Transcript
1968, No. 1
17
Low-pressure sodium lamps with indium oxide filter The further development of sodium lamps in recent years has led to a considerable increase in luminous efficiency. The higher efficiency has mainly been attained by improving the heat insulation of the lamps by means of heat reflecting filters. Most of the electrical energy which is supplied to a sodium lamp is wasted as undesired heat radiation from the discharge tube. These energy losses can be reduced considerably by means of a filter which has a high transmission for sodium light and a high reflectivity for the heat radiation from the discharge tube. The filter consists of a coating on the inside of the glass envelope which encloses the discharge tube. Suitable filters have been developed in which heavilydoped semiconducting tin oxide is used. These films have been applied in commercial sodium lamps for some years, giving luminous efficiencies up to 150 ImjW [1][2][3].The transmission of glass coated with such a tin oxide film is 87-89 % for sodium light (À. = 0.59 fLm)and this figure is only slightly less than that for uncoated glass (92 %). As the heat radiation of the sodium lamp is emitted almost completely in the infra-red region at wavelengths > 3 fLm the filter should also reflect strongly at À. > 3 urn. This condition is in fact fulfilled by the tin oxide films which attain a saturation reflection of 80 % at À. ~ 8 urn, The high infra-red reflection of the tin oxide filmsis related to their electrical properties. As can be shown from the dispersion theory of free charge carriers the concentration of free carriers must be higher than 3 X 1020jcm3 to obtain high infra-red reflection for wavelengths> 3 fLm.With the heavily-doped tin oxide films carrier concentrations of 6x 1020jcm3 can be achieved. An additional requirement for high infra-red reflection, which is also met by the tin oxide film, is a large value for the product ofthe effective electron mass and the mobility of the free carriers. The reflection increases as this product attains higher values (see reference [2],page 108). Semi-conducting indium .oxide films have recently been investigated [4]and these showed promise of even better results for sodium lamps than those obtained with tin oxide. Indium oxide films can be prepared in a similar way to tin oxide films. A mixture of InCh with an organic solvent such as butyl acetate is sprayed from an atomizer on to the hot glass. If the temperature is high enough, InCh is converted to In203 which forms a thin layer on the glass. Indium oxide films prepared
by the spray technique with no additional doping agent have carrier concentrations of only about 1019jcm3which are of course not sufficient to provide the desired high infra-red reflection. The carrier concentration can be increased considerably, however, by doping with tin, and for this purpose SnCl4 is added to the spray mixture. Fig. 1 shows the effect of different tin doping on the resistance per square of the indium oxide films. These films were prepared on polished borosilicate glass plates. The resistance per square RO is reduced from 170 Q to very low values of 7 to 8 Q if the spray mixture is doped with 2-3 atomic percent of tin. The flat minimum in the resistance curve for tin doping of about 2.3 atomic percent corresponds to a concentration of free charge carriers N = 5 X 1020jcm3
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and a mobility for the free carriers f..l = 50 cm2jVs. The concentration achieved in indium oxide can be seen to be nearly the same as for tin oxide (N = 6 X 1020jcm3)but the mobility is much higher than for tin oxide (f..l = 20 cm2fVs). These results led us to expect that the infra-red reflection of these indium oxide films would be higher than that of tin oxide provided that the effective mass of the free electrons, which has not yet been determined, was not much lower than it is in tin oxide. This expectation was confirmed by optical measurements. Fig. 2 gives a graph of the spectral transmission and reflection of such an indium oxide film. (The corre-
[1)
[2) [3) [4)
M. H. A. van de Weijer, Recent improvements in sodium lamps, Philips tech. Rev. 23, 246-257, 1961/62. R. Groth and E. Kauer, Thermal insulation ofsodium lamps, Philips tech. Rev. 26, 105-111, 1965. H. J. J. van Boort, Electrotechniek 43, 509, 1965. R. Groth, Phys. Stat. sol. 14, 69, 1966.
PHILIPS TECHNICAL
18 NoD
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VOLUME29
REVIEW
of the solution in the spray deduster the indium metal can be recovered by normal chemical processes. In commercial low-pressure sodium lamps of the. SOX-type the sodium discharge is generated in a Utube. This discharge tube is sealed into a cylindrical outer vacuum bulb. Coating this bulb with a tin oxide layer on the inner wall gave a luminous efficiency of 150 lm/W at an input power of200 W. When an In203 coating of adequate thickness is applied to this type of lamp instead of a Sn02 coating a luminous efficiency of 175 lm/W is obtained at an input power of 180 W. In Table I the efficiency of several SOX lamps which
8 10flm Fig. 2. Solid curves: spectral transmission T and reflection R of an indium oxide film. Spraying temperature 500°C; film thickness 0.31 (.Lm; concentration of free charge carriers 5 X 1020 cm=ê ; mobility of the free carriers 50 cm2/Vs. Dashed curves: the same quantities for a tin oxide film. Spraying temperature 460°C; film thickness 0.32 (.Lm; concentration of free charge carriers 6 X 1020 cm="; mobility of the free carriers 20 cm2/Vs.
sponding curves for the tin oxide film mentioned previously are given for comparison.) As the filters hardly absorb at all in the visible region, the transmission in this region is only modified by the interference. The film thickness ofO.31 (.Lmfor the indium oxide film has been chosen to obtain a maximum of transmission for the wavelength of the sodium D lines. For this wavelength the transmission ofthe glass coated with the film is 91 %, which very nearly corresponds to the transmission of uncoated glass (92 %). The infra-red reflection of the filter is higher than for tin oxide and reaches 90%. Films prepared on the inside of soft glass tubes showed the same filter characteristics as films prepared on polished borosilicate glass plates. A consequence of the spraying technique used is that only 10-15% of the indium metal in the spraying solution is actually used for the formation of the indium oxide film on the glass envelope. Since indium is more expensive than tin, it is necessary to reduce this loss of indium. It has been found that much of the vapour which contains the excess indium and leaves the spraying oven on the opposite side to the spraying gun can be dissolved in a spray deduster. After concentration
. Table I. Lamps from the SOX series. SOX with Sn02 coatin~ . 100 W 150 200
125 Im/W 137 150
~·I·«
SOX with In203 coating 90 W 135 180
1441m/W 157 175
have 1n203 coatings is shown alongside results for comparable lamps with Sn02 coatings. In general, the In203 coating gives an improvement in luminous efficiency of about 15 % when compared with a Sn02 coating, the dimensions, current and lumen output of the lamp remaining unchanged. This improvement considerably enhances the superiority in luminous efficiency of low-pressure sodium lamps over that of other gas discharge lamps. It is a well-known fact that sodium lamp installations are usually installed because of the need to keep down the cost of electricity consumption. The further reduction of power consumption by 15 % due to the improvement in the thermal insulation will therefore be an important factor in favour of the application of this light source.
H. J. J. van Boort R. Groth Drs. H. J. J. van Boort is with the Development Laboratory at Turn/zout (Belgium) of the Philips Lighting Division; Dr. R. Grotli is witb the Aachen laboratory of Philips Zentrollaboratorium GmbH.
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