Philips tech. Rev. 38, 83-88, 1978/79, No. 3 83

Amalgams for fluorescent lamps

J. Bloem, A. Bouwknegt and G. A. Wesselink

The luminous flux of a fluorescent lamp (low-pressure mercury type) depends to aconsiderable extent on the mercury-vapour pressure present in the tube. The pressureis determined by the temperature of the coolest part of the tube, which is usually thewall. The maximum luminous flux is reached when the wall temperature is about 40 oe,which for many fluorescent lamps corresponds to an ambient temperature of 25 oe.The wall temperature of lamps in closed luminaires or of speciallamps with high powerdensities can be very much higher. In such conditions a high luminousflux can still beobtained by using a suitable amalgam in place of the pure mercury. This has the effectof lowering the mercury pressure and also of keeping it more or less stable in a broadtemperature range. A difficulty with amalgam lamps is that they are generally relativelyslow in reaching their maximum luminous flux after ignition. Some amalgams have nowbeen found that can be used for making lamps that not only give a relatively high lightoutput at a high wall temperature but also reach their maximum luminous flux quickly.

Introduetion

A fluorescent lamp of the low-pressure mercury typeis a discharge lamp containing a mixture of inert gasesand a little mercury vapour. The discharge excites themercury atoms to a higher energy state, from whichthey return to the ground state, producing ultravioletradiation. This radiation is converted into visible lightby fluorescent powder applied to the tube wall. Themercury-vapour pressure (PHg) is an important param-eter in a fluorescent lamp. It is usually determined bythe temperature of the wall, the coolest part of thelamp. Ifthe pressure is too low, too few mercury atomsare excited, which means that not enough ultravioletradiation falls on the fluorescent powder. If the pres-sure is too high, the mercury atoms absorb much oftheir own radiation. This causes an increase in thenumber of mercury atoms in excited states, resulting ina greater probability of interactions involving non-radiative transfer of some of the excitation energy. Theoptimum mercury-vapour pressure is approximately0.8 Pa (6 x 10-3 torr) [1], a value reached when the walltemperature of the lamp is about 40°C. In most typesof fluorescent tubes in open luminaires this is the caseat an ambient temperature of about 25°C.The wall temperature of lamps in closed luminaires

or of lamps with higher power densities [2] can be sub-stantially higher than 40°C. To prevent the mercury-vapour pressure from becoming too high in such

Prof. Dr J. Bloem and Drs G. A. Wesselink are with PhilipsResearch Laboratories, Eindhoven; Dr A. Bouwknegt is with thePhilips Lighting Division, at Roosendaal.

lamps, the mercury can be introduced in the form of anamalgam. Apart from lowering the vapour pressure,the substitution of an amalgam for pure mercury hasanother useful effect. In the temperature range in whichthe solid and liquid phases of the amalgam coexist withthe gas phase, the mercury-vapour pressure above theamalgam is much less temperature-dependent than thatabove pure mercury. Fig. 1 shows the mercury-vapourpressure as a function of temperature for In-Hg, theamalgam that was first used in fluorescent lamps [3]. Inthe temperature range in which stabilization occurs themercury-vapour pressure above the amalgam does notdiffer much from the optimum value. Consequently theluminous flux does not differ greatly from the maxi-mum value over a relatively wide temperature range.Lowering the mercury-vapour pressure - which is

necessary at wall, temperatures higher than 40°C -adversely affects the starting of the lamp, the amalgamstill being at room temperature. When the vapour pres-sure above the amalgam is much lower than that abovepure mercury, lamp ignition is difficult and a relativelylong time elapses before the maximum luminous fluxis reached. We have therefore tried to find amalgams'which, while appreciably lowering and stabilizing themercury-vapour pressure during operation, do not havean unduly low vapour pressure at room temperature.

[1] J. F. Waymouth, Electric discharge lamps, M.I.T. Press,Cambridge Mass. 1971, p. 23.

[2] J. Hasker, J. IlIum. Engng. Soc. 6, 29, 1976.[3] C. J. Bernier and J. C. Heffernan, IlIum. Engng. 59, 801,

1964. '

84 J. BLOEM et al. Philips tech. Rev. 38, No. 3

Alloys of mercury with more than one other metal wereincluded in our investigations. The best results wereobtained with Bio.53Ino.47-6% Hg and, to a somewhatlesser extent, with Bio.47Pbo.29Sno.24-6% Hg.

We shall now first briefly discuss the temperaturedependence of the mercury-vapour pressure aboveamalgams. We shall then examine the reasons forchoosing the new amalgams, and finally deal with thebehaviour of these amalgams in experimentallamps.

Mercury-vapour pressure above amalgams

The solid and liquid phases of an alloy of a metalwith a little mercury coexist in a particular temperaturerange. This coexistence region lies below the meltingpoint of the pure metal. The liquid phase is in equilib-rium with both the solid and the gas phase. When thepartial vapour pressure of the metal is negligibly low,the gas phase above the amalgam consists almost en-tirely of mercury vapour. The vapour pressure of themercury depends on the mercury content in the liquid(Xl) and on that in the solid (xs). On an increase oftemperature within the coexistence region both Xs andXl become smaller because less mercury is dissolved inthe solid and in the liquid; seefig. 2. On the other hand,the same temperature rise causes an increase in theratio between the number of mercury atoms in the gasphase and the value of xi. Thus, with rising temperaturein the coexistence region the change in the mercuryvapour is determined by two opposing effects. Theresult is that the mercury-vapour pressure in this regionchanges relatively little.

The behaviour ofthe mercury-vapour pressure abovean amalgam as a function of temperature is shownschematically infig. 3. The stabilization begins at Tl,the temperature at which the first droplet of amalgammelt is formed, and ceases at T2, the lowest temperatureat which the amalgam is completely melted. In theexample given in fig. 3 the vapour pressure between Tland T2 goes through a maximum. It follows from ananalysis that for such a maximum to occur the mercurycontent of the amalgam must be about 5% or lower.The maximum then lies at about 20 oe below themelting point of the metal without mercury [41.

-New amalgams

Since indium has the relatively high melting point of156 oe, the stabilization above In-Hg occurs at suchhigh temperatures that the mercury-vapour pressure inthe stabilization range is on the high side (fig. I). Amore serious problem with In-Hg is the relatively highsolubility of mercury in solid indium. This has the effectof narrowing the stabilized region and of giving the

WPa.---------------------------------------------~/II//I

HglII/'IIIIII

1 100%

90%

I In-Hg I

10~0~~--~~--~--~~--~~~40 80 120 160°C

-TFig. 1. Mercury-vapour pressure PHg, as a function of tempera-ture T, above the amalgams In-6 % Hg and In-12 % Hg and abovepure mercury (dashed line). The mercury-vapour pressure atwhich the relative luminous flux of a fluorescent lamp is at amaximum is shown at the right, and also the pressure at whichit reaches 90 % of the maximum value.

Fig. 2. Schematic T-x diagram for an amalgam; X mercury con-tent, T temperature, Tm melting point of the metal withoutmercury. The solidus S indicates the temperature at which theamalgam begins to melt, and the liquidus L indicates the lowesttemperature at which the amalgam is completely melted. Anamalgam with a mercury content Xl begins to melt at Tl andbecomes completely molten at T2. Between these two temperaturesthe solid and liquid phases coexist. For an arbitrary temperatureT", in this coexistence region the mercury content in the solid(xs) and that in the liquid (Xl) can be found by drawing ahorizontalline and finding the points at which they interseet thecurves Sand L. With rising temperature the liquid-to-solid ratioincreases, while Xs and Xl both decrease.

mercury-vapour pressure at room temperature an un-desirably low value.An improvement over In-Hg is obtained by substitut-

ing bismuth for part of the indium [41.This has the

PhiIips tech. Rev. 38, No. 3 AMALGAMS FOR FLUORESCENT LAMPS 85

Sol sa-u«

Fig. 3. Schematic curve of the mercury-vapour pressure abovean amalgam as a function of temperature. Stabilization of thevapour pressure occurs between the temperatures Tl and T2, thecoexistence region of the solid (Sol) and the liquid phase (Liq)ofthe amalgam. In the situation drawn here, the mercury-vapourpressure in the coexistence region reaches a maximum. The mer-cury-vapour pressure mayalso continue to increase with risingtemperature in the coexistence region.

10Pa/ 12~!«/

I 90%IPHg H91

t 1 I 3~V/ 100%/ 0.75%//

90%II

10-1 III

Fig. 4. The mercury-vapour .pressure above Bio.5sIno.47-Hg forfive different mercury contents as a function of temperature.For the amalgam with 6% mercury there is a relatively widetemperature range within which the vapour pressure differs onlylittle from the optimum value (0.8 Pa). The vapour pressureabove this amalgam at room temperature lies much closer tothat above pure mercury than in the case of In-Hg (fig. I), whichimproves the starting of the lamp.

effect of reducing the melting temperature and oflowering the solubility in the solid. An alloy with thecomposition Bio.53Ino.47is a eutectic system of BiIncontaining a small percentage offree Bi, with a meltingpoint of 110 oe. The solubility of mercury in solidBio.53Ino.47is very low, because Bi and Hg atoms repel

each other very strongly. Fig. 4 shows the temperaturedependence of the mercury-vapour pressure aboveBio.53Ino.47-Hgfor five values of the mercury contentbetween 0.75 % and 12%. From about 20 oe to about80 oe the mercury-vapour pressure is independent ofthe mercury content. It is evident that the coexistenceregion of the solid and liquid phases of these amalgamsis already reached at room temperature. Just above80 oe the amalgam with the highest mercury content(12 %) is completely melted, as a result of which thepressure-temperature curve differs from the coexistencecurve, and the mercury-vapour pressure subsequentlyshows a marked increase with rising temperature. Atabout 100 oe the same occurs for the amalgam with6% Hg, and also, at somewhat higher temperatures,for amalgams with the lower mercury contents. We seethat for the amalgam with a mercury content of 6%there is a wide temperature range around 80 oe inwhich the mercury-vapour pressure differs very littlefrom the optimum value. In addition, the mercury-vapour pressure with this amalgam is reasonably highat room temperature, being in fact about a factor ofeight higher than that for In-12 % Hg.To determine whether there might perhaps be

another candidate for use in fluorescent lamps, wetheoretically compared the possibilities of a largenumber of amalgams [41.These contained one or moreof the metals Pb, Sn, Bi, In, Cd, Ga and Tl, most ofwhich have a not unduly high melting point and a lowvapour pressure. The criteria we used for the com-parisons were the melting point of the metal, the inter-metallic compound or the eutectic alloy without mer- ,cury, and the affinity between mercury and its possiblealloy partner or partners [51.These criteria made it pos-sible to decide whether the stabilization of the mercurypressure would occur at suitable temperatures andwhether a reasonably high mercury-vapour pressurewould be expected at room temperature.It was found that no improvement was to be

expected from the amalgams with one or two othermetals as compared with In-Hg and Bio.53Ino.47-Hg.Among the amalgams with three or four other metalsthe system Bi-Pb-Sn-Hg seemed to be the mostpromising. A eutectic mixture with the compositionBio.47Pbo.29Sno.24melts at a temperature as low as96 oe ,which means that the mercury-vapour pressureabove such an amalgam will stabilize at relatively low

[4] J. Bloem, A. Bouwknegt and G. A. Wesselink, J. IlIum. Engng.Soc. 6, 141, 1977.

[5] The data were obtained from R. Hultgren, P. D. Desai, D. T.Hawkins, M. Gleiser and K. K. Kelley, Selected values of thethermodynamic properties of binary alloys, American Societyfor Metals, Metals Park, Ohio, 1973; A. R. Miedema, R. Boomand F. R. de Boer, J. less-common Met. 41, 283, 1975;R. Boom, F. R. de Boer and A. R. Miedema, J. less-commonMet. 45, 237, 1976 and 46, 271, 1976.

86 J. BLOEM et al. Philips tech. Rev. 38, No. 3

temperatures. In addition, an amalgam with this alloymay be expected to have a higher mercury-vapourpressure at room temperature than Bio.53In0.47-Hg,because the attractive force of tin and lead on mercuryis weaker than that of indium on mercury.Experiments confirmed these expectations; seefig. 5.

In the case of the amalgam with 6% Hg the vapourpressure differs little from the optimum value over awide temperature range around 60°C. The mercury-vapour pressure above this amalgam at room tem-perature is only a factor of 1.7 lower than that abovepure mercury.

We shall now explain how the amalgams Bio.53Ino.47-Hg and Bio.47Pbo.29Sno.24-Hgbehave in an operatinglamp.

Experimentallamps with the new amalgams

The relative luminous flux @rel of a fluorescent lampat a particular amalgam temperature depends on thevalue of PHg at this temperature and on the relationbetween @rel and PHg. The temperature Topt at which@rel is at a maximum is higher for an amalgam than forpure mercury, the more so the lower the value of PHg

above the amalgam. The stabilization of PHg causes abroadening ofthe temperature range within which @rel

differs little from the maximum value. As an arbitrarymeasure of this broadening we have taken ~T90, thetemperature range within which @rel is more than 90 %of the maximum value. In Table !the values of Topt and~T90 for the new amalgams are compared with thosefor In-Hg and pure mercury. The Table also gives thevalues of the mercury-vapour pressure at room tem-perature, which are important for ignition of the lamp.

The relative luminous flux at a given ambient tem-perature Tamb is determined not only by the relationbetween PHg and the amalgam temperature and thatbetween @rel and PHg, but also by the relation betweenthe amalgam temperature and Tamb. Fig. 6 shows @rel

as a function of Tamb for lamps with Bio.53Ino.47-6%Hg,Bio.47Pbo.29Sno.24-6%Hg, In-12 %Hg and pure mer-cury. Comparing the curves of the amalgams withthat of pure mercury we note in particular thebroadening obtained with Bio.53Ino.47-6%Hg. Thebroadening obtained with Bio.47Pbo.29Sno.24-6%Hgis about the same as that obtained with In-12 %Hg.

To ensure prompt ignition ofthe amalgam lamp theamalgam should be situated near an electrode, where asuitable working temperature can quickly be reached.Normally the amalgam is applied at two or three places,indicated by A, Band C in fig. 7. The main amalgamis situated at position A behind the electrode, where thetemperature rises relatively slowly after the lamp isswitched on. A small amount of subsidiary amalgam is

1oPa....--------:,----------,

12~!//6%/ )3%

1.5%

90%

100%

90%

10-30~~~40~~-8~0~~~~~O~~~$~OOC-T

Fig.5. Mercury-vapour pressure above Bio.47Pbo.2oSno.24-Hgforfour different mercury contents as a function of température.Here again the amalgam with 6% mercury gives considerablestabilization of the vapour pressure near the optimum value.It can also be seen that the vapour pressure above this amalgamat room temperature is not much lower than that above puremercury.

Table J. Temperature Topt. at which the relative luminous fluxof the lamp is at a maximum, the temperature range dT90 withinwhich the relative luminous flux is more than 90 % of the maxi-mum value, and the mercury-vapour pressure PHg20 at roomtemperature for three types of amalgam and for pure mercury.

System Topt eC) dT90 eC) PHg20 (Pa)

Bio.53Ino.47-6% Hg 81 52-120 0.03Bio.53Ino.47-12% Hg 81 52-102 0.03Bio.47Pbo.29Sno.24-6% Hg 64 38-100 0.09Bio.47Pbo.29Sno.24-12%Hg 64 38- 85 0.09In-6% Hg 103 83-150 0.002In-12% Hg 95 72-130 0.004Hg 40 27- 58 0.16

BiO.47Pb0.29SnO.24 - 6%Hg

80

o 40 80-Tomb

6020

Fig. 6. The relative luminous flux tVr.! as a function of ambienttemperature T..mb for fluorescent lamps with Bio.53Ino.47-6% Hg,Bio.47Pbo.29Sno.24-6%Hg and In-12 % Hg and with pure mercury.The broadest curve is obtained with Bio.53Ino.47-6% Hg,

Philips tech. Rev. 38, No. 3 AMALGAMS FOR FLUORESCENT LAMPS 87

Fig. 7. Suitable locations for amalgams in a fluorescent lamp.At position A behind' the electrode E there is a relatively largequantity of amalgam (the 'main amalgam'), which determinesthe mercury-vapour pressure in a lamp operating in the steadystate. To obtain an acceptable mercury-vapour pressure soonafter the lamp is switched on, a small amount of subsidiaryamalgam is applied at position B to a metal ring R close to theelectrode; the amalgam generally used for this purpose is In-Hg.If necessary, some subsidiary amalgam mayalso be applied atposition C, on the glass between A and B. Shortly after the lampis switched on, the subsidiary amalgam reaches such a hightemperature that it is virtually depleted of mercury. After about6 to 10 minutes the mercury-vapour pressure in the lamp is fullycontrolled by the main amalgam. After the lamp has beenswitched off, the mercury redistributes itself among A, Band C.The time needed for complete redistribution is shorter ifthe mainamalgam has a high mercury-vapour pressure at room tempera-ture. M capsule for dosing the mercury.

(/Jrel90

t 80

{Bi0.53InO.47-6%HgBiO.47Pbo.29SnQ24- 6%Hg

500 2 3 4 5 6min

Q -t

100%",,/

90 '" <, _",/«; '/ ....._---'IBiO.47PbQ29Sn0.24-6%H9.,</

t 80 ~ /' In-12%Hg1'f--... BiQS3Ino.47-6%»>

./70 /

//

/

2 3 4 5 6min-t

Fig. 8. Relative luminous flux !lirel as a function of time t afterthe ignition of amalgam lamps with previous off-periods of 15hours (a) and 1 hour (b). A lamp with Bio.53Ino.47-6% Hg orBio.47Pbo.29Sno.24-6% Hg needs less time to reach an acceptableluminous flux than a lamp with In-12 % Hg, The difference isgreater the shorter the off-period of the lamps.

applied at position B to a metal ring close to the elec-trode, where the temperature rises very quickly. Ifnecessary an additional quantity of a subsidiary amal-gam may be introduced at position C on the glass be-tween A and B. The temperature of C lies in betweenthat of A and that of B. The subsidiary amalgam gener-ally used is In-Hg.

In practice the metal or the alloy is first applied without themercury. The appropriate amount of mercury is added usuallyby means of a capsule fixed to the metal ring close to the elec-trode; a thin electrically conducting wire is stretched over thecapsule. R.F. heating of this wire breaks the capsule open, thusreleasing the mercury to form the desired amalgam.

Immediately after the lamp is switched 'on the mer-cury in the discharge tube originates mainly from thesubsidiary amalgam. After 6 to 10 minutes the mainamalgam has warmed up and begins to control themercury-vapour pressure. Owing to the high tempera-ture, the subsidiary amalgam then has hardly any mer-cury left. When the lamp is switched off after sometime, the mercury is gradually redistributed between thetwo amalgams, and it is not until some considerabletime later that the original mercury distribution is re-stored. If the lamp is switched on again before thatpoint has been reached, the subsidiary amalgam willcontain too little mercury. Until the main amalgam hasagain warmed up sufficiently, not enough mercury willhave been released in the lamp to produce an accept-able luminous flux. The redistribution of the mercurybetween the two amalgams takes place faster if the mainamalgam has a high mercury-vapour pressure at roomtemperature. In this respect the new amalgams give aconsiderable improvement compared with In-Hg. Theimprovement is particularly noticeable when the lampis switched on again after having been switched off fora short time. As an example, fig. 8 shows the relativeluminous flux of our experimentallamps as a functionof time after ignition, for different switched-off inter-vals of 15 hours Ca) and 1 hour Cb). In the first case themaximum luminous flux is reached fairly quicklyfor Bio.53Ino.47-6% Hg and Bio.47Pbo.29Sno.24-6% Hg.In the case of In-12 % Hg a slight drop occurs afterabout one minute. This is an indication that the sub-sidiary amalgam has soon lost its reserve of mercuryand that the main amalgam has not yet warmed upenough to deliver a sufficient supply. The difference iseven greater when the off-period is not 15 hours butone hour. With the new amalgams an appreciableluminous flux is still reached quite soon after ignition,but this is no longer the case with In-12 % Hg.

88 AMALGAMS FOR FLUORESCENT LAMPS PhiIips tech. Rev. 38, No. 3

We may therefore conclude from the results obtainedwith the new amalgams that there are promising ap-plications in fluorescent lamps. An additional advant-age is that they use less mercury and also contain lessof the expensive metal indium.

Summary. The luminous flux of a fluorescent lamp is at a maxi-mum when the mercury-vapour pressure is 0.8 Pa. This value isreached when the temperature of the tube wall is about 40 °C,which is the case with many fluorescent lamps at an ambienttemperature of 25 °C. When an amalgam is used in the lampinstead of pure mercury the mercury-vapour pressure is lowered

and it also shows little variation within a fairly wide temperaturerange. As a consequence the temperature range within which thevapour pressure exhibits an acceptably small deviation from theoptimum value can be shifted to higher temperatures and alsowidened. This makes amalgams suitable for use in specialfluorescent lamps of the low-pressure mercury-vapour type inwhich the wall temperature is appreciably higher than 40 °C.The known mercury alloy In-12 % Hg has the drawback that themercury-vapour pressure above the amalgam at room temperatureis much lower than that above pure mercury. As a consequencethe maximum luminous flux is reached only very slowly afterthe lamp is switched on. The new amalgams Bio.53Ino.47-6%Hgand Bio.47Pbo.2DSno.24-6% Hg give a substantially higher andstabilized luminous flux at higher ambient temperatures, with-out the mercury-vapour pressure being too low at room tem-perature.