On the Candoluminescence of Phosphors in Carbon Monoxide and Methane Flames

O. G. Brik

(Presented by the scientific seminar of the Laboratory of Dielectrics and Semiconductors)

Gaydon [1, 2] notes that candoluminescence, i.e., the glow of certain solid bodies introduced into a flame, is characteristic of hydrogen and hydrogen-containing flames.

Sokolov and Gorban’ [3, 4] observed the candoluminescence of phosphors in an illuminating-gas flame and proposed a surface-recombination mechanism for this phenomenon [5]. According to this mechanism, candoluminescence arises as a result of the recombination of free atoms and radicals, formed during the combustion process [6], on the surface of the phosphor. The energy released during recombination is expended in exciting the phosphor.

In [7] it was proposed that the principal radical in an illuminating-gas flame that causes the candoluminescence of phosphors is free hydrogen atoms.

Since illuminating gas contains, in addition to hydrogen (51%), other components, including methane (30%) and carbon monoxide (8%), the question of the possibility of candoluminescence in the flames of these components is of interest. In the present work this possibility was studied with respect to methane and carbon monoxide flames (during combustion in air). The other constituents of illuminating gas (\(\mathrm{C_2H_2}\), \(\mathrm{C_2H_4}\), etc.) are present in small amounts and cannot have a decisive influence on candoluminescence.

Experimental Method

Luminescence was observed in the flame of a Bunsen burner 10 mm in diameter. The phosphor was applied as a suspension to a cooled surface. The concentration of combustible components of the illuminating gas was determined with an KhT-2M chromatograph. Natural gas from the Shebelinka field of the following composition was used as methane:

\(\mathrm{CH_4}\)—93.2%, heavy hydrocarbons—6.3%, \(\mathrm{CO_2}\)—0.1%, \(\mathrm{N_2}\)—0.4%.

Carbon monoxide was obtained by the reaction \(\mathrm{CO_2 + C \to 2CO}\); the hydrogen impurity amounted to 6–8%. (It is known [1, 9] that pure carbon monoxide without additions of initiating impurities of water or hydrogen ignites poorly.)

The temperature of the flame and of the surface of contact of the phosphor with the flame was measured with a chromel–alumel thermocouple 0.2 mm in diameter. The temperature of the phosphor in the flame was 60–70°C.

To measure the integral intensity of the radiation, an FEU-19M photomultiplier and an M-95 microammeter were used. To study the spectral composition, the radiation flux was passed through a UM-2 monochromator and an ISP-51 spectrograph with a constant slit at the output, with subsequent feeding of the signal to an FEU-19M, calibrated for spectral sensitivity (Fig. 1).

Fig. 1. Block diagram of the apparatus.
1—cylinder with phosphor, 2—burner, 3—ISP-51, 4—FEU-19M, 5—FEU power-supply unit, 6—M-95.

Industrial phosphors were used in the investigations: FK-102, K-35.

Experimental Results

Candoluminescence of phosphors was observed in methane and carbon monoxide flames. It was established that the emission spectrum of the phosphors in these flames coincides with the spectra in an illuminating-gas flame and in photoluminescence (Fig. 2).

Fig. 2. Candoluminescence spectrum of phosphor K-35 in a flame:
1—illuminating gas, 2—methane, 3—carbon monoxide, 4—under photoexcitation.

The integral intensity of candoluminescence in a methane–air flame is somewhat lower than the intensity in an illuminating-gas flame. In a carbon monoxide flame, however, the integral intensity is considerably lower than in an illuminating-gas flame.

Discussion of Results

According to the theory of chain combustion processes [6], in a hydrogen flame there is a significant concentration of free H and O atoms and OH radicals, and the ratio of their concentrations is expressed as follows [8]:

\[ (\mathrm{H}) : (\mathrm{OH}) : (\mathrm{O}) = 7500 : 50 : 1. \]

The combustion mechanism of methane [9] and carbon monoxide [10] also presupposes the presence of these same atoms and radicals. For a CO flame [11]: (H) : (OH) : (O) = 25 : 20 : 1.

According to measurements by Fenimore and Jones [12], at 1345°K the concentration of free hydrogen atoms in a methane–air flame at \(p = 1\) atm is \(44 \cdot 10^7\) mol/l, and in a hydrogen–air flame it is \(19 \cdot 10^7\) mol/l; under the same conditions the ratio \((\mathrm{H})_{\text{active}}/(\mathrm{H})_{\text{equil.}}\) is respectively equal to 1600 and 720. The concentration of O atoms and OH radicals is considerably lower [13].

The presence in these flames of a significant concentration of free hydrogen atoms indicates that the phosphor introduced into these flames undergoes candoluminescence due to recombination of hydrogen atoms on its surface. The intensity of candoluminescence should be a function of the concentration of free hydrogen atoms, as is evidenced by the observed difference in the intensity of the phosphor in methane and carbon monoxide flames.

The coincidence of the candoluminescence spectra of phosphors in an illuminating-gas flame and in the flames of its components indicates the common nature of the luminescence observed in these cases.

The experimental results, as well as the data given on the relative concentration of atomic hydrogen in methane and carbon monoxide flames, indicate that in these flames—and consequently also in an illuminating-gas flame—the free hydrogen atoms are mainly responsible for candoluminescence.

REFERENCES

1. A. Gaydon. Spectroscopy and the Theory of Combustion. IIL, 1950.
2. A. Gaydon. Flame Spectroscopy. IIL, 1959.
3. V. A. Sokolov, A. N. Gorban. Izv. AN SSSR, ser. fiz. 25, 424, 1961.
4. V. A. Sokolov, A. N. Gorban. Izv. AN SSSR, ser. fiz. 26, 514, 1962.
5. A. N. Gorban, V. A. Sokolov. Opt. i spektr. 7, 815, 1959.
6. N. N. Semenov. Chain Reactions. ONTI, 1934.
7. O. G. Brik. Izvestiya TPI 138, 265, 1965.
8. V. N. Kondrat’ev. Spectroscopic Study of Chemical Gas Reactions. Publishing House of the Academy of Sciences of the USSR, Moscow, 1944.
9. B. Lewis, G. Elbe. Combustion, Flames and Explosions in Gases. IIL, 1948.
10. V. V. Azatyan, V. V. Voevodskii, A. V. Nalbandyan. Kinetics and Catalysis 2, 340, 1961.
11. E. I. Kondrat’eva, V. N. Kondrat’ev. ZhFKh 21, 769, 1947.
12. C. Fenimore, G. Jones. J. Phys. Chem. 62, 693, 1958.
13. C. Fenimore, G. Jones. J. Phys. Chem. 62, 178, 1958.