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Produced by the NASA Center for Aerospace Information (CASI)
(NISA-CR-175440) TERRESTRIAL WHISKER GROVTH H84--20647

EXPERIMENTS NHICH ANTICIPATE SORE SPECIAL

EFFECTS OF A SPACE STATION ENVIRONMENT

Final Report (George Vashington Univ.) 21 p Unclas

HC A02/RF 101 CSCL 11D G3j24 00650

NASA Contract # NASW--3754

Entitleds

Terrestrial Whisker Groxth Erperisents Which Anticipate

Sam Special Effects of a Space Station Environaent

Submitted by:

Berson H. Hobbs

Professor of Physics

Principle Investigator

Contractor;

The George Washington University

Office of Sponsored Research

21U I St. N. W. Washington, D. C. 20052

Signeds

Herman H. Hobbs
2

possible to take over, with only minor codifications, the prooedures described

in Appendix I. That type apparatus, with suitable safety precautions, may be

used but with electric fields Just strong enough to promote alignment of the

whieker leaders. A strong pulse of electric field may be occasionally applied

to the plates to extract the whiskers ft on the molten substrate and greatly

facilitate harvesting the growth.
i

Part I: Summary of Results

Primary Goal:

The primary goal of this research me to determine, using methods and apparatus developed by the principal investigator, whether the absence of gravitationally driven thermal convection (to be expected in the space station environment) would prevent or drastically effect grofrth of whiskers by chemical

reduction of metal salts. This goal has been achieved in that it has been

possible to accomplish nearly complete suppression of such convection here in

our laboratory. The results of experiments-carried out under this project

show that suppression of the convection does .indeed effect the growth but

in subtle,-not necessarily detrimental ways: none of the changes observed are

such as to hamper efforts to produce whiskers in space!

Other Goals:

While all other goals in this project were addressed, and some measure of

progress made in each, it should be kept in mizA that the period of performance

of this contract was extremely short and required that precedence be )Pen to

the primary goal. Many aspects of this work are of profound and fundamental

scientific interest and it is hoped that NASA may see its way clear to extend

support on a more long-term and appropriate basis. The following is a short

statement on each of the remaining goals.

1.'Deteraine which materials show most promise for growth in gravity-free

environment"

The experiments, thus far, suggest that copper whiskers grown frog cuprous

iodide respond most positively to the suppression of convection; therefore, they

are strongly recommended for tests in the space environment. Ofi the other hand,

cobalt whiskers grown from cobaltous bromide show the greatest independence

from conditions of convection and applied electric fields of any material studied=

therefore, this medium is highly recommended.

2. 'Study the electric currents accompanying the growth in applied electric

fields".

This effect, quite possibly the most important from a long-term scientific

point of view, was necessarily given secondary importance in this project because

of the shortage of time. The most dramatic result is a clear demonstration that

these currents are not simply a side effect.of the large convection loop in this

growth apparatus. Currents were measured whenever possible and the effects on

them of suppression of the convection carefully noted. The dffects are, again,

subtle rather than drastic, and are shown in the body of this report.

3. " Formulate procedures for growth in gravity-free environment".

The lack of any drastic effect due to suppression of the convection makes it
3

Part II. Description of the Zxperi.sents

Appendix I of this report is a reprint of the paper which describes the

method developed by the author for growth of whiskers by chemical reduction in

applied electric fields. The results described herein were obtained using that

method with suitable modifications and extensions= for example, the convection

studies apply to the manvection loops occuring in the Vycor growth tubes described in that paper etc. It is therefore suggested that the reader first

peruse Appendix I unless already familiar with that work. Background and

contemporary references for this work will also be found in the bibliography

of that paper.

Evidence for Existence of Strong Convection Currents in the Growth Apparatus.

The author first became aware of the strong convection loops in this growth

apparatus upon noting the spiral patterns which occasionally appear at the

entrance and/or exit of the furnace. These patterns are produced only during

very long runs, are sometimes very faint, and are deposits of growth source

material (such as cuprous iodide). They .represent intersection of flow patterns

with the walls of the tube, but the impressive thing is that the cr3°omens of

the pattern which is laid down very gradually (runs lasting over I hour) suggests

that the currents must be very very stable! Figures 1 and 2 show such patterns

and also show that scale is not a factor, since the growth tube in Figure 2

has twice the diameter of that of Figure 1. The convection loops were studied

as a part of this project and a rough description of the loops gleaned from

visual observation is shown in Figure 3. The currents were rendered visible by

releasing small quantities of ammonium chloride at various locations inside the

furnace. This decomposed forming a white "smoke" which rendered the flow patterns

visible. Estimates of flow velocities were•made by observing the flight of individual particles of ferrous chloride released during the growth of iron whiskers.

These particles are thin plates which "float" along the flow lines and provide

very graphic views of the convection currents. The convection loop is roughly

the same at entrance and exit, but the division between the two loops is not

not necessarily at the exact center of the furnace. Figure . 5 illustrates the

kind of photographic data available in this apparatus. The flow had maximum

speed near the top and bottom of the tube in the region -labeled A in Fig. 30

Typically= Speed of flow in region A -----------------5 to 15 cm/sec

Gas in tube--------------------------------Nitrogen

Linear flow rate of gas-------------------- 0.062 cm/sec (3.7 cm/min)

Furnace temperature 700 oC

Tube diameter----------------------------- -45 mm
4

ORIGINAL PACE 19

OF POOR QUALITY

Figure 1. Spiral patterns at entrance to fumace

on Vycor growth tube ( 25 nun I. D. tube).

how

Figure 2. Spiral patterns at entrance to furnace

on Vycor growth tube ( 45 mm I. D. tube).
5

ORIGINAL PAGE ^"S

OF POOR QUALITY

A

Furnace

Gas

^ uas in

ou

H

Furnace

Figure 3. Flow lines deduced from observations of smoke patterns, and

larticles drifting with the flow. G.is the growth region.

Furnace

Gas

out

IFurnace I

Gas

in

Figure 4. Sketch of growth tube in furnace with porous plugs. (Porous

plugs are the stippled objects.) G is the Growth region.

Plugs are made of compressed alundum fibers.
Side View

Bottom View

5

ORIGINAL PAGE 19

OF POOR QUALIV

Figure 5. Patterns traced in downstream end of 45 mm Vycor growth tube

using "smoke" from ammonium chloride injected into the growth region.

In these pho-^ographs "downstream"is to the right: that is, gas exits

the tube to the right, and furnace with growth region is to the left.
7

OMG;NAL Pei •:„ :<.

OF POOR-'Y Y

Suppression of the Convection Loop.

The cause of the main convection loop is 1he tremendous thermal

gradient encountered by the gas as it enters or leaves the furnace.- During the

growth of cobalt whiskers, for example, the temperature drops about 500 degrees

in the last 10 cm before the gas exits the furnace. Suppression of the major

part of this loop is easily accomplished, in principle, by the insertion of

porous plugs in the growth tube as shown in Figure 4. The currentia between the

porous plugs were not observed but are assumed to be very small by comparison

to the unsuppressed currents. The thermal gradients in the central regions of

the furnace were measured &-id found to be less than 0.05 0C/cm with the porous

plugs in place.

In practice insertion of the plugs was found to be extremely difficult with

the original apparatus (as in Appendix I) because of the two thermocouple wells

and the delicate growth plates affixed to their ends; accordingly, a new configuration was developed using one cylindrical electrode which just fits inside the

growth tube and a single graphite-coated thermocouple well on the axis of the

cylindrical electrode. The growth material is placed on the bottom inside portion

of the cylindrical electrode and growth proceeds much as before. Of course, this

step complicates estimates of the applied electric field which increases in intensity

as the axis is approached. Electric fields quoted here are rough values and

apply to the region near.the surface of the cylindrical electrode (where the whisker

leaders appear). Whiskers will now be growing into stronger electric fields

as they increase in length. The porous plug is now moulded around the single

thermocouple well and inserted into the growth tube after the cylindrical electrode end

with its growth material is in place. The plug in the downstream4is easily inserted.

The porous plug which contacts the thermocouple well must be changed after each

growth run since it also contacts the wire which connects the cylindrical electrode

to the outside of the growth tube. It tends to become contaminated by the vapor

of the growth material as well as by reduced metal and gives rise to spurious

electric currents. More than 100 runs were made in various versions of the apparatus

with and without plugs, and with various growth materials. The results are

deemed sufficient to put to rest any notion that absence of the large convection

loop would stop or even hinder the whisker growth process. The effects were

clear and observable, but subtle and different for different growth material. A

summary of results are shown below. Effects of convection suppression on growth

currents will be shown in a later section of this report.

I :, t
8 

Effect of Suppression of Convection on Whisker Yield

Copper whiskers grown firm cuprous bromide ---------Yield of long fine whiskers

slightly better. Total

number in yield about the same.

Copper whiskers grown from cuprous iodide ----------Yield of long fine whiskers

dramatically improved.

Cobalt whiskers grown from cobaltous bromide--------Yield of all types about the

same with or without convection.

Other Methods of Suppressing Convection.

As anentioned in the proposal, it was also considered useful to try to change

the convection by using the growth tube and furnace in a vertical configuration.

An apparatus was constructed using a cut-down Vycor tube, vertically oriented

in a crucible furnace. Even though it worked, after a fashion, and actually

grew cobalt whiskers, it had special problems and was too time consuming (runs

took about 4 hours each) for the time frame of this project.

As a final check on the porous plug experiments, a few runs were made in

the original configuration (as in Appendix I) but with a Vycor cylinder sandwiched between the growth plates. This completely isolates the growth region

from any currents in the main tube (see Figure 6).The runs made with this

apparatus showed results quite similar to those with the porous plugs and so

will not be discussed further here. Although this scheme was conceived late

in the project and not used extensively, it is the neatest, the most convenient,

and will probably be used in future studies.

Attempts to Extend the Range of Available Growth Materials.

A number of new materials and procedures were tried and,while some showed

promise, the shortage of time required that they be set aside as temporarily

unsuccessful: these were, nickelous bromide, chromous chloride, and cobaltous

chloride. In addition, a procedure using methane as a reducing agent in place

of hydrogen was tried but set aside without notable success. Silver whiskers

were produced from silver bromide, and actually constitute a "first" since that

source material has not been used before, but again time did not permit further

development of that material.

- rf ^.^ J . J I ♦ ^. , -• _' - - - _sue
9

ORIGINAL PAGE 19

OF POOR QUALITY

!

r'

fi P ..

v ^ss^r

C1-f r -N.

Figure 6. Sketch of Vycor cylinder.V, enclosing the growth region. Cylinder

diameter. is 25 mm I.D. The metal plates P (as in Figure 1 of Appendix I) are

in contact with the ends of the cylinder. Gaseous exchange between the growth

region and the ambient gas flowing through the growth tube is by diffusion

through two slits (only one shown in sketch) in the side of the cylinder.

,
10

Production of Lon g Cobalt Whiskers.

Since Brenner's definitive work it is commonly assumed that cobalt is not

a good medium for whisker work because of the impossibility of growing cobalt

whiskers longer than about one or two millimeters (See Refs. 1, 2, 3, 4 of

Appendix I). This project has produced a new approach which produces cobalt

whiskers which are very fine and frequently longer than 10 millimeters. The

surprising thing about the cobalt is that once the crux conditions are established,

the drive to remain in the whisker mode is stronger than in many other materials.

The crux is twofold: first, the hydrogen must be withheld from the growth region

until the growth material (cobaltous bromide) has molted, and second, the hydrogen

is introduced only at very low partial pressure ( 5 % of normal atmospheric pressure, or less). The procedure developed here will be the subject of a paper to

be published as soon as possible, but for users of this report the detailed

procedure is as follows:

Using the cylindrical. electrode set-up described earlier, start with

140 milligrams of cobaltous bromide (Spex UHP) spread over 2 square centimeters

of the center portion of the cylindrical electrode (at bottom of growth tube).

Purge the growth tube with pure nitrogen (Carrier grade UHP) and with the growth

region outside the furnace (as in Appendix I) bring the furnace to 700 oC. Now

insert the tube so that the growth region is. centered. in the furnace and

monitor the temperature. As the temperature passes through the melting point of

cobaltous bromide (678 oC in nitrogen) st•^rt the flow of hydrogen at linear

flow rate of 3.7 cm/minute (in 45 mm I. D. tube). Growth will occur whether

or not the tube has porous plugs, and whether or not an electric field is applied 

across the electrodes. The best crops occurred in this laboratory with an

applied field of about 100 volts per cm (near the surface of the cylindrical

electrode).

As mentioned above, one very impressive thing about the cobalt growth is

the tremendous tendency to maintain whisker-like (high aspect ratio) growth

cnce proper conditions have been achieved. This tendency is perhaps exemplified

by some unusually long and not very pretty specimens which maintain the growth

in spite of kinks, branches, changes of orientation, and growth in a tight helix.

One such specimen required 30 photographs at the magnification of Figure 7 to

tell its story. It had a length of nearly 2 cm, which is something like 5000

times its diameter. Figure 7 is an example of this persistence of the whisker

form.
^,/€ .^^-^Y '. .^ tL- Y1 / •tea{^ `Y

OF POOR

11

Figure 7 . Highly magnified view of a portion of•a cobalt whisker.

At the left side the whisker has just changed from straight (not

shown) to a low pitched helix. After two pitch changes and a

short straight portion, there is a wiggle, then a l e ig straight

portion followed by several more helical modes etc.
12

Studies of Currents Accompanying the Growth.

These currants, Y:eferred to as "growth currents", remain a mystery as

discussed in Appendix I, but their behavior has been monitored wherever possible

and has been further explored during i,he course of this work. The most valuable

contribution of this project to their understanding is, of course, the clear

demonstration that they are not a direct result of the convection loop. Sup<<-ession

of the convection changes them subtly, but they persist and in fact are somewhat

stronger in the absence of convection. They remain an excellent indicator of

the state of the growth process during a given run: virtually no growth has been

observed after the current passes through its maximum point. The shape of the

"growth current cur.-, " has a shape characteristic of the so;irce material: a sort

of "signature". It is even sensitive to the "batch" of source material, as is

the whisker growth itself. The cylindrical version of the growth apparatus is

better suited to studying these currents than the parallel plates and gives

more --, eproducible results. The currents shown in "Figures 8 through 13 illustrate

some of these effects. They are "typical" currents: that is, rer-roducible providing

everything else stays the same and there are no "glitches", g,.ch as leakage ac ;^-._s

insulators, whiskers growing across the gap and shorting. open circuits along

the graphite coating on the thermocouple well, etc. Some 100 runs were made and

the tapes are on hand for further study.
13

parks on Figures 8 through 13.

;ores 8, 9, and 10 are tracings of dual pen recorder tapes of runs

M ^^, W z25, and # 2270 ?that order. A11 started WITM 140 milligrams of

cuprous bromide in hydrogen flowing at a linear flow rate of 3.7 cm per minute.

All were with applied electric field of roughly 100 volts/cm directed vertically

apward (bottom plate positive), and all in the 45 mm I.D. growth tube. Figure

8 is with the convection loop present, Figure 9 is with the convection loop

suppressed ( porous plugs in place) and Figure 10 (which should be the same as

Figure 8) is with convection loop preser.t but using cuprous bromide from a different

batch of starting material.

Figures 11, 12, and 13 are a similar series. They are runs # 252, # 255,

and # 279• Each used 140 mg of cobaltous bromide in diluted hydrogen admitted

in accordance with the schedule described in the text. Figure 11 is with

convection loop present, Figure 12 is with the convection loop suppressed (plugs

in place), and Figure 13 is same conditions as Figure 11 but using a different

batch of material. The material in the runs of Figure 11 and 12 is Spex U.H.P,

and grew excellent whiskers, That in run of Figure 13 is Fisher Purified and

grew a very poor crop.

In all of these figures the temperature pen is offset from the current

indicating pen by 1.1 cm which amounts to about 1 minute at the speeds used

in these runs. This means that the temperature graph is 1 minute ahead of

the corresponding current reading. To synchronise temperature and current the

temperature graph must be displaced 1.1 cm to the left.

In the cobalt run of Figure 12 the large residual current remaining

after normal growth time has expired is caused by contamination of the alundum

of the porous plug by the growth material and by reduced cobalt. This is a

serious drawback to any critical analysis of currents with the plugs present.
W

14

ORIGINAL PAGE 19

OF POOR QUALITY

Tempejlture (Each unit represents 100 oC) (Broken Line)

Growth Mbrrent (Each unit repsasents 3nC& 7 amperes) Solid Line

r
,%Vv

15

OPIC31 iraL Fad

OF FLOOR Q AU i Y

Temperature (Each unit represents 100 00 (Broken Line)

Growth current (Each unit represents 3X10 amperes) (Solid Line)
N

O^

OJ

N

O

F-^ N

N

rn

OD

16

ORIGEN!AL P,,qGE r3 OF POOR QUALITY

Temperature X 100 0 (Broken line)

Growth Current(3 X 10-6 amperes)(Solid line)

N W ^ In O%

1

1

1

1

p

' ON

O ^n

1

i

i red, Fisher batch #2

J
1

17

r OF Pool Q, ,-,AL ,1-y

Teaperature ( K 100 0g) (Broken Line)

Gr6wth Current ( X 10 amperes) (Solid Line)
18

ORIGINAL PACA: [S'

OF POOR QUALI i Y

Temperpture (X 100 ° )

Growth Current (X 10 amperes) (Solid line)
N

ON

CO

N

O

N

N

Cn

N

CO

^T 19

ORIGINAL PACE 6

OF POOR QUALITY

Temperature ( X 100 oC ) (Broken Line)

Growth_Cu =ent (-X 1076 amperes) (Solid Line)
20

as

w.Lj.%;= ouyyrcooivu viL bile convection subtly modifies but does not prevent, or

even noticeably hinder, whisker growth, we may conclude that the absence of

gravitationally driven thermal convection in the space station environment presents

no obstacle to experiments such as those conducted in this project. Clearly,

the procedures outlined in Appendix I could be followed without serious modification.

The applied electric fields can be weaker by, perhaps, an order of magnitude; just

sufficient to provide alignment of the whisker growth leaders. One additional

step, which has been tried in this laboratory, is to apply a strong pulse cf

electric field to "snatch" the whiskers from the molten substrate. The "snatched"

whiskers rise to the top plate and stick there (probably spot welded by their own

residual charge) in a perfect position for study or for "harvesting".

The ready vacuum which is available just outside the space laboratory could

be a real asset, especially in the case of growth of cobalt whiskers. These

whiskers grown in highly diluted (low partial pressure) hydrogen. Such dilution

is the cheap and safe way to achieve low partial pressures in the terrestrial

laboratory. On the space station it is recommended that the growth vessels, with

their furnaces be mounted outside the laboratory; communitating with the inside

by means of double vacuum valves (airlocks) and gas service lines. The pressure

in the vessels is then controllable by simple reducer valves exhausting to the

zero pressure of space. Note that this is quite safe, even compared to earth

laboratories, since all gas pressures are positive and all leaks are to space!

On earth working with hydrogen at reduced pressures and high temperatures is

highly dangerous because all leaks are air and are into the apparatus itself.

Locating the growth vessels outside the spacecraft also has the advantage of

lowering the power requirements of the furnaces. Another safety consideration

in the space laboratory is release of noxious flumes ( all of these growth

experiments release poisonous gases) which will be neatly and completely taken

care of by mounting the growth vessels in space. Exhaust at positive pressure

and at the elevated temperatures involved in this growth will drive all gases

clear of the vicinity of the spacecraft.

Whisker growth experiments in space will take their priorities from many

scientific as well as non-scientific considerations. From the point of view of

this worker they are highly recommended and his services are offered herewith

in any capacity if the decision is made to include them in the program.

RW .