on a node of the crystal grating and speed of the reverse process - desorption; - the constant of the recombination rate of the adatoms on a surface of solid.
Let's enter identifications for the concentrations into an instant t: [RL] N1(t); [R2L] N2(t). For the above kinetic mechanism RRL a system of differential equations (the mathematical model) will be written to a following kind:
The received set of equations allows to determine the intensity of a luminescence at any moment of time:
For the shock mechanism of the atoms recombination by Rideal-Eley (1) the intensity of HCL is proportional to a stream of atoms from a gas phase on the semiconductor’s surface:
Here , the quantum yield of HCL, excited in the given reaction, j(t) is the flux of the atoms from a gas phase, is the cross-section of the impact recombination, N1(t) is the number of adatoms on the luminescence centers of the semiconductor’s surface at given period of time. Using the given dependence of the chemiluminescence intensity from the atoms stream from a gas phase the construction of a responsive inertialess luminescent sensor for measuring the atoms concentration in a gas phase (in the stream of atoms) is principally possible.
By modern photoelectronic multiplexers it is possible confidently to register luminescence intensity conformable to 104 - 105 quantas/sm2 seconds. At a quantum yield of a luminescence = 10-2 - 10-4 (crystal phosphors on the basis of semiconductors ZnS, Zn2SiO4 - Mn etc.), cross-section of recombination = 10-17 sm2 and filling of the semiconductor’s surface by the atoms N1 = 1013 sm-2 for the registration the atoms streams j = 1010 - 1011 sm-2s-1 (concentration of atoms accordingly 106 - 107 sm-3) are accessible.
The efficiency of the HCL excitation depends on the grade of exciting gas and such as the semiconductor operating in the experiment. It is connected with the discrepancies in the micromechanisms of the HCL excitation in miscellaneous gas - surface systems, in the speeds of adsorptive - desorptive and recombination processes, in catalytic activity of a surface of different semiconductors. Due to this HCL (by the group of signs) can be used in the analytical purposes - for the determination of the impurities in binary and multicomponent gas mixtures.
In the conjecture, that the excitation of the heterogeneous chemoluminescence takes place in the reaction of the atoms recombination on the semiconductor’s surface by the mechanism of Langmuire-Hinshelwood (2), the intensity of HCL is proportional to quadrate of the atoms concentration on the semiconductor’s surface and is complicatedly connected with the atoms concentration in a gas phase:
where is the quantum yield output on one recombination of adatoms.
The knowledge of the atoms recombination mechanism on a surface of a luminescence sample of a semiconductor sensor, thus, is indispensable for the presence of sensors samples, on which the recombination by the mechanism of Rideal-Eley will be realized. The retrieved semiconductors there fore, can be used for the determination of the atomic particles concentration in gases. For the determination of mechanisms of atoms recombination reaction on a surface and mechanism of HCL excitation it is important to know the filling of the semiconductor’s surface by atoms (radicals) during the experiment at any moment of time.
The N1 determination takes place by the modulation of magnitude of flux j on Δj (Pic. 1). At such variation the RRL intensity is changed to magnitude , defined by an alteration of speed of the atoms recombination only by the Rideal-Eley’s mechanism, as during the modulation ( 2 sec) the atoms concentration on the semiconductor’s surface remains constant. It allows to determine at any moment of time and at preset temperature of a sample Т the atoms concentration on a surface applicable to a stationary level of excitation of a semiconductor sample: .
The experimental dependence I from N1 is indicated in a Fig. 1. For the miscellaneous Т 3 experiments are given (Fig.2). In the whole investigated interval of the concentrations N1 it is linear, i.e. the RRL excitation of the semiconductor’s surface ZnS-Tm takes place as a result of the impact atoms recombination by the Rideal-Eley’s mechanism. For the given conditions of the experiment the contribution of the mechanism of Langmuire-Hinshelwood (4) in general RRL intensity does not exceed 10 %.
As we can see from (3), I(t) nevertheless depends from , which can change during the experiment owing to plasmachemical modification of a semiconductor sensor (for example, 2H + ZnS → H2S↑ + Zn). For the same reason also will be changed with a certain period of time. The magnitude N1 depends from j and also is changed in due course. It makes impossible to use the stationary methods and the methods based on a preliminary graduation of samples of semiconductor sensors for measuring atomic particles. The possible output is the usage of impulsive methods in construction of semiconductor chemical sensors, in which the limitations on the stability of the sample of the CL semiconductor measuring device are peeled, and there is a waste preliminary sensor grading.
At the impulsive method of probing of sounding the second atoms stream of the same chemical composition as well a source gravity j2 in reactionary volume with the semiconductor is entered. For these streams accordingly we shall record the luminescence intensites (5), , (6)
where it is possible to see, that in this case the proportion of (5) to (6) does not depend on time-varying magnitudes , , N1. From (5) and (6) we can determinate partial pressure of atoms in gases at any significances of filling of the semiconductor’s surface by adatoms N1 at any moment of measuring of t magnitude.
The temperature reduction of the semiconductor is made for the exception of diffusive recombination (thus the atoms recombination rate by the mechanism of Langmuire-Hinshelwood is reduced). The usage of the additional (the second) atoms stream by the gravity j2 completely removes the question on the CL semiconductor sensor preparation, also the limitations on the choice of measuring moment are declined, and also there is no necessity for a procedure of preliminary titrating and storage of an active element of a semiconductor sensor. For the creation of nonequilibrium semiconductor chemical sensors on the basis of the HCL phenomenon, the choice of such semiconductors, on which the HCL excitation in reaction of atoms recombination by the shock mechanism of Rideal-Eley will be realized, is indispensable. It’s retrieved, that to this wide band gap semiconductors AIIBVI (ZnS, CdS) satisfy.
Text for figures
Fig. 1 Time function of the HCL intensity of a luminophor ZnS, CdS-Ag from time of excitation by H atoms. Т=300K, j=1015 sm-2s-1.
Fig. 2 Experimental dependence of HCL intensity ZnS-Tm in the Н atoms stream from the filling of a surface by the Н atoms N1, j=1015 sm-2s-1. (1) - T1 – 300K, (2) - T2 – 340K, (3) - T3 – 450K. Validity to aproximations is showed for each experiments.
|The purification of gases using high-speed vortical flows||Block "radiator-fan" of cooling system of the car’s engine|
С after a 17-hours storage on the open stand at an ambient temperature minus 20 єС; reduces the engine wear on 300%, amount of harmful...
|Block "radiator-fan" of cooling system of the car’s engine|
С after a 17-hours storage on the open stand at an ambient temperature minus 20 ºС; reduces the engine wear on 300%, amount of harmful...
1. /МЕД__курс_ МОДУЛЬ 2/Alimentary tract/Alimentary tract elbЕромянц.doc