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140

DETAILED TECHNIQUES FOR

PREPARING AND USING

HARD GALLIUM ALLOYS

GEORGE G. HARMAN

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
National Bureau of S': AUG 5 1965

THE NATIONAL BUREAU OF STANDARDS.

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March 3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent

with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,- devices, and structures; advisory services to government agencies on scientific and technical problems; invention and development of devices to serve special needs of the Government; and the development of standard practices, codes, and specifications. The work includes basic

and applied research, development, engineering, instrumentation, testing, evaluation, calibration services,

and various consultation and information services. Research projects are also performed for other government

agencies when the work relates to and supplements the basic program of the Bureau or when the Bureau's

unique competence is required. The scope of activities is suggested by the listing of divisions and sections on the inside of the back cover.

Publications

The results of the Bureau's research are published either in the Bureau's own series of publications or

in the journals of professional and scientific societies. The Bureau itself publishes three periodicals avail- able from the Government Printing Office: The Journal of Research, published in four separate sections,

presents complete scientific and technical papers; the Technical News Bulletin presents summary and pre- liminary reports on work in progress; and Basic Radio Propagation Predictions provides data for determining

the best frequencies to use for radio communications throughout the world. There are also five series of nonperiodical publications: Monographs, Applied Mathematics Series, Handbooks, Miscellaneous Publications,

and Technical Notes. A complete listing of the Bureau's publications can be found in National Bureau of Standards Circular 460, Publications of the National Bureau of Standards, 1901 to June 1947 ($1.25), and the Supplement to National Bureau of Standards Circular 460, July 1947 to June 1957 ($1.50), and Miscellaneous Publication 240,

July 1957 to June 1960 (Includes Titles of Papers Published in Outside Journals 1950 to 1959) ($2.25); avail- able from the Siq)erintendent of Documents," Government Printing Office, Washington 25, D. C.
NATIONAL BUREAU OF STANDARDS

technical ^^ote

140

APRIL 1962

DETAILED TECHNIQUES FOR PREPARING AND

USING HARD GALLIUM ALLOYS

George G. Harman

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

Price 75 cents

DETAILED TECHNIQUES FOR PREPARING AND USING HARD GALLIUM ALLOYS

George G. Harman

This report presents an expansion and clarification of techniques for preparing and using dentalamalgam-type gallium alloys for industrial and scientific

uses that were previously published in the Review of

Scientific Instruments . In addition, new material,

such as hardness of the alloys and discussions of the

bonding mechanism are presented. A different class

of bonding alloys based on the same principals but

not incorporating gallium are described, along with

application details.

1, Introduction

In the year or so since issuing the various publications, on

"Hard Gallium Alloys", ^>2>3 there have been hundreds of inquiries

about the details of alloy mixing techniques, bonding applications

and the sources of gallium. Thus it is desirable to discuss in

detail the frequently occurring problems in order to assure that

the techniques achieve their maximum scientific and industrial usage.

From the questions asked it is apparent that many workers are un- familiar with such procedures as making eutectics, mixing liquid

metal with powders, and in general, with the problems and procedures

of handling gallium.

Although some new material is presented, this publication is

primarily an expansion and clarification of the three preceeding

ones on the subjectlj2,3 and reprints of these are available upon

request to this author at the National Bureau of Standards, Washington

25, D. C. The first article^ gave general techniques, listed tables

of alloys, gave expansion coefficient curves, and included several

inserted "Additional Notes" pages. These notes have been revised

twice during the various reprintings of the article. The NBS

Technical News Bulletin article^ was essentially a summary of the

first article and its "Additional Notes". The third article^ was

a brief explanation of some problems one may encounter when using

the alloys near their maximum useful temperature range and dis- cusses some comments made by another worker.

1. G, G. Harman, Rev. Sci. Instr. 31 717 (July 1960).

2. NBS Tech. News Bui. 45 5 (Jan, 1961).

3. G, G. Harman, Rev. Sci. Instr.

^

465 (April 1961).

4. H, M. Ferrari, Rev. Sci. Instr. 32 465 (April 1961).
(2)

The bonding properties of these alloys which have been of

interest to many readers have been investigated only cursorily and

as a secondary result of the work and applications described in

Reference 1. However, all details and techniques that this writer

is familiar with are presented here in an effort to assist readers.

Many of the procedures described below are standard metallurgical

practices or are normally used in the preparation of dental amalgams.

A great deal of cooperation was received from the NBS Dental Research

Section and their papers^*" and reports were used as background

material in references 1, 2, and 3.

The basic procedure to use the alloys is to mix liquid gallium

(or one of its eutectics) with a readily alloying metal powder, then

tamp the mixture in holes, or, place it between metal (or some types

of ceramic) plates and apply pressure. In time, the mixture hardens

by an alloying process similar to that of a dental amalgam, and,

depending on various conditions, will produce a bond to the material

that will withstand quite high temperatures.

The procedures have been extended to include other, less ex- pensive, materials that require higher hardening temperatures, some- what similar to that of a single component epoxy resin. These are discussed in Part II.

2. Gallium Based Alloys

2.1 Making the Gallium-Tin (or Indium) Eutectic

One of the most frequent questions received from persons desiring to use gallium alloys is vaihy, how, and when to mix tin (or

indium) with the gallium. The purpose of this procedure is to

inctease the ease of mixing the gallium with other metal powders

and to decrease the necessary mixing temperature, thus prolonging

the useful working life of the mixed materials. Gallium (Ga) melts

at i\J 30°C (86°F) but its melting point may be depressed to /V 20 C

(/V 65°F) by adding 10-12 weight 7„ of tin (Sn) or 20-25% indium (In)

to it. Also, by using these Ga + Sn or In mixtures, some lengthening of the working time (pot life) is achieved, as was discussed in

the additional notes of Reference I.

Incidentally, it is worth commenting that gallium is available

in purities ranging from 99% to 99.9999"^.. There is no apparent

technical advantage in using the higher purities, and they may cost

5. D. L. Smith and J. J, Caul, J. Am. Dental Assoc. 53 315 (Sept. 1956)

6. D. L. Smith, H. J. Caul and W, T, Sweeney, J„ Am. Dental Assoc.

53 677 (Dec. 1956).
(3)

several times the lowest grade. In general, specify the lowest

purity available, as long as the lead (Pb) content is not a significant part of the impurity. (Lead has detrimental effects in various

mercury-silver alloys and by analogy might also effect gallium alloys.)

The Ga-Sn (or Ga-In) mixture is most easily made by placing

solid lumps of tin (not tin powder^) or indium in contact with the

appropriate amount of gallium. The tin is simply placed in contact

with the solid gallium. A little finger pressure will initiate the

reaction, which can immediately be seen as bright liquid droplets

forming around the edges of the tin. This can then be left over- night to complete the reaction. If the ingredients are heated to

/v 50°C ( ro 120°F) or higher, the reaction will occur more rapidly,

within minutes. Once formed it will be a liquid (similar in appearance to mercury) above its freezing point, as mentioned previously.

This liquid is then used directly in place of the gallium according

to the weight percentages given in Table I of Reference 1 (reproduced

on the following page).

If desired, a larger amount of Ga-Sn mixture than will be used

at one time can be made. Since this mixture freely wets almost

anything it pouches, some care should be given to the choice of a storage container. A teflon container has proven to be best, but

a satisfactory alternative is to coat the inside of a glass bottle

with paraffin. Some wetting will take place but usually the entire

alloy can be transferred by "dumping" or shaking out.

In order to separate a small portion of the eutectic from a larger quantity , the simplest procedure is to transfer the entire

amount onto a piece of paper (it does not freely wet paper) and

'cut' a section of it off with a teflon "policeman" (shown on the

extreme right of Figure 2). An ordinary knife or spatula can also

be used.

The presently described work utilized the Ga-Sn mixture, but

there is no apparent reason why the Ga-In mixture could not be used.

Both indium and gallium readily form alloys with the metal powders

listed in Table I. Since the lowest melting point mixture (eutectic)

incorporates /^^ 25% indium, a significant cost reduction can be

achieved. It has been reported that the In-Ga eutectic hydrolizes.

This is true if it is stored in 100% humidity but appears to be of no consequence under normal storage conditions. Such mixtures have

been used continually in this laboratory for six years (for use as liquid-metal electrodes), stored in small plastic containers that

7, Each tin powder particle is apparently protected by an oxide

coat and this severely slows its reaction with the gallium.
(4)

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(5)

were not hermetically sealed, and no adverse effects were observed.

Therefore experimentation with this eutectic is recommended.

In addition to indium or tin, other metals might add desirable

properties. For the case of dental amalgams, various quantities

of zinc, copper and tin are added to the basic mercury-silver com- position for such purposes as reducing oxidation, increasing hardening time, controlling expansion, and increasing the hardness. Most

likely, similar types of additions will be necessary for the gallium

alloys before they will be practical in many applications.

2.2 Selection of the Proper Alloy

The selection of the proper alloy for a particular use is

generally based upon the maximum anticipated operating temperature.

However, whenever possible a gold-based gallium alloy should be

chosen (see Table I). Alloy #5 is probably the most desirable.

It is very difficult, even for the beginner, not to succeed in

making a proper mixture when using precipitated gold (Au) powder.

With gold the problems arising from oxidation, such as with copper

and some other powders (discussed in Reference 3), do not exist.

Also, the Au alloys all expand significantly upon hardening thus

locking themselves into cavities. ° The weight percentage of gallium

is not critical and it can be varied continuously from about 15%

(a very "dry" mixture) up to 41% (a "wet" mixture).

Several workers have had difficulties when making the 41%

alloy (#6, Table I). Apparently it has not always hardened properly.

The explanation for this trouble is apparent from the phase diagram

of Au-Ga, reproduced for convenience in Figure la. About 42% Ga

results in an Au-Ga intermetallic compound. Additional gallium

will not modify this compound, thus leaving free gallium in the

interfacial region, resulting in little or no alloy strength. As

explained in Reference 3, some metal powder is usually lost before

mixing and some more of it is usually not wet with gallium during

the mixing. Thus the alloy may end up gallium-rich and never completely harden, or contain pockets of unhardened material in a generally hardened alloy. Also the addition of tin to the gallium

may modify the phase diagram. Therefore it is no longer recommended

8. Although gold-gallium alloys may expand 10% upon hardening, this

does not necessarily apply high pressure on the walls of a hole. It

appears that after filling all voids the expansion pressure is, in

part at least, relieved in the upward direction and usually produces

a mound at the top of the hole. The "wet" alloys appear to be better

in this respect than the "dry" ones. Some "dry" alloys such as #4, Table I, have been observed to crack ceramic tubes while hardening,
(6)

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that alloy number 6 (41% Ga + Au) be used unless both the measurement and mixing process are thoroughly controlled. The range of

30-35% of the Ga mixture + Au seems to be the most desirable when

using a precipitated Au powder. If the 41% mixture (#6) is used,

then any free Ga that is squeezed to the edge (or top) when applying

pressure (or packing in holes) should be removed as discussed in

Reference 3, thus accomplishing a decrease in the Ga percentage.

There is an additional precaution to be observed when using

the Au + Ga (or Ga-Sn mixtures) alloys. Large batches of this mixed

material, particularly when containing /v 41% of the gallium mixture,

may form an exothermic reaction and harden almost immediately upon

packing or applying pressure. Smith and Caul (NBS Dental Research

Section) measured temperatures as high as 450 C ( /Ay840 F) during

such a reaction on a/ 1 gram pellets of 40% Ga mixture + 60% gold.

This reaction is not likely to occur under the normal usage in

filling small holes (A^O.l gram capacity) or bonding flat pieces

of metal together, since there is ample opportunity to conduct away

the generated heat.

As a word of caution, one worker reported trouble with a similar

exothermic reaction when he mixed his gallium and gold powder in a shaker- type device. The violent shaking apparently generated enough

heat to start the reaction. Since no one has reported such a reaction

in the copper based alloys, a possible solution would be to use powdered gold-copper alloys. These can be obtained with various percentages of copper (Cu) and are sold as brazing alloys.

Regardless of the chosen metal or the percentage of gallium it

is necessary to consider the particle configuration (Precipitated,

spherical, or flake type) to arrive at the final properties of the

bond. This is discussed more fully under "Pot Life"o

2.3 Techniques of Mixing the Alloys

After choosing the desired alloy and weighing the ingredients

they must be properly mixed before a reaction can take place. Because

gallium wets almost anything it touches, a teflon beaker and stirring

rod are preferable but not essential. Figure 2 shows some of the

tools used for mixing and applying the alloys.

The beaker is shown in the center and the stirring rod is

immediately to its right. Note that the upper end of the stirring

9. Such powders used in present experiments were obtained from Wastern

Gold and Platinum Co., Belmont, California. These powders have a spherical particle configuration.

k
(8)

rod has been cut off flat. This was done since mixing is best

accomplished by tamping the gallium into the metal powder with only

occasional stirring. Several workers using only stirring, have

taken as long as 10 minutes to mix the ingredients. The total time

required to hand mix a typical batch (r\/0.5 grams) of Ga + Au

powder should be less than one minute for a beginner and ^v 1/2 minute after practice, although this will vary with the particle con- figuration. Gallium + copper powder usually takes about twice this

time.

The freshly mixed (tamped) alloy has a silvery appearance,

much the same as the original gallium mixture, although gold con- taining alloys will darken and turn bluish as they harden. The

object of mixing is to insure the wetting of each small particle

with the liquid metal, much the same way that a piece of copper is

tinned with solder. When this wetting is accomplished there is

intimate contact between the gallium and powder, and the alloy

reaction commences immediately.

Mixing of the ingredients can also be accomplished with a dental type automatic mechanical amalgamator. Smith and Caul^

reported that mixing by this means improved some properties that were of dental interest.

When freshly mixed the alloys will be of a granular nature,

depending upon the powder particle size, its configuration, and

the tendency of the particles to agglomerate. Alloys containing

a high Ga mixture (/V40%) might be described as being similar to

moderately wet cement. Others with a/ 25% gallium are simply grainy

and contain large agglomerates, similar to very dry, freshly mixed

cement. There is little tendency even for "wet" mixtures to flow

into cracks or voids similar to melted solder and brazing material.

When any of the freshly mixed alloys are packed into cavities

a portion of the gallium is squeezed to the top giving the mixture

the appearance of being much "wetter" than before packing. Thus,

even though a particular mixture appears too "dry" or grainy, it

may be completely satisfactory after the packing operation commences. When packing in the cavity has been completed it is usually desirable to scrape away or otherwise remove the excess gallium at

the top. This lowers the gallium concentration, which particularly

for copper based alloys, raises the melting point. This can be

seen from the phase diagram of Figure lb.

2.4 Pot Life and Metal Powder Configuration

After the components are thoroughly mixed there is a certain

amount of working time (pot life) before the mixture is too stiff
(9)

FIGURE 2 - Tools that are frequently used for

mixing and packing gallium alloys. From left

to right these are tweezer with a bent tip,

three standard dental tools, teflon beaker,

teflon stirring rod with the top cut off, and

a teflon policeman
(10)

for further use. For any given mixture this time is an inverse

function of temperature. However, each different metal powder

(gold, copper, nickel) has a different pot life. Also it appears

that powder made either by different processes or by different

manufacturers can have a greatly differing pot life. For example,

the gold used in the investigations of Reference 1 was a precipitated powder obtained from Fisher Scientific Company. It took

about 40 seconds to mix with the gallium (alloy #5) and had a pot

life of about 1/2 hour at room temperature. Subsequently, another

gold powder^, made by a different process, which could be mixed in

about 20 seconds (alloy #5) has been tested and gave a pot life of

2 to 3 hours with proportionally extended hardening times. Furthermore, use of the latter material resulted in a satisfactorily "wet"

mixture with only 20% of gallium, (With such a mixture no excess gallium squeezed out upon pecking and the pot life was about 1/2

hour.) There is a reasonable explanation for this. The conventional

type of precipitated powder has a mossy configuration and thus offers

a large surface area for a given nominal particle size. The other

powders consisted of spherical particles which offer a minumura of

surface area. The reader should determine which particle type is

best for his particular application. The spherical ones would be

best where higher ultimate melting points, longer pot life, or "wetter"

mixtures are required. Some manufacturers offer metal (powders)

that consist of tiny flakes. If these are not lubricated, they

would probably be a good compromise between the precipitated and

the spherical powders.

2.5 Packing the Alloys Into Cavities

Normally, portions of the freshly mixed alloys are removed

from the beaker (or amalgamator) with tweezers and dropped or pushed

into the cavity. Then a pointed metal tool is used to pack the

alloy firmly. Dental tools of the type shown in Figure 2, to the

left of the beaker, are convenient for this use but any long thin

instrument can be used. Some workers have used the shank end of

small drill bits. Others have successfully used blunted scribes

or small sized jewelers screwdriver blades. Wooden objects, such

as toothpicks, have been tried by several persons but without much

success.

At times it may be necessary to pack the alloys into delicate

samples that are suspended at both ends, such as when mounting

samples for thermoelectric measurements. It is rather difficult

to support the sample from the back side if ordinary dental tools

are used to pack the alloy, A simple and completely satisfactory

tool for this purpose can be made by bending one side of a pair

of tweezers as shown at the extreme left of Figure 2.
(LL)

FIGURE 3 - Method of preparing and mounting crude ceramic and

other samples. Figure 3A shows a 2x view of the samples held

in position with double-faced-adhesive tape. The wires are placed in holes that were cut with an S. S. White Industrial

Abrasive Unit, Figure SB shows the samples after they have

been packed with a gallium-gold mixture. The probe at the

bottom of the figure is a dental tool used in packing the

alloys

.
(12)

When planning to imbed wires in cavities, the samples should

be completely prepared before mixing the alloys. Figure 3A is a close-up (magnification 2X) of two crude polycrystalline samples

prepared for applying the soft alloy. A nickel wire is held in one cavity on each sample and a Chrome 1-Alumel thermocouple is in

the other. The samples are held in place with double- adhesive

tape. Figure 3B is an overall view showing final packing of the

ingredients. As the alloy hardens, it will expand and lock itself

and the wires firmly in the cavities, as discussed previously. The

prepared sample can be left for several hours to harden at room temperature or put in an oven at a/ 100 C for several minutes.

2.6 Bonding Surfaces Together

There are two types of bonding for flat surfaces of metals

and for some non-metals. The first is for metals that freely alloy

with gallium (obtained from Table I.), Of these, silver, copper,

and gold have been the most successful. It is possible to obtain

good bond strengths between such surfaces. A rough, clean surface

produces the best results. The surfaces must be completely wet by

the alloy. This is best accomplished by smearing the soft alloy over the surface. A flattened teflon, polyethylene, or other plastic

stirring rod (Fig. 2) is quite adequate for this. When properly wet,

the surface will have a silvery luster. The surface can also be

pre-wet with the gallium-tin mixture alone, if care is taken to wipe

off the excess (leaving only a very thin film) before applying the

mixed alloy.

The layer of alloy between metal pieces should be moderately

thick ( /V 0.01") in order to assure complete coverage. Very thin

uniform layers are difficult to obtain. Usually, attempts to bond

with such layers produced areas containing numerous air pockets and

thus weak bonds.

After the mixed alloy is placed on the metal surfaces they are firmly ©lamped together with a vice, c-clamp or other means and

allowed to harden. The best results (joint strength of 3000 - 4000

Ib/sq. in. measured in a tear test) were obtained when joining copper

pieces, using a gallium and gold alloy. Heat, (150 C, 300 F) was applied for 24 hours. This was done to facilitate gallium diffusion

into the metal surfaces. No tests have been run to determine the

optimum time and temperature for sufficient diffusion, and possibly

only an hour or so would yield similar results. It is possible that

the gallium diffusion into copper follows the usual exponential- timetemperature relationship common with other types of diffusion. This

would imply that good results could be obtained at room temperature

in an appropriate length of time. See the section on Bonding

Mechanism for a further discussion.
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High bond strengths do not result when materials that are not

readily wet with gallium (e.g., stainless steel and most ceramics)

are used. Here the bond is apparently of a mechanical nature and

will depend upon such features as surface roughness to obtain

adhesion. Abrasion, sandblasting, and etching are customarily used

to obtain such surfaces, but even with this the resulting bond

strength will probably not exceed 100 or 200 Ib/sq. in. in test.

An obvious alternative is to plate the uncooperating surface

with gold, copper, or silver. Only a minimum amount of work has

been done to verify this, but one precaution is evident. If the

plating is very thin (/VO.l mil) then there is a possibility that

the coat will separate from the substrate during the alloying process,

Probably a one or two mil (0.002") coat is safe, but verification

experiments should be run at the maximum anticipated operating

temperature .

Most non-metals can be treated in the same class with stainless

steel. However, both forsterite ceramic and glass-like lithium

fluoride crysta^.s developed stronger bonds than anticipated when

joined with a gallium-gold alloy. In both cases the samples were heated to 150 C for 24 hours. The forsterite had a rough surface

but the lithium fluoride was polished. One test on joining pieces

of dense alumina ceramic resulted in very low bond strength.

Many people have inquired about making aluminum-aluminum bonds

with gallium alloys. A number of attempts have been made to achieve

this result but without any success.

As a general procedure when using more or less brittle bonding

agents one should avoid simple butt joints. However, tongue and

groove (scarf and landed scarf) are satisfactory. Corner joints

should be of the end-lap or mitered joint type with spline. Right

angle joints that give rise to either peel or cleavage stress should

be made in the 'L' or 'T' design. Cylindrical slip joints (copper

plumbing pipes are a good example) are ideal for this type of bonding. Various lap-type joints are satisfactory. The limited number

of bond-strength tests have been made with the latter type.

2.7 Hardness of the Alloys

A few hardness tests were made on hand-packed hardened specimens of alloys but at best these are only indicative. Most of the

test samples contain discontinuities in sufficient quantity to lower

the hardness. (Machine or vice-compressed samples should be better).

Also the uneven distribution of gallium throughout a hand-packed

sample can result in hardness gradations along the profile. With

these qualifications in mind the following table is presented. All
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are Vickers hardness values measured with a 5 kg load by Wo T.

Holshouser.

Alloys

1

2

6

7

8

TABLE II

Constituents, Wt

447o Cu + 24% Sn, 32% Ga

50% Cu + 18% Sn, 32% Ga

59% Au + 41% Ga

33% Au + 33% Cu + 33% Ga

49% Au + 21% Ag + 30% Ga

(Footnote 10) 65% Au + 16% Cu + 1% Zn + 18% Ga

Annealed Copper

Annealed Yellow Brass

^Annealed Silver

Approximate

Hardness

68

58

40

40

56

128

73

112

38

The copper, brass and silver (annealed at 450 C, 1/2 hour)

are presented for comparison purposes. As an example of the variations encountered, values for alloy No. 6 ranged from 34 to 46, and one anomalously high value (96) was obtained for alloy No. 1. All

of the alloys appear at least as hard as brass by the scratch test.

This may mean that individual particles are harder and it is the

particle boundary that is weakest. Possibly because of this the

alloys are usually somewhat brittle; more so than would be indicated

from their measured hardness. If this is the case then appropriate

heat treatment should improve the ductility.

2.8 Mechanism of Alloying

No studies are known to have been made on the mechanism of

gallium alloy binding of the type discussed above. However, a few

brief comments can be made, based on the established concepts of

liquid phase sintering.

10. This experimental alloy was made by using spherical powders con- sisting of an alloy of 20% Cu + 80% Au (obtained from Reference 9.

These powders were mixed with 20% of a liquid made by mixing 5%> Zn and 5% Au in Ga. Probably as a result of the spherical particles

the maximum spread in Vickers hardness was only 5%.
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This sketch shows two gallium coated gold particles pressed together.

The short lines indicate the depth of alloying. The more gallium

the greater this depth. Thus, it is apparent that there can be a bond over a wide continuous variation in the amount of gallium present.

The interfacial bonding should be the result of (1) a small amount

of gold dissolving in the liquid gallium and (2) a crystallization

of some of the gold (as a gold-gallium alloy) in the interfacial

region, when the majority of the gallium has diffused into, and thus

alloyed with, the bulk of the gold particle. Thus a two step process

occurs, in which the gallium preferentially diffuses into (and alloys

with) the gold particle, leaving behind a gold-rich gallium solution.

When the dissolved gold concentration becomes too great, crystallization in the form of a gold-gallium alloy occurs. This nucleates on the particle surface and fills the interfacial region. Assuming

this mechanism is correct in the present case, bonds made at elevated

temperature should be stronger than those made in the room ambient,

because of the greater solubility of gold in gallium at elevated

temperatures. Therefore, better more uniform interfacial layers

would form and thus the bond strength would be greater. Such crystallization would be primarily dependent upon the maximum temperature

rather than time. For a good discussion of such mechanisms see for

instance the book Fundamental Principals of Powder Metallurgy by

W. D. Jones (I960) Chap. IV, the book Transistors Vol. I, P 28-31

and 112-120, Published by RCA (1956) or other texts on similar subjects.

Assuming this is the correct mechanism for the gallium alloying,

then for greatest bond strength the gallium-tin mixture should actually

be a gallium-tin-gold mixture containing the maximum amount of gold

soluble at room temperature. Also, alloys made from spherical particles, with their longer hardening time, can be raised to higher temperatures before hardening. Therefore, more gold would dissolve in the

gallium and a stronger interfacial bond should result.

The above comments may serve to help persons who are not metallurgists to better understand the present alloying processes. The

comments apply equally well to the non-gallium containing alloys

described in Section II.

2.9 Sources of Gallium

Gallium is a rare, little known element, thus the most frequent

question received was for a list of suppliers.

Five grams of gallium is probably the absolute minimum required

to evaluate the present techniques and 10 or more is desirable. It

is not recommended that 1 gram be purchased for this purpose. Some

manufacturers will not sell less than 100 grams and others will sell

as little as 5 grams but in general the prices drop significantly for
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amounts of 100 grams or more. The following is an alphabetical

listing of the manufacturers (or suppliers) of gallium. It represents

all of the sources in the United States that this writer is familiar

with, although there may be others. Presumably any metal or scientific

supply house will obtain gallium for special orders but few have it as a stock item.

AIAG Metals Inc.

9 Rockfeller Plaza

New York 20, New York

Fisher Scientific Company

711 Forbes Avenue,

Pittsburgh 19, Pennsylvania

Aluminum Company of America

2900 Missouri Avenue,

East St. Louis, Illinois

Materials for Electronics, Inc,

152 - 25 138th Avenue,

Jamaica 34, New York

Eagle-Picher

Rare Metals Department

Miami, Oklahoma

United Mineral and Chemcial Corp,

Metals Department

16 Hudson Street,

New York 13, New York

2.10 Physiological Properties of Gallium

One of the more frequently asked questions pertains to tne

physiological properties of gallium metal. Since this is a liquid

metal, many people associate it with mercury and assume it to be

similarly dangerous. No tests have been performed in this laboratory

to verify or contradict this, and it is not our function to do so. However, gallium metal has been used freely without precautions

(except perhaps to wash the hands after physical contact) in this

laboratory by many persons for over 6 years with no apparent harmful

effects. There have been a number of studies on the subject that

are more authoritative than uncontrolled personal observations, and

the interested reader may refer to them. ^1> ^2, 13, 1h, 15, 16

11. Mineral Yearbook, 1310, (1949).

12. Dudley & Levine, Studies of Acute Toxicity of Gallium, Naval

Medical Research Institute Report #3 (1/13/49).

13. Brucer et. al,, A study of Gallium 72, Radiology 6JL 534-613 (1953)

14. Meek, Harrold, and McCord, Toxicity of Gallium, An Exploratory

Investigation; Industrial Medicine, 2^, 7 (Jan. 1943).

15. J, L. Hartley and N.O, Harris, A Comparative Evaluation of Tissue

Tolerance to a Gallium Alloy, Air University, School of Aviation

Medicine, USAF, Randolph AFB, Texas: Report #58 - 148, (Nov. 1958).

16. H. C. Dudley and M. D. Levine, Studies of the Toxic Action of

Gallium, J. Pharmacology & Experimental Therapeutics V.95, 487 (1949).
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3. Alloys That Will Harden in Higher Ambient Temperatures

Since gallium is expensive and its alloys must be mixed immediately before use (or else refrigerated), it is desirable to consider

other possibilities, where higher forming temperatures are permitted.

In such cases premixed ingredients, which will keep on the shelf for

years can be used. These can be in the form of prepared metallic

strips and other preformed shapes which can easily be inserted between

components, the appropriate pressure and heat applied, and a bond

will be formed.

If ambient temperatures up to about 250 C can be tolerated,

then a large number of mixtures suggest themselves as capable of

being used as the solder. In each case, however, the metals chosen

must alloy with one another and one must be liquid and one solid at

the chosen bonding temperature. Again, the solid constituent of the

bonding materials will be a finely divided powder. For best results,

the metals to be bonded should be those which will actually alloy

with the liquid constituents of the bonding agent. The following

table sets forth some examples of satisfactory bonding materials,

where the liquid constituent is in liquid phase at temperatures

significantly above room temperature.

TABLE III

1.

Metals

In-Au

Useful % Range

of Low m.p. Met£

537o - 30%

il Ha: Approximaterdening Ambient

1750c

2, In-Cu 52% - 30% 175°C

3. In-Ag 33% - 30% 175°C

4. Sn-Cu 55% - 30% 250°C

5, Sn-Au 50% - 30% 250°C

6, Sn-Fe 68% - 30% 250°C

7. Sn-Ni 55% - 30% 250°C

Approximate Useful

Temperature Range

4950to 350OC

700°to 650°C

600°to 550°C

750°to 400°C

600°to 400°C

900°to 800°C

1250°to 1150°C

In general the best results are obtained by using combinations

in which the low melting component of the bonding agent makes up

from 40 to 60% of the entire composition with 30% set as a minimum

limit. This is done to assure uniform wetting of both the higher

melting powder and the pieces to be fastened. This percentage may

possibly be lowered by further experimentation.

Just as with gallium, suitable preparations of indium or tin

may be desirable in order to lower the curing temperatures. Typical
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examples of this eutectic type depressing of the curing ambient are

as follows:

(52% In + 48% Sn) plus Au, Cu or Ni powder cures at

about 130°C.

(32% Cd + 68% Sn) + Au, Cu powder cures at about 200°C.

Ternary and quaternary alloys) + appropriate metals.

There are undoubtedly other combinations that may be useful under

various circumstances.

The procedure is to mechanically mix the two (or more) components

at room temperature. Where indium is used the high melting metal

powder may be kneaded into a solid strip of indium , or the powders of

indium and other metals may be mixed and pressed into thin strips.

Either the strip or the mixed powder is placed between the metals

to be bonded. An appropriate low temperature soldering flux (not

a brazing flux) is applied. In some cases it may be desirable to

"wet" or "tin" the metal pieces with a very thin coating of the

low melting poinfe constituent. To insure the best reaction the

components are clamped under pressure and the entire unit is placed

in an oven at the appropriate temperature for the chosen alloy.

With two exceptions, the above materials have received only

cursory tests and therefore additional experimentation would be

required before their use. The exception was alloy #1 and by infer- ence alloy #2. Two one-half inch square bars of copper were bonded

together using precipitated gold powder which was kneaded into a strip of soft indium metal (50% indium and 50% gold). The resulting

strip was placed between the copper bars, which were wet with a soft

soldering flux (Division Lead Company flux #335). The bars were held together with a C-clamp and placed in an oven at A/ 200 C overnight. The resulting bond was considerably stronger than 5000 Ib/sq.

in. and very obviously broke by peel rather than tensile failure

since the bars bent in the testing machine. The melting point from

the phase diagram (assuming m 50 wt% of both In and gold in the

bonding alloy and no more than 30% In available for alloying with

the copper surfaces would be a/ 500 C. Indium has a tensile bond

strength of only a hundred or so Ib/sq. in. thus it is obvious that

both the gold and copper formed hard alloys with the indium resulting

in the strong bond. There was no effort to determine the minimum

time and temperature possible for forming such a strong bond.

4. Conclusions

This report with Reference 1, 2, 3, and 4 represent all of the

published knowledge presently available on industrial and scientific
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uses of dental- amalgam- type gallium alloys for bonding purposes.

There are undoubtedly many applications where the use of the alloys

may not be practical, but for the many others where it is, sufficient

details have been presented to allow persons with any type of background to evaluate the use of these alloys in their particular problem.

In addition it is hoped that these works will create an interest in

fundamental studies of the type of bonding mechanisms involved and

result in improved characteristics and simplified techniques. With

greater consumption the price of gallium should decrease and make

these processes available to a wider range of uses.

U. S. DEPARTMENT OF COMMERCE

Luther H, Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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