PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV

Vol. 162　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1967

SOME FEATURES OF THE CANDOLUMINESCENCE OF CALCIUM OXIDE

M. B. KONDRATENKO

(Presented by the scientific seminar of the Department of Experimental Physics)

Calcium oxide belongs to a series of oxides whose bright glow in hydrogen-containing flames was associated with candoluminescence. However, in the works of V. A. Sokolov and A. N. Gorbanya [1], [2] it was shown that at high temperatures (>600°C) the radiation of CaO in a flame of hydrogen and illuminating gas has an equilibrium character and obeys Kirchhoff’s law, whereas at temperatures below 600°C true candoluminescence is observed on some samples of natural lime. Smith [3] observed the candoluminescence of CaO activated with small amounts of bismuth, manganese, praseodymium, and others.

In the present work the following tasks were set: to determine what causes the luminescence of natural lime; to obtain phosphors by activating calcium oxide with the elements found in natural lime and to investigate the glow of the phosphors obtained; to determine whether radical-recombination processes affect the character of CaO emission in a flame at high temperatures.

Experimental Results

1. Samples of natural lime giving candoluminescence were selected, the spectra of candoluminescence were recorded, and the temperature limits for the existence of luminescence were determined. The spectra were recorded with an ISP-51 spectrograph with a photoelectric attachment, in which FEU-19M and FEU-22 photomultipliers were used.

On these samples a weak photoluminescence with a maximum at 5600 Å was found, observed at temperatures below 250°C, and a bright candoluminescence with a maximum at 5750 Å, observed in the temperature interval from 550 to 600°C. Quantitative spectral analysis revealed the following impurities in the lime: Pb—0.0001%; Mn—0.03%; Si—1%; Ti—0.006%; Cu—0.006%; Fe—0.01%.

As a result of activating calcium oxide with the corresponding amounts of the elements found, it was established that weak photoluminescence below 250°C is produced by CaO activated with copper, while bright candoluminescence at temperatures of 550–600°C is produced by CaO activated with manganese. The temperature dependence of the candoluminescence of the CaO—Mn phosphor is shown in Fig. 1.

The phosphor CaO—Mn at room temperature, under cathode bombardment, gives a bright broad luminescence band with a maximum at 5900 Å. As the temperature is raised, the maximum shifts toward shorter wavelengths. At a temperature of about 600°C the luminescence disappears.

Comparison of the cathodoluminescence spectra at a temperature of 550–600°C with the candoluminescence spectra of the CaO—Mn phosphor and of natural lime containing manganese shows complete coincidence of these spectra (Fig. 2). This indicates the same mechanism of luminescence emission under different methods of exciting the phosphor.

Fig. 1. Dependence of the candoluminescence intensity of the CaO—Mn phosphor on temperature

Fig. 2. Spectra of cathodo- and candoluminescence of the CaO—Mn phosphor and natural lime:
1 — cathodoluminescence of CaO—Mn at 20°C,
2 — cathodoluminescence of CaO—Mn at 570°C,
3 — candoluminescence of CaO—Mn at 570°C,
4 — candoluminescence of lime at 570°C

However, when luminescence is excited in flames, the sum of the recombination energy of radicals on the surface of the phosphor, which is responsible for the luminescence, and the thermal energy of the electrons proves insufficient to excite the emission of CaO—Mn at temperatures below 500°C, since candoluminescence of CaO—Mn is observed only at temperatures from 500 to 600°C. At temperatures above 600°C, temperature quenching of luminescence occurs in this phosphor, and with a further increase in temperature visible thermal radiation of CaO arises, which under flame conditions is distinguished by certain features.

1. To study the features of the glow of calcium oxide at temperatures above 600°C, samples of CaO 1 (Fig. 3) were placed in a quartz tube 4.

Burner 5 was used for heating. Small burner 6 served as the source of the flame. The temperature was determined with a platinum–platinum-rhodium thermocouple 2. The flame was drawn into tube 4 by means of a water-jet pump producing a vacuum of several cm of water column. The temperature was regulated by the size of the flame of burner 5. Spectra were recorded at identical temperatures in an atmosphere both containing radicals and not containing them. Destruction of radicals in the stream entering tube 4 was accomplished by covering the outer end of the tube with a previously heated copper mesh 3, which

is a strong catalyst for the recombination reaction of radicals [4].

In these experiments it was found that, in the temperature interval from 750 to 1300°C, the presence of radicals exerts a strong influence on the distribution of the spectral intensity of the thermal radiation of CaO in the visible region. With increasing wavelength of the visible light emitted by the sample in an atmosphere containing radicals, the intensity of the temperature radiation decreases. The color temperature of the body becomes considerably higher than the true temperature. The radiation of CaO in an atmosphere

Fig. 3. Apparatus for studying the temperature glow of CaO in various atmospheres

Fig. 4. Temperature glow of CaO: 1 — in an atmosphere without radicals; 2 — in an atmosphere containing radicals

of radicals also obeys Kirchhoff’s law, only in this case the emissive power $\varepsilon_{\lambda T}$ decreases with increasing wavelength

waves. The corresponding curves for a temperature of 1100°C are shown in Fig. 4.

A similar effect, although much less pronounced, is observed on magnesium oxide. The thermal radiation of TiO₂, ZnO, and ZnS does not change in the presence of radicals.

1. If not combustion products but a combustible mixture (hydrogen and air) is directed into tube 4, combustion occurs directly on the incandescent CaO sample. In this case the temperature rises so much that the quartz tube covering the thermocouple softens and is deformed (the melting temperature of quartz is >1700°C). This indicates that conditions may be created under which a catalytically active body in the flame can be heated to a temperature higher than the flame temperature. The catalytic activity of CaO and of natural lime is very considerable. Table 1 gives the recombination coefficients of CaO and various lime samples measured by us by the thermoelectric-probe method [5]. As is seen from the table, the recombination coefficients for all samples are substantial and approach unity. This is the main reason not only for the additional heating of lime under flame conditions, but also for the good candoluminescence of lime phosphors.

Table 1

Recombination coefficient of lime and calcium-oxide-based phosphors

Discussion of the Results

On the basis of the concepts of the electronic theory of catalysis, F. F. Volkenshtein, A. I. Gorban’, and V. A. Sokolov developed a radical-recombination mechanism of candoluminescence [6]. According to this mechanism, the process of excitation of candoluminescence consists in the following: an adsorbed atom forms a local acceptor level C in the forbidden band (Fig. 5), to which an electron passes from the valence band (transition 1). As a result of recombination of the adsorbed atom with an atom from the gas phase, or with a second adsorbed atom, the electron receives energy and passes into the conduction band (transition 2). Upon recombination of the electron with an ionized activator atom A (transition 3), a luminescence quantum is emitted.

The processes shown in Fig. 5 are written in the form of equations:

\[ C + L \to CL, \tag{1} \]

\[ \text{transitions 1}\qquad CL \to CeL + pL, \tag{2} \]

\[ \text{transitions 2}\qquad C + CeL \to C_2 + eL, \tag{3} \]

\[ \text{transitions 4}\qquad AL + pL \to ApL, \tag{4} \]

\[ \text{transitions 3}\qquad eL + ApL \to AL + h\nu, \tag{5} \]

where \(C\) is the symbol of the radical participating in the reaction; \(L\) is the symbol of the lattice; \(CL\) is a neutral adsorbed radical; \(CeL\) is an adsorbed radical that has captured an electron; \(eL\) and \(pL\) are a free electron and hole; \(ApL\) is an ionized activator atom.

The fundamental difference between the radical-recombination mechanism of excitation and other mechanisms consists in the fact that only an electron possessing energy sufficient for transition from the valence band to level \(C\) can be thrown into the conduction band, which is entirely unnecessary for other types of excitation.

Excitation of luminescence occurs when the inequality

\[ E_T + E_p \geq E_{0T}. \tag{6} \]

is satisfied.

In our case, at a temperature of 300°K the recombination energy of atomic hydrogen is 4.35 eV [7], and the width of the forbidden band is 7.7 eV [8]. At this temperature there is no appreciable number of electrons with an energy of 3.35 eV, and inequality (6) is not fulfilled.

With increasing temperature, as is known, the width of the forbidden band narrows and the thermal energy of the electrons increases. Proceeding from these considerations and experimental data, it may be assumed that in our case, at temperatures above 550°C, inequality (6) is fulfilled for a considerable number of electrons, since at this temperature an appreciable number of quanta with energy greater than 1.5 eV appears in the spectrum of thermal radiation [9].

Fig. 5. Band scheme of candoluminescence: \(E_{0T}\)—energy of the forbidden band, \(E_T\)—thermal energy of the electron, \(E_p\)—energy of radical recombination.

Above 600°C, temperature quenching of luminescence sets in, i.e., instead of transitions 3, radiationless transitions become more probable.

Adsorption and recombination of radicals can occur directly on activator atoms; in this case, a considerably smaller energy than the width of the forbidden band is required for the electron transition. In our case, excitation of luminescence occurs at a temperature \(>500^\circ\)C, when the sum of the energy of radical recombination and the thermal energy of the electron is close to the width of the forbidden band. Proceeding from this, it may be assumed that, in the case of the phosphor CaO—Mn, appreciable adsorption and recombination of radicals on activator atoms does not occur.

The effect of a decrease in the intensity of the long-wavelength edge of visible light at temperatures between 600 and 1300°C can be qualitatively explained with the aid of this same excitation mechanism. It may be assumed that adsorbed atoms form local levels only in the interval \(2\Delta E\) (Fig. 5). In this case, electrons thermally excited to an energy \(E_T \pm \Delta E\), instead of returning to the initial state with emission of a light quantum, may, through radical-recombination processes 1, 2, be thrown into the conduction band and thence return to the valence band by a radiationless transition 6, which should lead to a decrease in the radiation intensity in the energy interval \(E_T \pm \Delta E\).

To explain the strong increase in temperature in the case of direct combustion of gas on incandescent calcium oxide, let us consider the mechanism of hydrogen combustion [10]. This mechanism includes a considerable number of reactions, among which reactions involving a third partner proceed with a large release of heat (equations 7, 8, 9):

\[ \mathrm{H}+\mathrm{O}_{2}+M \to \mathrm{HO}_{2}+M+45000\ \text{cal/mol}, \tag{7} \]

\[ \mathrm{O}+\mathrm{H}_{2}+M \to \mathrm{H}_{2}\mathrm{O}+M+114000\ \text{cal/mol}, \tag{8} \]

\[ \mathrm{H}+\mathrm{H}+M \to \mathrm{H}_{2}+M+103000\ \text{cal/mol}. \tag{9} \]

In the present case, combustion occurs directly on the surface of calcium oxide, which plays the role of the third partner \(M\) in reactions (7)—(9) and which, as our measurements have shown, is catalytically active with respect to radical-recombination reactions; as a result, the probability of the occurrence of processes (7)—(9), with the release of a large amount of heat, increases substantially.

Analogous studies were carried out by us for the Auer-Welsbach mixture (99% thorium oxide and 1% cerium oxide), known for its remarkable optical properties, but we did not detect any influence of the active atmosphere on the emission spectrum of this mixture. The high catalytic activity of the mixture (recombination coefficient \(\simeq 0.9\)) leads only to a higher temperature of the mixture in the flame.

The emissive power of thin meshes differs greatly from the emissive power of thicker rods made from this mixture at the same temperature. This effect of the dependence of emissive power on the size of the body has found a good explanation through the application of Bouguer’s law for light absorption to the radiation of partially transparent bodies [11].

Conclusions

1. In this work, the cando- and cathodoluminescence of natural lime and calcium oxide activated with manganese have been studied. Owing to the large width of the forbidden band, the phosphor has a high luminescence quenching temperature and is comparatively little sensitive to luminescence-quenching impurities. It has been shown that the radical-recombination mechanism of excitation of candoluminescence, based on concepts from the electronic theory of catalysis on semiconductors, can be applied to explain the candoluminescence of phosphors with wide forbidden bands.

2. It has been established that CaO and natural lime possess high catalytic activity, which accounts for the good candoluminescence of lime luminophores and is also the cause of the peculiarities of the thermal radiation of lime under flame conditions.

3. A previously unknown effect has been discovered in the heating of solid bodies in a high-temperature flame, and an attempt has been made to give a qualitative explanation of this effect from the standpoint of the radical-recombination excitation mechanism.

4. Using lime as an example, it has been shown in this work that, under special conditions in a flame, a catalytically active substance can be heated to a temperature noticeably exceeding the flame temperature. Thus, the nature of “Drummond light,” i.e., the bright glow arising when lime cylinders are heated in an oxyhydrogen flame, is connected not only with the selectivity of the thermal radiation of CaO, but also with the catalytic activity of lime toward radical-recombination reactions under flame conditions.

References

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