US2951767A

US2951767A - Phosphor and method

Info

Publication number: US2951767A
Application number: US770522A
Authority: US
Inventors: Norman C Beese
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1958-10-29
Filing date: 1958-10-29
Publication date: 1960-09-06
Anticipated expiration: 1977-09-06

Description

SePt- 6 1960 N. c. BEEIsE 2,951,767

PHOSPHOR AND lldEI'l-{OD Filed Oct. 29, 1958 FIG. l.

Io 22 Is I4 26 f |l` l I/ T' 24 l h: --t x I I Is Is 2o 2s :so I2 2o Ia Is FIG. 2

E n 1 In l L9 0- I I; E I I PERFORMANCE AFTER 3: 'I 25oo'HouRs OPERATION l IMPROVEMENT ovER 5 |15 I i CONTROL LAMPS n- 2 I O l m o z I Lz E I L- 3 o I T 0.05 OJO OAS O2 PER CENT BARIUM coMPouND (EXPRESSED As EouIvALENT eARIuM oxIDEJ ,u I zeno E D 5 50c gg 1 z 240C D O I- I 9 230e HouRs `oF I AMP OPERATION INVENTOR NORMAN C. BEESE.

. to lamp.

PHSPHR AND WTHD Norman C. Reese, Verona, NJ., assigner to Westinghouse Electric CorporatiomEast Pittsburgh, Pa., a corporation of Pennsylvania Filed Uct. 29, 1958, Ser. No. 770,522

12 Claims. (Cl. 'M7-33.5)

This invention relates to fluorescentlamps and, more particularly, to phosphor material for fluorescent lamps and to methods for improving the fluorescent response of. both phosphor material and fluorescent lamps.

The maintenance of the light output of fluorescent lamps (called lumen maintenance) is an important factor in measuringthe performance of such lamps. In `actual practice, the lumen output of a fluorescent -lamp is measured at G-hours operation, since the outputduring the first 100 hours is apt to vary somewhat from lamp In the usual fluorescent lamp, the light output decreases something in the order of 4% to 5% during the first 100 hours of operation and after 500 hours of operation, the light output of an average lamp will have dropped approximately 7% to 8% from the zero-hour output. Thereafter the light output drops off more slowly.

It is the general object of this invention to avoid and overcome the foregoing and other difficulties of and objections to prior-art practices by the provision of a method for improving the `lumen maintenance of fluorescent lamps.

It is another object to provide a method for stabilizing the lumen output of finely-divided phosphor material for use in iiuorescent lamps.

It is a further object to provide phosphor material for use in fluorescent lamps which phosphor, when incorporated into such lamps,-wi1l display improved lumen output. i

It is an additional object to provide permissible and preferred ranges for speciiic phosphor additive material which will improve the performance of fluorescent lamps.

The aforesaid objects of the invention, and other objects which will become apparent as the description proceeds, are achieved by depositing onto finely-divided phosphor material selected amounts of selected barium compound. There is also provided the improved phosphor material having selected barium compound added thereto as well as a method for processing a fluorescent lamp incorporating such phosphor material.

For a better understanding of the invention reference `should be had to the accompanying drawing wherein:

Fig. 1 is an elevational vieW,partly in section, of a fluorescent lamp having an envelope coated with phosphor in accordance with the present invention;

Fig. 2 is a` graph oflamp performance wherein lumens per watt gain is plotted versus percent of barium compound (expressed as equivalent barium oxide) Vdeposited .Qu the phosphor of the lamp;

-Fig. 3 is a graph of lamp light'output versus hours of lamp operation, further illustrating -the improved performance which is obtained through the use of the present invention.

With specific` reference Ato the form of the` invention illustratedin the drawing, the numeral A10 in` Fig. lA illusf 2,951,767 E E Patented Sept.- d, i-w50 the end of the envelope 12 with lead conductors 18 sealed therethrough and supporting at their inwardlyextending extremities refractory metal coils20, which are fabricated of tungsten for example. lfhese coils are normally of a coiled-coil construction or of a triple-coil construction, such constructions being well known, and contained Within the turns of the inner coil or coils is a filling of electron-emitting material 22. Such electronemitting materials are well known and normally comprise a mixture of alkaline-earth oxides which may have other materials such as zirconia added thereto. As a specific example, the electron-emitting material comprises a mixture of 60% by weight barium oxide, 30% by weight strontium oxide and 10% by weight calcium oxide and the electron-emitting material may have an additive such as 0.9% by weight of zirconia if desired. This specific electron-emitting material is `given only by way of example and many other mixtures of alkalineearth materials or even single alkaline-earth materials can be used if desired, as is Well known. Electrical connection for the lead conductors 'lis normally eiectedby contact pins 24 which project from the supporting base caps 26 at either end of the lamp envelopell The envelope l2 has coated on its interior surface a phosphor material Z8, which previously has had selected amounts of selected barium compound substantially evenlydepositedthereon, as will be explained inldetailhereinafter. The envelope also contains a small filling of argon or other inert, ionizable i gas at a pressure of 4 mm., for example, in 4order to facilitate starting, a1- though other starting gas ll pressures can be used,:as is Well known. 'Also contained within the envelope is` a small charge of mercury 30, as iscustomary. In the operation of such a lamp', the phosphor responds to the 2537 A.U. resonant radiation 'of the mercury discharge `to produce longer Wavelength radiations, 'as is Well 'known The phosphor material can comprise any `ofthe usual Vphosphors asused with a liuores'cent 1amp,the mostcomrnonly used being categorized as a halophosphate phosphor. Halophosphate Vphosphor materials are `de scribed in US. Patent No. 2,488,733, dated' November 22, 1949, are generally analogous to the natural mineral apatite and Will display substantially the: same X-ray diffraction pattern as this mineral. These'halophosphate phosphor materials can be represented by the matrix 3M3 (1)0.,)2- 1M'L2 where L represents a halogen or mixture of halogens and M and M represent either dderent or Videntical bivalent metals or m'nrtures of such metals. In practice, the primary constituent for most halophosphate phosphors is calcium orthophosphate although strontium orthophosphate is used in some limited cases. The halide constituent normally comprises calcium chloride or fluoride or strontium chloride or fluoride or mixtures of both and the activator materials are normally antimony or antimony plus manganese.

The details for preparing halophosphate phosphors are generally well known andthe raw-mix constituents cornprising the phosphor are normally thoroughlymixed'or blended and then tired, preferably incovered crucibles, at temperatures Which can vary from about 1100 C. to about 1260i" C. for example, with the maximum firing temperature being determined by the hardness of the resulting fired batch. As 'a first specific example,` 583 grams of calcium oxide are admixed with 538 grams of phosphorus pentoxide, 10.17 grams manganous carbonate, 31 grams antimony trioxide, 48 grans strontium `chloride -and 77.3 grams calcium tiuoride. 'This admixture `is red at a temperature of about 1185 C. for a period Vide, 21.5 grams manganous carbonate, 31 grams antimony trioxide, 48 grams strontium chloride and- 77.3 grams calcium fluoride are admixed and fired at a temperature of about 1190 C. for about two and one-half hours to produce what is known as a warm-white halophosphate phosphor. As a fourth example, 569 grams calcium oxide, 538 grams phosphorus pentoxide, 16.25 grams manganous carbonate, 31 grams antimony trioxide, 48 strontium chloride and 77.3 grams calcium fluoride are admixed and ired at about ll30 C. for about three hours to produce what is known in the art as a 3500 K.

halophosphate. To complete the processing for the foregoing examples, the phosphors are all lightly ground after nal tiring to reduce them to a iinely-divided status suitable for coating. The state of division Vof the phosphor is not critical and as an example, the finely-divided phosphor has an average particle diameter of 7 microns.

Any of these specific examples or any other halophosphate-type phosphor can be rst prepared and then processed `in accordance with thepresent invention, as explained in detail hereinafter. Phosphors other than the so-called halophosphate phosphors such as zinc silicate activated by manganese can also be irst prepared and then processed later as per thepresent invention.

InV accordance with the present invention, selected amounts of selected barium compound are substantially evenly deposited on the nely-dividedphosphor material after it is completely processed, desirably before the phosphor is coated onto the lamp envelope 12. These barium compounds desirably are quite soluble in liquid medium Vin order to facilitate their being deposited on the finelydivided phosphor lin a substantially even fashion. The barium compounds found to be suitable are barium chloride, barium acetate, barium azide, barium hydroxide and barium trimethyl acct-ate. The latter four of these specitied barium compounds are all vreadily convertible to barium carbonate when heated toca temperature of from 300 C. to 675 C. in the presence of carbon dioxide and moisture. It has been found that the permissible amounts of the specified barium compound which can be added, with the barium content therein expressed as equi-valent barium oxide, constitutes from 0.005% to 0.14% by weight of the phosphor. Thepreferred amount of ,the specified barium compound which is added to the phosphor, with the barium content therein expressed as equivalent barium oxide, constitutes from 0.035% to 0.07% by weight of the phosphor.

As a specific example, 100 cc. of a 1.33 molar percent solution of barium acetate can be added to 400 grams of iinely-divided 4500 K. halophosphate phosphor as specified hereinbefore. This mixture is stirred into a phosphor slurry which is then evaporated to dryness to produce on the finely-divided phosphor a substantially even deposit of 'barium acetate, wherein the bariumrcontent of the acetate, when expressed in terms of equiv-alent barium oxide, constitutes approximately 0.05% by weight of the phosphor. To facilitate drying, the liquid vehicle used in forming the slurry can comprise 97 cc. of ethanol Vand 3 cc. of Water, with the barium acetate content reand chloride, to provide such equivalent barium oxide content as falls within the foregoing permissible or preferred ranges.

The equivalent barium compound can also be added in dry-powder form, but it is ditiicult from a production standpoint to obtain a substantially even deposition of the compound on the finely-divided phosphor when using such a dry-additive technique. From a production standpoint, it is desirable to add the specied barium compound in the form of a rinse, thereby eliminating the necessity for drying a substantial amount of liquid vehicle from the phosphor. With such a technique, suflicient residual barium compound can be deposited in solution form onto the finely-divided phosphor particles to provide for adequate residual barium compound after the phosphor is completely dried.V

After the phosphor material has the specified barium compound deposited thereon, it is coated onto the vitreous lamp envelope in `accordance with the usual techniques. To form a coating paint, any of the foregoing phosphor materials having the specified barium compound deposited thereon can be mixed with a vehicle such as butyl acetate and a small amount of binder material such as nitrocellulose. As a specific example, 200 kilograms of a 4500 K. halophosphate phosphor material having the specified barium compound deposited thereon are admixed with 75 liters of butyl acetate and 25 liters of butyl acetate having included therewith 2% by Weight of nitrocellulose, with the 25 liters of butyl acetate and nitrocellulose having a number 7 Parlin cup viscosity of 65-75 seconds. This phosphor-vehicle admixture is pebblemilled to form a homogeneous suspension which constitutes the so-called paint and this paint can be further thinned if desired. The paint is then flushed over the inside of a fluorescent lamp envelope to coat same, after which the nitrocellulose binder is volatilized by lehring the coated envelope at a temperature of about 665 C. for a period of 2 or 3 minutes, for example. The lehring temperature is limited by the tendency of the vitreous envelope to deform and for the usual soda-lime-silica glass, the maximum lehring temperature Yshould not exceed 675 C. The primary purpose of the lehring operation is to Volatilize the binder material from the phosphor. During the foregoing lehring operation, the deposited nitrocellulose binder material, which amounts to about 0.05 grams for a four foot T12 lamp envelope, breaks down to form carbon dioxide and moisture and other products of, decomposition and under such. condiother than barium chloride, will readily convert in the presence of such moisture and carbon dioxidev to barium carbonate. The usual 40 w. T12-type uorescent lamp will have approximately 4 to 5 grams of phosphor coated therein and the additive barium compound will be deposited in a substantially even manner on the finely-divided phosphor material.

To complete the lamp fabrication, the mounts are sealed into the ends of the vitreous envelope and the cathodes are treated by passing an electric current through the cathode coils in order to convert the Yalkaline-earth carbonatesV to oxides, as is usual. Simultaneously during cathode treating, the envelope is baked to a temperature ofat least 300 C. and preferably about 450 C. in order to remove any residual traces of moisture which inherently are present. Thereafter the proper mercury charge is introduced along with the starting gas fill, the `envelope issealed off and the base caps aixed to the ends of the envelope.

As outlined hereinbefore, the specified bariumcompound which is added in the specified amount comprises barium chloride or barium compound which is convertible to barium carbonate at temperatures of from 300, C. to 675 C. in the presence `of carbon dioxideand moisture. The carbon dioxide will always be present in small amounts, both from decomposition of the nitrocellulose binder and breakdown of the alkaline-earth carbonates'to form the electron-emissive material. Small amounts of moisture a're also present at all times, because of the breakdown of the nitrocellulose binder and because of the absorption of such moisture on the surface of the phosphor, which moisture is substantially driven off during lamp baking just before the envelope is sealed. The amounts of carbon dioxide and moisture which are required to cause the very small amounts of the specied barium compound to convert to barium carbonate are in Ano way critical and even when lehring out the binder material, an excess of carbon dioxide and moisture will always be present over that required to effect such a conversion. These barium compounds will convert to the carbonate at temperatures considerably less than 300 C., but the lamps are required to be baked at least at this temperature to remove residual moisture and the lehring or baking temperatures cannot normally exceed 675 C. because of the tendency for deformation of the lamp envelope.

In Fig. 2 are illustrated performance characteristics after 2500 hours operation for lamps which have deposited onto the phosphor the specified barium compound `such as the acetate in the specied amount. The ordinate values in the graph shown in Fig. 2 are expressed in lumens per watt gain over the control lamps which had no barium compound added thereto, but which were otherwise processed in an identical fashion. The abscissa values represent the percent of barium compound which is substantially evenly deposited onto the finely-divided phosphor material, with the percent by weight of specified barium compound expressed as a percent by weight of the phosphor in terms of equivalent barium oxide. As shown 'in the curve in Fig. 2, in order to have any appreciable improvement over the control lamps, the amount of additive barium compound, with the barium therein expressed as equivalent barium oxide, should constitute from 0.005% to 0.14% by weight of the phosphor. In order to obtain best output, the amount of specified additive barium compound, with the barium expressed as barium oxide, constitutes from 0.035% to 0.07% by weight of the phosphor.

`In Fig. 3 is shown a graph which represents the actual performance characteristics for iiuorescent lamps having added thereto the specified barium compound versus control lamps which did not incorporate theban'um compound, but which were otherwise processed in an identical fashion. The improved lamps have their performance plotted as the heavy curve. The ordinate values are expressed in terms of light output in arbitrary `units and the abscissa values represent hours of lamp operation. After G-hours operation, the lamps having the specified barium compound added thereto have an` increased output of approximately l lumen per watt. After 500 hours, the output increase is approximately 1.8 lumens per watt and after 2500 hours, the increase in output is approximately 2 lumens per` watt and this latter increase continues through the life of the lamps.

The reason for the improvement which is realized through the addition of the barium compound as specified is not clearly understood. As one possible explanation,

however, very thin layers of barium oxide are known to be transmissive to 2537 A.U. radiations while simultaneously acting as a filter for 1850 A.U. radiations.` The phosphor materials which are used with fluorescent lamps respond primarily to 2537 A.U. radiations to generate visible light and the 1850 A.U. radiations which are generated by the mercury discharge are known to be detrimental to the phosphor. The amount of barium compound which is deposited onto the phosphor is extremely small and is suflicient to form on the phosphor a film or layer of a thickness in the order of about ve angstrom units. While the specified barium compounds, other than barium chloride, are readily converted to barium carbonate during lamp processing, the energy contained in the radiations generated by the mercury discharge is sufficient to convert barium carbonate to barium oxide. ASuch a conversion has been proved experimentally. This theoretical explanation is substantiated by the experimental observations that only extremely limited amounts of barium compound can be utilized. The action of barium chloride, however, is not clearly understoo-d and it is not known whether barium chloride acts as a lilter for 1850 A.U. radiations or whether the very thin deposit of barium chloride is converted to barium carbonate under the conditions of lamp processing.

As another possible explanation, the barium compound may react with any sodium sulphate present in order to form barium sulfate. While sodium sulfate is detrimental to lamp performance, barium sulfate is a stable compound and will probably not react with the mercury present within the lamp, as in the case of sodium sulfate. While such a theoretical explanation encompasses all of the additive barium compounds, it does not explain why only limited amounts of barium compounds are required in order to realize the present benefits and why, if the specified limited amounts are exceeded, no increase in output is obtained. Whatever the mechanism involved, the improvements to be obtained from the specified amounts ofthe specied deposited barium compounds are appreciable.

In the preferred method for processing the uorescent lamps, the speciiied barium compound has been deposited on the phosphor before it is coated onto the lamp envelope. This procedure is preferred from a production standpoint. It is possible, however, to deposit the barium compound on the phosphor after the phosphor has been coated onto the lamp envelope and this is readily achieved by pouring a water solution, for example, of the specified barium compound through the phosphor after it has been coated onto the lamp envelope.

It will be recognized that the objects of the invention have been achieved by providing a method for improving the lumen maintenance of fluorescent lamps and a method for stabilizing the lumen output of finely-divided phosphor material for use in liuorescent lamps. In addition, there has been provided a phosphor which will have a stabilized output as well as permissible and preferred ranges for specific phosphor additive materials which improve the performance of liuorescent lamps.

As a possible alternative embodiment any of the speciiied barium compounds can have admixed therewith any light-transmitting inert material such as equal parts by Weight of silica, in order to disperse such barium cornpound in a more uniform manner. The amount of added silica is not critical. Also, while numerous examples of suitable barium compounds have been given, other barium compounds which will convert to barium carbonate under .fthe specified conditions of temperature and atmosphere can be substituted for those barium compounds specified.

While best-known embodiments have been illustrated 'and described in detail, it is to be particularly understood that the yinvention is not limited thereto or thereby.

I claim:

l. The method of stabilizing the lumen output of tinely-divided phosphor material for use in fluorescent lamps: comprising substantially evenly depositing on the phosphor material substance of at least one of the group consisting of barium chloride and barium compound convertible to barium carbonate at temperatures of from 300 C. to 675 C. and in the presence of carbon dioxide and moisture; and the amount of said substance deposited on the said phosphor, with the barium therein expressed as barium oxide, constituting from 0.005% to 0.14% by weight of said phosphor.

2. The method of stabilizing the lumen output of iinely-divided halophoshate phosphor material for use in liuorescent lamps: comprising substantially evenly depositing on the phosphor material substance of at least one of the group consisting of barium chloride and barium compound convertible rto barium carbonate at temperatures of from 300 C. to 675 C. and in the presence of carbon dioxide and moisture; and the amount of said substance deposited on the said phosphor, with the barium therein expressed as barium oxide, constituting from 0.035% to 0.07% by Weight of said phosphor.

3. The method of stabilizing the lumen output of nely-divided halophosphate phosphor material for use in iluorescent lamps, comprising substantially evenly depositing barium chloride on the phosphor material in such amount that the bariumcontent thereof when expressed as equivalent barium oxide constitutes from 0.005 to 0.14% by Weight of said phosphor. n

4. The method of stabilizing the lumen output of iinely-divided halophosphate phosphor material for use in fluorescent lamps, comprising substantially evenly depositing bariumV acetate on the phosphor material in such amount that the barium-content thereof when expressed as equivalent barium oxide constitutes from 0.005% to 0.14% by weight of said phosphor.

5. The method of stabilizing the lumen output of finely-divided halophosphate phosphor material for use in uorescent lamps, comprising substantially evenly depositing barium azide on the phosphor material insuch amount that the barium-content thereof when expressed as equivalent barium oxide constitutes from 0.005% to 0.14% by Weight of said phosphor. v

6. The` method of stabilizing the lumen output of finely-divided halophosphate phosphor material for use in fluorescent lamps comprising substantially evenly depositing barium hydroxide on the phosphor material in such amount thatlthre barium-content thereof when expressed as equivalent barium oxide constitutes from 0.005% to 0.14% by weight of said phosphor.

7. The method of stabilizing the lumen outputof tinely-divided halophosphate phosphor material for use in fluorescent lamps,.comprising. substantially. evenly depositing barium trimethylacetate on the phosphor material in such `amount .that .the barium-content Athereof when eX- pressed as equivalent barium oxide Y constitutes from 0.005 %.to 0.14% by weight of said phosphor.

8. A phosphor material for. userin uorescent lamps, a substancesubstantially evenly deposited on said phosphor material, said substanceselected from at least one of thegro'up consisting of barium chloride and barium compound convertible to `barium carbonate at. tempera- Vtures offrom 300` C.- to.675 C. and in the presence of carbon vdioxide and moisture, and the amount of Ysaid deposited substance expressed inV terms of equivalent barium oxide constituting from 0.005% to 0.14%.. by Weight of said phosphor.

9. A halophosphate phosphor material for use in uorescent lamps, a substance deposited on said phosphor material, said substance selected from at least one of the group consisting of barium chloride and barium cornpound convertible to barium carbonate at temperatures of from 300 C. to 675 C. and in the presence of carbon dioxide and moisture, and the amount of said Vdeposited substance expressed in terms of equivalent barium oxide constituting from 0.035% to 0.07% by weightof said phosphor.

10. The method of improv-ing the fluorescent response of fluorescent lamps, which method comprises: substantially evenly depositing on phosphor material before incorporation into iluorescent lamps, substance comprising at least one of the group consisting of barium chloride and barium compound convertible to barium carbonate at temperatures of from 300 C. to 675 Cfandrin the presence of carbon dioxide and moisture, with the barium content of the substance in said group expressed in terms of equivalent barium oxide constituting from 0.005 to 0.14% by weight of said phosphor; coating saiddepositcarrying phosphor material onto the interior surface of a lluorescent lamp vitreous envelope; and heating Said phosphor-coated envelope to a temperature of from 300 C. up to the deformation temperature of said vitreous envelope in an atmosphere containing carbon dioxide and moisture while simultaneously removing any products of decomposition from the interior of said envelope.

11. The method of improving the lluorescent response of'fluorescent lamps, which method comprises: substantially evenly depositing on halophosphate phosphor material before incorporation into fluorescent lamps, substance comprising at least one of the group consisting of barium chloride and barium compound convertible to barium carbonate at temperatures of from 300 C. to 675 C. land in the presenceV of carbon dioxide and moisture, with the barium content of the substance in said group expressed in terms of equivalent barium oxide constituting from 0.005% to 0.14% by weight of said phosphor; coating said deposit-carrying phosphor material onto the interior surface o f a fluorescent lamp vitreous envelope; and heating said phosphor-coated envelope to a temperature of from 300 C. to 675 C. in an atmosphere containing carbon dioxide and moisture while simultaneously removing any products of decomposition from the interior of said envelope.

l2. The method of improving the fluorescent response of fluorescent lamps, which method comprises: substantially evenly-depositing on halophosphate phosphor material before incorporation into fluorescent lamps,substance comprising at least one of the group consisting of barium chloride andbarium compoundconvertible to barium carbonate at temperatures of from 300 C. to 675 C. and in the presence of carbon dioxide and moisture, with thenbarium content of the substance i ns"aid groupexpressed in terms of equivalent barium oxide constituting from 0.035% to 0.07% by Weight ofsaid phosphor; coating said deposit-carrying'phosphor material onto'the interior surface of `a fluorescent lamp vitreous envelope; and heating said phosphor-coated envelope to a temperature of from 300 C. up to the deformation temperature of said vitreous envelope in an atmosphere containing carbon dioxide and moisture While simultaneously` removing 'any products of decomposition from the interior of said envelope.

References Cited inthe lerof this patent UNITED STATES PATENTS 2,650,884 Paksvver et al. Sept. 1, 1953 2,662,830 Halstead Dec. 15, 1953 2,684,306 Brewer July 20, 1954 2,733,163A Steadman Ian. 31, 1956

Claims

1. THE METHOD OF STABILIZING THE LUMEN OUTPUT OF FINELY-DIVIDED PHOSPHOR MATERIAL FOR USE IN FLUORESCENT LAMPS: COMPRISING SUBSTANTIALLY EVENLY DEPOSITING ON THE PHOSPHOR MATERIAL SUBSTANCE OF AT LEAST ONE OF THE GROUP CONSISTING OF BARIUM CHLORIDE AND BARIUM COMPOUND CONVERTIBLE TO BARIUM CARBONATE AT TEMPERATURES OF FROM 300*C. TO 675*C. AND IN THE PRESENCE OF CARBON DIOXIDE AND MOISTURE, AND THE AMOUNT OF SAID SUBSTANCE DEPOSITED ON THE SAID PHOSPHOR, WITH THE BARIUM THEREIN EXPRESSED AS BARIUM OXIDE, CONSTITUTING FROM 0.005% TO 0.14% BY WEIGHT OF SAID PHOSPHOR.