US2450854A - Method of coating by evaporating metals - Google Patents

Description

ot.5,1948. w. H. CQLBERT ml. 2,450,854

METHOD OF COATING BY EVAPORATING METALS Filed Deo. 3, 1946 l INVENToRs' 3 killhlrlllolel.

Patented Oct. 5, l

'y -METHOD 0F COATING BY EVAPORATING METALS William E. Colbert and Arthur R. Weinrich,

Brackenridge, Pa., asslgnors to Libbey-Owens- Ford Glass Company, Toledo,` Ohio, a corporation of Ohio [application December 3, 1946, Serial No. '713,700

M Claims. (Cl. 117-107) l, V Our present invention relates to a novel method of coating by evaporatlng metals. It has to do, more particularly, with the coating or wetting,

by capillary attraction, of a filament which, for

example, may be formed from a coil of ordinary tungsten wire, tantalum, molybdenum or columbium wire, by various metals in the form of an alloy with small amounts of manganese which metals to be evaporated in pure form normally do not wet thesemetallic iilaments, and the application .of saidr` metals or metal alloys by deposition resulting from thermal evaporation, to the face or surface of an article, such as a piece of glass, porcelain, plaster, metal, plastic, Cellophone, paper, or the like, to provide a reflective or metallized surface coating forV said article. The invention also has to do with securing wet ting and with thermal evaporation of such metals from tungsten, tantalum, molybdenum, or columbium filaments alloyed or coated with manganese by the application to the filaments of the pure metals desired to be evaporated.

Thisapplication is a continuation-impart of our copending application, Serial No. 552,290, iiled Septemberl, 1944 now Patent No. 2,413,606. Methods and apparatus have previously been employed to apply coatings of metals by thermal evaporation to the faces or surfaces of such articles to produce mirrors, reflectors or metallized materials for other purposes. In these methods it is desirable to effect the thermal evaporation of the metal, such as silver, copper, gold or aluminum, by applying the metal directly to an electrically energized and thus heated tungsten or other metallic illament which is preferably located within a vacuumized chamber. The metals which 4may be used vas iilaments ior such evaporations must obviously be of high melting point and also of low vapor pressure at the elevated temperatures at which the metals applied to the laments evaporate. Thus, tungsten, tantalum, molybdenum and columbium have represented the only practical materials for such use. Platinum also has been used to a small degree but its high cost is generally prohibitive. While iron and nickel are of relatively low vapor pressure, they are of such relatively low melting point that filaments made from them rapidly burn out. With these filaments many of the metals can be readily evaporated. Thus, i`or example, aluminum, magnesium, vanadium, barium, strontium, iron, nickel, cobalt, manganese, thorium, chromlum and titanium, when applied to laments of tungsten, tantalum, molybdenum or .columbium, will on heating in a vacuum, melt and spread over the lament by capillary attraction and satisfactory evaporation of these metalsl then oocurs fromV the large amount of surface' which the molten metal covers.A

I However, with a large number of metals which it is desirable to be able to thermally evaporate cODDer While readily lending themselves to thermal evaporation from a Crucible, cannot be evaporated readily from a coil of tungsten, tantalum, molybdenum or columbium when applied to a I illament of these metals and heated by electrical resistance. The silver or copper on melting shows no amnity for .the metallic iilaments and almost immediately after melting collects vinto a drop and falls oil of the iilament. This lack of ability to wet tungsten, tantalum, molybdenum and' columbium occurs also with the metals gold, zinc,

tin, antimony, cadmium, bismuth, lead, thallium and indium. With each of these metals the use of the four available coil illaments as a means cf evaporating these metals has not been possible, and less desirable means of heating have been necessary where it became necessary to evaporate these under practical conditions repeatedly in the commercial production of mirrors and metallic coated articles. As each of these metals, after melting, pulls together into droplets andfalls oi the iilarnents, there has resulted a wastage of the metal whenever it has been attempted to evaporate them from these iilaments and there have been continuous failures of the apparatus to function due to the toss of the metals from the heated Wires; and where any metal has been evaporated the amounts so evaporated have always been uncertain and without control.

We have found that' we may use tungsten, tantalum, molybdenum or columbium as laments for the evaporation of metals which do not wet these filaments by causing them to Wet such laments by the application to such iilaments, or the forming thereon, of alloys of those metals whichwe desire to evaporate with small amounts of manganese such as amounts ranging from 0.1% to 5%, which is characterized in that it will also form an alloy with tungsten, tantalum, mo-

lybdenum and vcolumbiuxn in the presence of the metal we desire to evaporate. Thus, for example, We may add small amounts of manganese to silver and when-such alloys areA melted on a tungsten iilament the silver will 'be found to wet the tungsten filament and to spread itself by capillary attraction over the surface of the tungsten Not all metals have been-found to act in this manner.- Ailoying the normally non-wetting metals among themselves, such as adding lead to silver.

does not seem to bring about any desirable improveinent in the wetting characteristic. In each case, however. it is found thatithe manganese readily forms alloys with tungsten, tantalum, molybdenum and columbium and also forms alloys with copper, silver, gold, zinc, tin. antimony, cadmium. bismuth, lead, indium and thallium. It has also been found that manganese when heated reduces tungsten oxide, molybdenum oxide, tantalum oxide. and columbium oxide, to the metals,

normally present upon filaments composed of tungsten. molybdenum, tantalum. or columbium, aids in securing the desired wetting since silver. copper. gold, zinc, tin, antimonyfcadmium, bismuth. lead, thallium and indium do not wet the oxide-coated filament. Thus. the manganese readily brings about the desired wetting and it appears clearly that this is accomplished through -the reduction of the oxide coating upon the filament and upon the mutual alloying tendency which these metals possess. The manganese may also be added to the filament either as an`alloy with the tungsten, tantalum, :molybdenum or columbium. or preferably it may be employed as a surface coating or surface alloy with such filaments. Filaments containing small amounts of manganese are found to wet readily when the pure otherwise non-wetting metals are fused thereon. Thus the wetting is brought about in the presence of the manganese and it is immaterial as to whether the manganese is supplied in l an alloy applied to the filament or exists inthe Vfilament surface.

One ofthe objects of our invention is to provide an improved and satisfactory method or process of evaporating metals rwhich normally do not wet'heater filament coils of tungsten, tantalum, molybdenum, or columbium, by allQyillg the metal so as to cause the metals to wet the coils of such filaments and to coat the coils by capillary attraction so that thermal evaporation can then be carried out.`

-Another object-of our invention is to apply to a tungsten, tantalum, -molybdenum or columbium filament a metal to-be evaporated which does not wet such filament coils.,alloyed with a suitable 'proportion of manganese which alloys with the Tillament and brings about a proper wetting or -coating of the filament wires by capillary attraction under the inuence of heat applied tothe filament.

As another object of our invention there is provided an improved method or process 'whereby a metal which is to be evaporated and which does not normally wet heater filaments of tungsten. tantalum, molybdenum and columbium is alloyed with maganese and applied to such filament, and by securing a wetting and coating of the filament by capillary attraction of the metal desired to be evaporated may be deposited upon the face or surface'of an article, by thermal evaporation. to v'provide such article with a reflective or metallized surface coating.

As further object there are provided filaments which have beenpre-alloyed with small amounts of manganese which may be used directly to evap- Y 4 orate the normally non-wetting metals since such alloyed filaments are found to be wetted readily by the molten pure metals desired to be evaporated.

A further object of our invention is to apply to a tungsten, tantalum, molybdenum or columbium A filament, a metal such as copper, silver. gold. zinc.

, tin, antimony, cadmium, bismuth. lead, indium or and that the removal of Vcoatings of these oxides.

ting 'or coating of the filament metal by capillary attraction under the influence of heat applied to the filament and thus permits thermal evaporation of the metals.

Generally speaking, and in accordance with our present invention. the metal to be evaporated which normally does not wet the heater filament is applied alloyed with small amounts of manga'-I nese to the extent of 0.1% to 5% or more, providing wetting characteristics tothe coils of a filament which may be formed from tungsten. tantalum. molybdenum or columbium. Thus in order to thermally evaporate copper, zinc, gallium or arsenic, which are metals of the chemical periodic table arrangement found in series 5 or the metals silver. cadmium, indium, tin and antimony, which include metals of series 7, or the metals gold, thallium. lead and bismuth which in the periodic arrangement include series 11. all of which metals do not wet filaments made of tungsten, tantalum, molybdenum or columbium, we flrst bring about a satisfactory wetting and adhesion of these metals to the filaments by applying the metals 'to the filaments as an alloy with small amounts of manganese and then by energizing the filamentand thus heating it we cause a melting of the metals. When the applied metal alloys are thus melted they react Iwith any oxide lcoatings upon the filaments and reduce such to -the metals tungsten, tantalum, molybdenum or columbium and then they apparently alloy to some degree with the metal comprising the heater filament wire and-by reason of such the molten metals wet the filament wires and by capillary attraction are drawn out over the surface of the coils. The molten metal which lhas thus covered considerable surface of the heated coil and is held theretoby capillary adhesion is thereafter evaporated uniformly from the heater coils to apply a surface coating of a metallic or reflective nature to an article such as a piec'e of glass, porcelain, silica, mica, plastic, metal, Cellophane, resin, or other support material, by deposition resulting from the thermal evaporation of the metal from the filament. The operations of thermal evaporation'may with some of the metals, be carried out at normal pressure but generally are preferably carried out in vacuum chambers known to the art and within a high vacuum, which may be of the order of one millimeter down to 10 to the minus 5 millimeters or better. It is very necessary that the metal to beevaporated wet and coat the coil surfaces in order that the metal will evapcrate uniformly in all directions. By securing such wetting action the thermal deposition of these metals, in addition to being made possible, has been found by our process to give uniform coatings.

We may also proceed to secure the objects within the scope of this invention by supplying manganese in the filament either as an alloy with the tungsten, tantalum, molybdenum or columbium, or as a surface coating; or as a surface alloy on such filaments. When the pure metals such as silver, copper or gold which will not normally wet the bium filaments are applied to these filaments containing some manganese they will onmelting readily wet the manganese-containing filaments.

Thus, in general, the invention comprises melting the metal desired to bev evaporated upon a tungsten, tantalum, molybdenum or columbium filament in the presence of manganese which brings about the desired Wettingand adherence of the molten metal to the heated filaments.

As will be shown more fully later, pieces of the metal to be evaporated 'and which have been previously alloyed 'with the metal which brings about the alloying with and wetting of the iliament, may be hung onto the loops or coils of the lament. f

The foregoing and other objects and advantages of the present invention will appear-,from

the following description and appended claims when consideredin connection with the accompanying drawings forming a'part of this speciiication wherein'similar reference characters designate corresponding parts in the several views.

In said drawings:

Figure 1 is a perspective view, partly broken away, illustrating one suitable apparatus for carrying out our improved method or process.

Figure 2 is a perspective view of a fragment of an electric filament showing the application of a suitable metal alloy to several of the coils or convolutions thereof, and illustrating one phase of the method or process of wetting or coating the iilament by said alloy; and

Figure 3 is a view similar to Figure 2 showing the filament after the completion of the wetting process by the alloy of Figure 2.

Figure 4 is an enlarged; cross-sectional view of v a tungsten, tantalum, molybdenum or columbium electric resistance filament precoated with a thin layer of manganese.

Figure 5 is another enlarged, cross-sectional view showing a iilament of tungsten, tantalum, molybdenum or columbium which is alloyed only in the surface ofthe filament with manganese.

Figure 6 is also an enlarged, cross-sectional view i of a filament formed from tungsten, tantalum, molybdenum or columbium, which is/ alloyed throughout with a small amount of manganese and upon which the pure metals, on melting,

will directly show a good wetting action.

Figure '7 is a perspective view of a laxnent containing manganese such as shown in Figures 4, 5 and 6 and upon which pieces of theI puremetal to be evaporated have been hung.

Figure 8 is a perspective view after the filament and pieces of metal-of Figure '1 havebeen heated to effect a wetting of the iilament.

Before explaining in detail the present invention it is to be understood that the invention is not limited in its'application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and ofbeing practiced or carried out in various ways. It is to beunderstood also that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

Referring now to the drawings, we'have shown a suitable apparatus for carrying out our improved method or process, aswell as one suitable metal alloy and the steps of applying the alloy or a filament by a wetting 'action resulting from capillary attraction. We have also shown suit able alloyed filaments and the steps of applying the pure metal to be evaporated to such filaments and the wetting action upon the'lar'nent resulting from capillary attraction after the pure metalx is melted.A

Suitable apparatus employed by us, and illustrated in Figure 1, comprises, as shown, a supporting base il) upon which is mounted a housing, shown as a whole at ii. The housing Il maybe in the form of a bell-jar or the like having a dome-like or semi-spherical top portion or enclosed end andv a bottom open end having a surrounding flange or projectionA i2 which is adapted to rest upon the top face or surface of the supporting base i0.

Within the chamber provided by the housing l I, we have shown a suitable work-piece support i3 for supporting a work piece, such as a plate orv piece of glass, plastic, plaster, paper, porcelain,

. metal, or thelike it, in upright position.

tantalum, molybdenum or columbium, or an alloy of these Withmanganese or precoated with a layerr of manganese, whose opposite ends are attached to brackets il mounted upon the supporting posts i5 and adjustable thereon so as to vary the position or location of the filament it with relation to the supporting base i0.

The chamber provided by the housing il may, if desired, becompletely evacuated of air through outlet pipef'or conduit Illa 'and have a high vacuum created therein by means of suitable air evacua-ting and vacuum creating means, such as a pump` (not shown).

In accordance with one method embodying our invention which is to be performed or carried out within the chambered housing li, we preferably provide a metal alloy which may consist of silver. copper or gold or other normally non-wetting metal and approximately 0.1% to.5% or more of manganese. Pieces of this preformed silver and manganese alloy, for example, or copper 'and manganese alloy, for example, or gold and manganese alloy, for example, several of which are shown at IE, in Figures land 2, are bent and hung on the loo'ps or convolutions ita of the lamenti 6 composed of pure tungsten, tantalum, molybdenum or columbium in the manner shown.

lt is known that silver. copper and gold lend themselves admirably to thermal evaporation but they have no wetting aiinity for tungsten, tantalum, molydenurn or eolumbium surfaces and therefore silver, copper or gold alone is unsatisfactory for coating the nlament it formed from either of these metals by a wetting action effected by capillary attraction. Wetting of the filament wire is essential to secure a maximum of evaporating surfaces to provide evaporation uniformly in all directions, to the securing of uniform deposits.

and also to avoid the dropping of the molten metal ofi the heater wires. We have found' that manganese readily alloys with silver, gold and copper and the alloys have a wetting amnity for the four above-mentioned metals, any one of which may be used for making the larnent i6, and thus manganese is particularly useful in securing the wetting of the iilament by capillary attraction. Therefore, by including a certain percentage of manganese, preferably 0.1% to 5% or more, with the silver, copper or gold to form the alloy i8, the manganese will serve to bring about wetting or coating of said iilament by the molten metal by action of the filament I6 by the alloy I8 is shown generally at I9, Figure 2. As the wetting action by capillary attraction continues, the two metals of the alloy will proceed to wet the coils of the filament I6 and in fact, will substantially wet or coat and cover the surfaces of the fllament. In Figure 3, we have illustrated several of the coils or loops I6a of the filament as being coated at 20 by the alloy from which the pieces I8 are formed.

Thus, by including manganese with the silver, copper or gold, as'an alloy, it is possible to quickly and effectively coat or wet the filament I8 by capillary attraction. Since, therefore, the alloy builds up onto the surfaces of the filament in substantially the manner illustrated in Figure 3, there will be a relatively uniform coating or wetting of the filament and a uniform dependable evaporation of the silver, copper or gold. Heretofore, when attempts were made to wet the filament by the use of the silver, the copper or the gold alone. only small portions of the molten metal would cling to the filament as droplets hanging from the lower ends of the coils of the filament, with the major portion of the molten metal dropping or falling off the coils.V This was particularly undesirable since it was practically impossible to produce by thermal evaporation, an even surface coating by deposition, or to control the deposition to desired coating deposit thicknesses on the vsurface of an article, such as the article I4, to which it was desired to apply a reflective surface coating. By virtue of the fact that the silver, copper or gold did not properly wet the tungsten or other metal filament but had a tendency to drop E said filament, the process of coating with these metals by deposition was unsatisfactory, slow and painstaking because only a small portion or percentage of the filament received the metal coating. Considerable shutting down and starting over again was required when most of the gold, silver or copper on first melting dropped ofi the coils and no evaporation was secured. Thus great waste occured, the purpose was considerably slowed down, and the coating produced by deposition, if any, was uneven or spotty and unsatisfactory because of such uneven character thereof on the surface of the article coated. Commercial production under such uncertain conditions was impossible.

It is to be understood that in carrying out our method or process as described above, in the chamber of the housing II, the chamber depending upon the `metal being evaporated, may be at atmospheric pressure, or it may be evacuated of air and a vacuum created therein. Thus, after the pieces I8 of the silver and manganese alloy or copper and manganese alloy or gold and manganese alloy, as the case may be, have been applied to the coils of the filament I8 and the work piece I4 mounted upon its support I3 within the chamber, a vacuum of l0 to the minus 3 millimeters or better is created and the filament I6 is then energized and therefore heated so as to melt the alloys and to cause the hot manganese to reduce any oxide coatings on the filaments and to start in motion the wetting action of the filament by capillary attraction, as explained above. After the wetting action has been completed as illustrated generally at 20 in Figure 3, the filament I6 is heated further, whereby silver, copper or gold in the coating 20 of the 8 alloy will be thermally evaporated and transferred by deposition to the face or surface of the work piece I 4 which, as shown, is disposed in a position opposite the filament I8.

We have found that the desired wetting may also be brought about by applying pure metals as pieces, as shown at 23 in Figure 7, to a filament of tungsten, tantalum, molybedenum or columbium, indicated at I6 which has been precoated at 24, Figure 4, pre-alloyed at 25, Figure 6, or surfacealloyed at 26, Figure 5, with manganese. In carrying out the operation, the filament I8 is mounted as in Figure 1 and the, pure metal pieces 23 of Figure 'I are applied thereto. Afterthe ob- :lect I4 to be coated has been placed in position the bell jar II is lowered, the vacuum is created, andelectric current is applied to the filament through the electrodes I5 to cause melting of the applied -pieces ad a wetting of the coil as illustratcd in Figure 8. By continued application of heat to the coil the applied metal will be ther ilrally evaporated and deposited upon the object During the evaporation of the higher boiling 'metals such as gold, silver and copper, some of the manganese is also distilled while in the case of the lower boiling metals such as lead and zinc, where the filaments are operated at relatively low temperatures, little of the manganese whether present originally as an alloy with the metal to be distilled or present in or on the filament is distilled. 'Ihus. in the latter case practically pure lead or zinc coatingsl are deposited whereas in the case of gold, silver or copper an alloy of manganese and these metals is thermally deposited upon the object being coated.

A further advantage of very practical importance in the thermal evaporation of the various metals has been secured through our securing good wetting of the filaments in that the metals being evaporated show very little explosive boiling or spitting which by reason of small chunks of metal blown over onto the article being coated has caused spoilages. This appears to have been accomplished by the decrease of surface tension forces accompanying the wetting and also in the elimination of conditions leading to superheating by getting the metal to spread out in a thin coating over most of the filament surfaces.

The precoating of filaments with manganese as shown in Figure 4 may be accomplished by thermally evaporating in a vacuum a .coating of the manganese upon the cold filaments. If the filaments also carry the metal to be evaporated, such as silver, etc., the latter also would become coated with the manganese and upon heating the coated filaments, good wetting by silver, etc., is secured with subsequent evaporation. Such manganese-coated filaments may also be heated to cause the manganese to surface-alloy with the filament to produce structures as illustrated by Figure 5. Where it is desired to introduce manganese into the filament as a uniform alloy this may be accomplished by introducing manganese into a molten alloy and thereafter in the known manner producing wire by pulling such alloy through dies.

From the foregoing it will be seen that we have provided an improved method or process for applying certain metals to a filament of tungsten, tantalum, molybdenum or columbium by causing a wetting resulting from capillary attraction through the presence of small quantities of manganese and applying heat from the filament, and have thereby been able to carry out evaporaface or surface of a work piece toprovide metallized or reflective surface therefor. It will also be seen that while we secure the desirable requisite of wetting of the filaments of tungsten, tantat lum, molybdenum or columbium, by metals which normally do not wet these, by the presence of manganese this may be accomplished in several ways. Thus, we may apply separate pieces of a preformed alloyof such metals with manganese and these will wet the pure metallic filaments. or we may apply the pure metals to be evaporated to a filament containing some manganese either in its surface or throughout. l

While we have referred tothe use of tungsten, tantalum,molybdenum, or columbium as suitable metals from which the coiled fllamenaor element i6 may be formed, other suitable metals may be used for this purpose. We have mentioned these metals particularly since vtheir high melting points and low vapor pressures at the boiling temperatures of other metals make these the practically desirable metals for use as such filaments.

'We have described "our improvedemethod or process as preferably being carried out in a vacuumized chamber in which the step of wetting the filament takers place, as does also the step of thermal evaporation of the metal to effect its deposition upon the work piece to provide a reective coating thereupon. l

Obviously also in the case oi' the most readily volatile metals, such as cadmiuni and zinc, the melting of the metals and the wetting of the iliaments, as well as the evaporation of the readily volatile metals, such as cadmium or zinc, may be carriedout under atmospheric conditions ofpressure if desired, while employinga suitable inert atmosphere. y

Having thus described our invention, what we claim is:

1. Themethod of making mirrors by the deposition of a metal on a polished support material, comprising evaporating a metal from a filament made of a metal selected from the group consisting of tungsten, tantalum,molybdenum and columbium wherein the` metal is heated on such dlament as an alloy with manganese which causesl the metal desired to be evaporated to wet, to 'adY here to, and to spread out over the filament surfaces and by the continued application of heat.

to evaporate, and depositing a coating thereof on said polished support material. n 2. The method of coating surfaces which comprises evaporating metals from a filament made -of a metal selected from the group consisting of tungsten, tantalum, molybdenum and columbium wherein the metal is heated on such filament as an alloy with manganese which causes the metal desired to be evaporated to wet, to adhere to, and to spread out over the filament surfaces and by the continued application of heat. to evaporate and to deposit upon said surfaces.-

3. The method of coating articles by evaporating silver from a filament selected fromthe group consisting of tungsten, tantalum, fmolybdenum and columbium wherein the silver is alloyed with v manganese and is heated 'on such filament and wherein said manganesepcauses thesilver desired to be evaporated to wet, to adhere to, and tol spread out over the filament surfaces and by the continued application of heat, to-evaporate and to coat the articles byA deposition oi' the silverk tion of heat, to evaporate within the vacuum and to coat by deposition said support material.

5. The method of coating a"support by evaporating copper from a filament selected from the group consisting of tungsten, tantalum, molybdenum and columbium wherein the copper is heatedy on such filament in the presence of manganese and wherein said manganese causes the copper desired to be evaporated to wet, to adhere to, and to spread out over the filament surfaces and by the continued application of heat, to evaporate and to coat by deposition thereof on a surface of the support.

6. The method of coating a support by evaporating copper. within a vacuum from a filament selected from the group consisting of tungsten, tantalum, molybdenum and columbium wherein the copper is alloyed with a relatively small amount of manganese and is heated on such filament and wherein said manganese causes the cop. per desired to be evaporated to wet, to adhere to,l and to spread out lover the filament surfaces and by the continued application of heat, to evapf orate within the vacuum, and to coat the support by deposition.

7. A method according to claim 4 wherein gold is substituted for silver.

8. A method according to claim-1, wherein the manganese is present in a relatively small amount.

9. A method according to claim 2, wherein the manganese is present in'an amount less than 5%.

10. A' method according to claim 3, wherein the manganese is present in an amount under 5%.

11. A method according to claim 4, wherein the manganese is present in a relatively small amount. A f

12. A method according to claim 4, wherein gold is substituted for silver and where the manganese is present in an amount less than 5%.

13. The method of coating a support material by evaporating a metal within a vacuum from a filament selected from the group consisting of tungsten, tantalum, molybdenum and columbium wherein the metal is heated on said filament in the presence of manganese and wherein said manganese causes the metal desired to be evaporated to wet, to adhere to, and 'to spread out over the filament surfaces and by the continued application of heat, to evaporate within the vacuum and to coat bydeposition said support material.

14. A method according to claim 13,4 wherein e support material is a polished support mate-- rial. l

WILLIAM H. COLBERT. ARTHUR R. WEINRICH.

REFERENCES crrEn The following references are ofsrecord in the l ille of this patent:

UNITED STATES PATENTS v Name Date Colbert Dec. 31, v1946 OTHER REFERENCES` Number

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