A significant reaction is reported between gold surfaces and an indium-gallium eutectic liquid alloy often used to transfer heat away from optical elements in high power synchrotron radiation beamlines. This reaction proceeds at the slightly elevated temperatures (70{degrees}C) typical of conservative bakeouts used to achieve ultrahigh vacuum in the chambers which house these optical elements (mirrors, gratings, and crystals) without damaging their highly precise figure and finish. The nature and extent of this reaction is discussed, based mostly on the experience gained recently during the vacuum commissioning of two mirror chambers for a VUV wiggler-based synchrotron radiation beamline. 2 refs., 5 figs

Hulbert, S.L.

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Submitted to: Fourth International Conference on L-OA/ - If*-' '-*£ ' /Synchrotron Radiation InstrumentationManchester, England 7-15 to 7-19-91BNL—46496DE91 017480Interaction of InGa Liquid Alloy Coolantwith Gold Coated Optical MaterialsAU6 2 6 1991S. L. HulbertNational Synchrotron Light SourceBrookhaven National LaboratoryUpton, NY 11973The submitted manuscript has been authored under contract DE-AC02-76CH00016with the Division of Materials Sciences, U.S. Department of Energy. Accordingly, theU.S. Government retains a non-exclusive, royalty-free license to publish or reproducethe published form of this contribution, or allow others to do so, for U.S. Governmentpurposes.DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.ummDISTRIBUTION OF TH«S DOCUMENT IS UNLIMITEDInteraction of InGa Liquid Alloy Coolantwith Gold Coated Optical MaterialsS. L. HulbertNational Synchrotron Light SourceBrookhaven National LaboratoryUpton, NY 11973AbstractA significant reaction is reported between gold surfaces and an indium-galliumeutectic liquid alloy often used to transfer heat away from optical elements in highpower synchrotron radiation beamlines. This reaction proceeds at the slightly elevatedtemperatures (70°C) typical of conservative bakeouts used to achieve ultrahigh vacuumin the chambers which house these optical elements (mirrors, gratings, and crystals)without damaging their highly precise figure and finish.The nature and extent of this reaction is discussed, based mostly on the experi-ence gained recently during the vacuum commissioning of two mirror chambers for aVUV wiggler-based synchrotron radiation beamline.- 2 -I. IntroductionFor the last decade or so, liquid indium-gallium eutectic alloy has been used asan efficient heat transfer medium from optical elements (e.g. mirrors, crystals, andgratings) to water-cooling blocks in the ultra-high vacuum environments of synchrotronradiation beamlines. The eutectic is gallium-rich (Ga 75.5%, In 24.5%), melts at12 115.7°C, and has a negligibly small vapor pressure (-10 Torr at 300°C) . Success-ful implementation of cooling schemes using this alloy requires that InGa not interactsignificantly with the surfaces it contacts. For example, aluminum is a very poorchoice of mirror or cooling block material because the gallium in these alloys isknown to react rather strongly with aluminum via grain boundary diffusion at room2 1temperature and above . In 1983, Hunter and Williams demonstrated the dramaticextent of the Ga-Al reaction, which extends to a depth of 2.5mm below the aluminumsurface, causing severe embrittlement: the surface turns to a grayish powder and flakesaway. Hunter and Williams reported the results of a multi-year study of the interac-tions of these indium-gallium eutectic alloys with a variety of different metal surfaces,at room and elevated (300°C) temperatures. In addition to the strong Ga-Al reaction,the conclusions of their study were as follows: (1) The slight reaction of these alloyswith Cu-Be, brass, Si, Ag, and Mg is most likely slow enough that their use with thesemetals is acceptable. That is, they don't significantly diffuse aways from tbe contactarea, either laterally or into the bulk material. (2) The Ga-Cu reaction is stronger thanwith the metals listed above, so contact between InGa eutectic alloy and copper is notrecommended. (3) No measurable interaction was fuund between InGa eutectic alloysand W, 304 S.S., Ni, graphite, pyrolytic carbon, Ge, Invar, SiC, B, Pb, Pt, or Rh.II. Mirror results and discussionThe purpose of this note is to report the existence of a rather strong reactionbetween InGa eutectic alloy and Au at approximately 70°C both in ultrahigh vacuum- 3 -and at atmospheric pressure. This reaction was discovered during the initial low-temperature (70°C) bakeout of the fully-assembled MQ and Mj mirror boxes for thewiggler beamline U13U-A on the VUV storage ring at the NSLS.The MQ mirror box contains three Au-coated (900A) SiC minors (350 x 50 x 35m m ) and Mj contains one Au-coated (900A) SiC mirror (500 x 60 x 35 mm3). Allfour mirror segments are cooled by water-cooled Cu plates (-1" x 1/4" x length ofmirror) which are spring loaded against the long sides of each mirror segment as closeas practical (<lmm) to the mirror front surface, with a very small amount (£1 drop) ofInGa alloy spread out uniformly in between as the thermal transfer medium. The sideof the Cu block facing the mirror is Ni-coated to avoid Ga-Cu reaction. Also, theAu-coating on the SiC mirror extends down the unpolished sides of the mirror wherethe Cu cooling plates make contact. It turns out to be significant that M» is a horizon-tally deflecting mirror (the M» surface lies in a vertical plane) and M. is a verticallydeflecting mirror (the M. surface lies in an horizontal plane).As shown in Fig. 1, the surfaces of the horizontally-deflecting MQ mirror seg-ments suffered only minor diffusion of Ga across the mirror surface, extending at most-5mm from its starting location at the edges of the mirror surface. This diffusionoccurred during the initial 70°C bakeout of these mirrors and chamber, which lastedfor approximately two days. Fig. 1 shows the maximal extent of the Ga-Au reactionfor the amount of Ga in the system, because subsequent bakeouts (at 70°C) have notexpanded the reacted surface area. On the other hand, -25% of the surface area of thevertically-deflecting M, mirror, shown in Fig. 2, is covered by Ga-Au reaction pro-duct, which extends completely across the width of the minor in a couple of places.Since no greater thickness of the InGa layer was used for M, compared to Mg (andthe bakeout temperature was the same), it must be concluded that orienting a mirrorsurface perpendicular to gravity (mirror surface in an horizontal plane) enhances this- 4 -reaction.Mirror M. has been subsequently stripped and recoated with Pt, and the thermalcontact is made with a thin indium foil rather than InGa alloy. This conservative treat-ment may not be necessary, since the Ga-Pt reaction has been shown to be negligible .During the stripping process, the entire Ga-Au reaction product (Ga-Au alloy) liftedfrom the SiC substrate as a film, indicating that the full thickness of the Au coating(900A) was reacted with Ga in the affected regions of the mirror surface.III. Test slide results and discussionIn order to understand the nature of the Ga-Au reaction, three Au-coated (400Athickness) glass microscope slides (1/2" x 1/2") were subjected to the following treat-ments:Treatment 1: a drop of InGa alloy was placed near the middle of a slide and baked to75°C in vacuum (~10*3 Torr).Treatment 2: same as treatment 1, except the vacuum vessel was backfilled to approxi-mately 600 Torr of dry nitrogen before baking.Treatment 3: a drop of InGa alloy was placed near the middle of a slide and allowedto sit in air at room temperature.For this study, the orientation of the slides relative to gravity was not varied: all threeslides were kept in an horizontal position.The treatment 1 slide began to expand after one hour at elevated temperature(75°C) and continued to expand for the next two days, at which point the heat wasturned off. The original drop of InGa alloy had expanded to a diameter of 12mm,nearly covering the microscope slide. The treatment 2 slide began to expand sometimewithin the first three hours at elevated temperature (75°C) and continued to expand forthe next three days, at which point the heat was turned off. Again, the drop of InGa- 5 -alloy had expanded to a diameter of 12mm. The treatment 3 slide did not show visi-ble changes for three weeks in air at room temperature. Clearly, elevated temperature(7O-75°C) is sufficient to initiate and maintain a significant interaction between InGaalloy and Au, and there is no visible dependence on pressure (at atmospheric pressureor below).In order to determine the nature of the interaction of the InGa alloy with the Aulayer, scanning electron microscopy (SEM) with x-ray elemental analysis was per-formed on the three slides after treatment. A representative SEM photograph of theoutside edge of the reacted area of the treatment 1 slide is shown in Fig. 3. The InGa"puddle" was expanding from the upper right to the lower left in Fig. 3 when the heatwas turned off. The front edge of the expanding InGa puddle is marked by the lowerleft edge of the dark region (the narrow path running from upper left to lower right),and contracted to the upper right edge of the dark region upon cooling from 75°C toroom temperature, leaving a small "mountain" of InGa in one spot. The lower leftregion of Fig. 3 is unreacted Au surface, as determined by x-ray elemental analysis,which shows only the characteristic emission lines of Au, the glass substrate (Si.Ca),and Cr (which is used as a binder to prevent gold island formation). X-ray elementalanalysis of the dark region reveals the presence of substrate, Cr binder, and unreacted*InGa alloy, but no evidence of Au. This indicates that InGa alloy reacts completelywith a 400A Au layer at 75°C and that the reaction products (presumably In and Ga-Au alloy) cling to the InGa puddle as it contracts during cooling. The main InGa pud-dle (upper right portion of Fig. 3) and the island left in the dark region show only Inand Ga x-ray emission lines in the ratio characteristic of bulk InGa alloy. SimilarSEM topography and elemental composition was observed around the exterior of theInGa puddle created via treatment 2.A somewhat different and more instructive situation arises around a small- 6 -droplet of InGa alloy which was inadvertently deposited in a region of the treatment 1slide which was destined to lie outside the maximum extent of the expanding InGapuddle. The CEM photograph of this droplet after being subjected to treatment 1 isshown in Fig. 4. The edge of the large InGa "puddle" can be seen at the top of thisfigure. The region outside the oval-shaped rings is unreacted Au layer, while the darkregion surrounding the odd-shaped bright features in the center of the oval showsmainly glass substrate (Si,Ca) x-ray emission lines, binder (Cr) lines, and a small frac-tion of unreacted alloy (In.Ga) lines, identical to the dark region in Fig. 3. The con-centric oval-shaped rings which surround the odd-shaped bright features and the darkregion can be observed, apparently, because the amount of InGa in this droplet wassmall enough that it did not expand to cover its own reaction front. X-ray elementalanalysis of these rings show that they consist of Ga and Au, probably as an alloy reac-tion product; only a small fraction of substrate lines are observed, and there is nomeasurable In intensity above background. The indium, which does not react stronglywith gold, remains concentrated in some of the odd-shaped bright features in the centerof the oval region; these features exhibit mostly In and Au x-ray emission lines. Oth-ers of the bright features show mainly Ga and Au lines, much like the oval-shapedconcentric rings, and are assigned to active Ga-Au reaction zones.For comparison, a small drop of InGa alloy was placed on a clean, unpolishedCu surface and baked (150°C) in vacuum (10 Torr) for three days. Recall that in thestudy by Hunter and Williams , copper interacted more strongly with InGa than anyother material studied except aluminum. In contrast to the gold results describedabove, the drop of InGa on a copper substrate did not expand significantly. Fig. 5 isan SEM photograph of the boundary of the drop, which shows the unexpanded drop inthe upper left corner, a granular reaction region covering most of the figure, and barecopper surface in the lower right corner. X-ray elemental analysis of the granular- 7 -reaction zone shows Ga and Cu emission lines but no indium lines, which indicatesthat, as for gold and aluminum, the InGa-Cu reaction is dominated by the Ga-Cu reac-tion.V. ConclusionsInGa eutectic alloy is an ideal thermal transfer medium in ultrahigh vacuumenvironments, except for the reactive properties of its Ga component The list of unac-ceptable mating materials has now been increased by one (Au), and the list of accept-able materials has been reiterated (see Introduction). Of course, any materials not onthese lists that might come in contact with InGa eutectic alloy should be checked forunacceptably strong reaction before use.AcknowledgementsThe expert technical assistance provided by A. Lenhard during assembly of themirror holders, installation of the mirrors, and the microscope slide tests is gratefullyacknowledged. The SEM photographs and elemental x-ray analysis were provided byR. Sabatini. This work was supported by the Division of Material Sciences U. S.Department of Energy under Contract No. DE-AC02-76CH00016.References1. W. R. Hunter and R. T. Williams, Nucl. Instrum. Methods 222, 345 (1984), andreferences therein.2. W. R Groggin and J. W. Moberly, Trans. Am. Soc. Metals 59, 315 (1966).Figure Captions1. Normal incidence photograph of the central portio%of one of the three segments ofthe horizontally-deflecting Au-coated (900A) CVD SiC MQ mirror (350mm x- 8 -50mm surface) as installed at NSLS beamline U13U-A. Note the reflection of thecamera and the eight-inch viewport through which the photograph was taken. Thewater-cooled copper blocks are spring-loaded against the top and bottom faces ofthe mirror by stainless steel pins held in a large aluminum U-channel. Note thatsmall semi-circular shaped regions of the surface have reacted with the InGa alloywhich forms the interface between the mirror and the top copper cooling block.2. End (a) and side (b) perspective photographs of the vertically-deflecting Au-coated(900A) CVD SiC M, mirror (500mm x 60mm surface), after being removed fromits chamber and holder for stripping and recoating. Note that InGa alloy hasreacted with a large fraction (- 25%) of the Au-coated mirror surface.3. SEM photograph of the edge of a "puddle" of InGa alloy on a Au-coated (400A)microscope slide created by baking (75°C) a small drop of InGa in vacuum (10Torr) for two days. Baking in dry nitrogen (600 Torr) produces similar results.The puddle was expanding from the upper left to the lower right in this photographwhen the heat was turned off. The front edge of the expanding InGa puddle con-tracted from the lower left edge to the upper right edge of the dark region (thenarrow path running from upper left to lower right) upon cooling from 75°C toroom temperature, leaving a small "mountain" of InGa in one spot. The lowerleft region of the photograph is unreacted Au surface. X-ray elemental analysis ofthe dark region shows no evidence of Au, indicating that InGa reacts completelywith a 400A Au layer at 75°C. 200x magnification, lOOjim scale bar shown.4. SEM photograph of a small droplet of InGa alloy which has reacted with a Au-coated (400A) microscope slide by baking (75°C) in vacuum (10" Torr) for twodays. The edge of the large InGa "puddle" shown in Fig. 3 is shown at the top ofthis photograph. The amount of InGa in this droplet was small enough that it didnot expand to cover its own reaction front, which appears as a set of concentricoval-shaped rings surrounding the bright odd-shaped remains of the initial droplet.- 9 -X-ray elemental analysis of the concentric oval rings shows no evidence of indium,indicating that the Ga-Au reaction dominates the interaction of InGa alloy with Au.The unreactive indium is left concentrated in some of the odd-shaped brightfeatures in the center of the oval region, which exhibit a thirty-fold increase in theratio of In to Ga x-ray emission intensity compared to bulk unreacted InGa alloy.500x magnification, 10|im scale bar shown.S. SEM photograph of a small drop of InGa alloy which has reacted with a clean,unpolished Cu surface by baking (150°C) in vacuum (10 Torr) for three days. Incontrast to the Au surface (Figs. 3 and 4), the drop does not expand significantly;the boundary of the InGa "puddle" in the upper left corner of the photograph isclose to its original location. The InGa-Cu reaction is dominated by the Ga-Cureaction: x-ray elemental analysis of the granular reaction zone (which covers mostof the area of photograph) shows Ga and Cu emission lines but no indium lines.300x magnification, 100/im scale bar shown.

Interaction of InGa liquid alloy coolant with gold coated optical materials

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