AMERICAN INSTITUTE OF MINING AND METALLURGICAL ENGINEERS Technical Publication No. a301 - - Class B, Metals Technology. June, 1948

DISCUSSION OF THIS PAPER IS INVITED. It should preferably be presented by the contributor in person at the Philadelphia Meetirig. October 1948, when an abstract of the paper wjU be read. If this is impoa- slble, drscuss?on rn wntlng (a coples) may be sent to the Secretary. Amer~can Inatltute of Mining and Metal- lurgical Englnqers, a9 West 39th Street. New York 18. N. Y. Unless specld arrangernent.is made, discussion of this paper w1U close Dec. I. 1948. Any discussion offered thereafter should preferably be In the form of a new paper.

An Electron Diffraction Study of Oxide Films Formed on Copper-nickel Alloys at Elevated Temperatures BY J. W. HICKMAN* MEMBER AIME AND E. A. G ~ R A N S E N *

(Philadelphia Meeting. October 1948)

INTEODU~~ION nickel at which composition Uhlig2 (quot- b RECENT work by the authors1 on the ing La (he's data) states that the alloy

oxide films formed on alloys of titanium and Sy~tem t~comes passive. Magnetic and

zirconium nickel and copper indi- specific heat data indicate that the d band

cated that an investigation of the oxides of co~~er-nickel alloys is filled at 60 at. Pet which form on alloys of copper and nickel copper or 4O at. Pet nickel. might permit the relative rates of forma- Matt and JonesS have shown that if, on tion and diffusion of the ions of these metals alloying nickel with copper, one considers a

to be determined. This binary system is of copper atom to be identical with a nickel - further interest since the alloys which atom except for an extra electron (nickel form are of the solid solution type over-the has the co!!figur!!tion: 3d8 4s"nd - copper: - .-

entire range. In addition nickel is mu& 3 dl0 4s) then a t 60 at. Pet copper or 40 at-

e more oxidation resistant than copper. pct nickel enough electrons are contributed A comparison of the free energies of by copper to fill the d band of the alloy.

formation of the simple oxides may At this composition the magnetic suscepti- occur on these alloys indicates that bility approximates zero according to oxide should be observed over the entire Grew.' time and temperature ranges of oxidation Although the electron diffraction data for all alloy compositions.~ In order for wil l not determine the rates of the several nickel oxide to predominate on the surface, reactions which may occur during it is necessary that nickel ions form tion, yet a determination of the structures and diffuse to the surface. Two factors of the surface oxides may ~ i e l d informa-

* which may influence the formation of tion concerning the relationships, if any, nickel oxide may be the ionic sizes, where which may'exist between the oxide strut- Cu+ is o.g6A and cNi++ is o.70A, and tures which form and that composition

A the charges on the ions. On the basis of where the magnetic susceptibility approxi-

size, Ni++ should diffuse more than mates zero. I t is quite possible, of course, Cu+ while on the basis of charge, c u + that the mechanisms of the oxidation and should form more readily than Ni++. corrosion reactions may be so widely

According to Uhlig2 passivity to corro- different that no correlations between them sion in an alloy appears a t a critical compo- may be possible.

sition which for copper-nickel alloys is SURVEY OF LITERATURE to the of the d band of ~h~ metals, and ni&el, which are

electronic energy states to fill with elec- face-centered cubic, form solid ,solution trans- This composition in the copper- which are face-centered cubic with 4' at. pct no superstructure for all compositions.

Westinghouse Research Laboratories. Three oxides of nickel have been investi- East Pittsburgh Works. East Pittsburgh, Pa.

a References are at the end of the paper. gated structurally by X ray diffraction. Of Copyright, 1948, by the American Institute of Mining and Met-allurgicd Engineers. Inc.

Printed in USA

D N STUDY OF OXIDE FILMS 2 . ~ . . . . . . AN ELECTRON: DIFFRACTI(

these N ~ O is the most stable over a wide temperature range. Preston' has investi- gated the structure of oxide films on nickel using the electron diffraction reflection technique. The only oxide observed was NiO with a lattice parameter of 4.1oA. Darbyshire6 found that films of oxide separated from the metal by electrolytic corrosion were structurally identical, with ' NiO in the massive form. The transmission patterns gave a lattick paiameter of 4 . 1 ~ 4 in' good agreement with the results' ob-

, tained by Preston.' Gulbransen and Hick- man7 have studied the oxidation of nickel from 300 to 7o0°C and report that NiO is observed over the complete temperature range.

Germers has identified the oxidation product occurring on copper as cuprous oxide. Smith0 has investigated the struc- ture of thin films of cuprous oxide and has found the oxide to be cubic with a lattice parameter of 4 . d which is slightly higher than the reported X ray value of 4.24A. A systematic study of the oxides which form on copper over the temperature range 100 to po°C, reported by Gulbransen and Hickman7 shows only the presence of cuprous oxide with an average lattice parameter of 4.32A.

No systematic study of the oxides occur- ring on copper-nickel alloys has been made using the electron diffraction technique. Miyakelo has studied the oxides which form on the following alloys: 70 pct Ni, 30 pct Cu; 40 pct Ni, 60 pct Cu and 7 pct Ni, 93 pct Cu. He has reported that only a CuO film was found on these alloys but forma- tion of NiO beneath the uppermost layer was suggested.

Frohlich,"* who has studied the oxida- tion of pure and alloyed copper, states that the rate of oxidation of pure and alloyed copper is independent of the method of

The authors feel that Frohlich's work has a connection with their experimental results since the statement is made that alloys of copper and nickel form a copper rich zone containing oxide inclusions of nickel.

preparation and pretreatment and of the oxygen pressure between 45 and 760 mm of mercury. Apparently copper diffuses into the CuxO layer since normal CusO does not have all the copper lattice positions occupied while analysis shows an excess of copper in the oxide layer formed by diffu- sion. In most alloys the region between the base metal and the oxide layer is enriched . in the alloying element. However, Ca, Cr, Li, Mn, Si, and Ti form a well-defined layer of their own oxide. Alloys with Al, Be and Mg are easily oxidized while alloys with B, Si, Sn or Zn, whose oxides restrain diffusion, resist oxidation. Frohlich" states that metal diffuses into the oxide layer but oxygen also diffuses into the alloy as is shown by the formation of a copper-rich zone containing oxide inclusions of Mn, Ni, Si, Ti or Sn.

A complete description of the apparatus used in this study is reported by Gul- . bran~en.'~.'~ When the reflection electron diffraction technique is used to study oxide surfaces, the preparation of the metallic surface prior to oxidation is important in determining the quality of the patterns obtained. Experiments have shown that a clean, flat, slightly matted surface, which can be penetrated by the electrons gives sharp, intense patterns.

- Samples, as received, 'in the form of rods

of 0.375-in. diam are cut to 0.375-in. lengths. These are given a surface finish *

using the precision abrader13ending up with 4/0 emery paper. Oxidations are carried out

.

after the abrading treatment without any other pretreatment such as annealing or etching. P d e d oxygen under approxi- mately I mm pressure is used in all the oxidations. Electron diffraction photo- graphs are taken in the vacuum of the camera before oxidation; after I, 5, 30 and 60 min. oxidation; and after cooling to room temperature under approximately 0.05 atm pressure of hydrogen.

J. W. HICKMAN AND E. A. GULBBANSEN-TP 2391

OXlqE STRUCTURE

10 x NIO

TYPE OF PAlTEW o = ORIENTED $-.SHARP

I U* MEDIUM D-DIFFUSE

I I I I I I 10 2 0 3 0 40 50 60

TIME (MIN.)

FIG I-OXIDE FILMS ON NICKEL.

TYPE OF PATTERN 0 . ORENTED s = SHARP Y = MEDIUM D = DIFFUSE

FIG OXIDE FILMS ON 29.30 CU-NI.

Oo0-

loo-

-- soo-

1%

400-

3w-

200-

100-

XX X X

XX X X

xx X X

03 @

a 0 @

@@ 0 0

OXIDE STRUCTURE frPE OF PATTERN X =NIO o - ORIENTED

8 -SHARP 0 =GuLO u =MEDIUM o =DIFFUSE .yo

I I I I 0 I o m 3 0 4 0 # ) 6 0

TIME (uln.)

FIG 3-OXIDE FILMS ON 19.94 CU-NI.

'700-

600-

m-

1%

400-

zoo-

200

KX)-

X X x x x;S: Y Y

X% x x a3 $5. + a (3 @

0 0 -

o x w STRUCTURE TYPE OF WTTERN X -NIO o = ORIENTED

s SHARP 0 . capo U - MEDIUM D DIFFUSE + - coo

I I I I I) 2 0 3 0 4 0 5 0 6 0

TIME (MINI

J. W. HICgMAN AND E. A. GULBBANSEN-TP 2391

'I" TYPE OF PATTERN

0 ORIENTED S - SHARP Y MEDIUM

- 0-DIFFUSE- -

nk OF PATTERN 0 -ORIENTED - s- SHARP + -CUO M MEDIUM D OIFFUSE , . x -r lo I I ' I I I

0 D B O X ) 4 0 5 0 6 0 TIME (MIN.) .

AN ELECTRON DIFPRACTION STUDY OF OXIDE FILMS

TYPE OF WTTERN 0 ORIENTED 3 'SHARP Y ' MEDIUM 0 'DIFFUSE

I I I I 0 10 2 0 3 0 4 0 50 a

TlME (MW)

FIG '/--OXIDE PKMS ON 59.56 CU-M.

I PXlDE STRUCTURE D P E OF P A T T a )( a NiO

00..

Om ORIENTED s= S W P u= MEDIUM D= DIFFUSE

I I + = P o I I I I I 0 10 20 3 0 4 0 5 0 6 0

TlME (MIN.)

FIG OXIDE FILMS ON 68.78 Co-M.

J. W. HICKadAN AND E. A. GULBUNSEN-TP 2391

M "WEDIUM 0 DIFFUSE I

TlME (MINI

FIG OXIDE FILMS ON 80.52 CU-M.

OXIDE STRUCTURE TYPE OF PAlTERN

0 0 ORIENTED

10 s =SHARP Y MEDIUM D - MFFUSE

0 D 20 30 40 !SO 60 TlME WIN.)

FIG IO-OXIDE FILMS ON 88.74 CU-NI.

8 AN ELECTRON DIFFRACTION STUDY OF OXIDE FILMS

Table I lists the samples investigated. METHOD OF INTERPRETATION together with their heat numbers and The electron diffraction interplanar dis- analyses. All alloy samples are obtained tances are compared with those obtained from Driver Hams Co., Harrison, N. J. by other workers using X ray diffraction. through the courtesy of Mr. F. E. Bash. No attempt is made to determine the

TYPE OF PATTERN o - ORIENTED

,

FIG 11-OXIDE FILhiS ON COPPER. I The alloys are prepared from electrolytic copper and electrolytic nickel with a small amount of magnesium deoxidizer being used. Analyses are made for copper, the balance being nickel. No analyses for minor ingredients such as carbon, silicon, man- ganese or iron are made.

TABLE I-Analysis of Copper-nickel AUoys* HEAT NUMBER PER CENT CU

s 4 6 s 10.00 4246 s 19.94 4146 S 29.30 4246 S , 38.90 4346 S 49.37 4446 S 59.56 4746 S 68.78

i 4846 S 80.31 49 6 s 88. 4

F. E. Bash, &river Harris CQ.. Jarcison. N. J.

I n a subsequent paper we shall present micrographs and X ray mraction data of the alloys.

orientation of oxide fdms which show evi- dence of such effects by the appearance of 4

'intense arcs or spots; nor is any study made I

of the orientation of the metaltic interface. The data from the oxidations are presented - 1 in the form of existence diagrams of the oxides plotted on a time-temperature scale (Fig 1-11). The terms oriented, sharp, medium and diffuse occurring on the

I existence diagrams and represented by the letters 0, S, M, and D, respectively, refer to the types of diffraction patterns. Those patterns are termed "oriented " where intense arcs or spots occur as the result of I preferential growth of the oxide crystals on the surface. The remaining three classifica- tions are made on the basis of mraction I

line widths.

J. W. HICKMAN AND E. A. GULBRANSEN-TP 2391 9

RESULTS AND DISCUSSION Table 2 gives a synposis of the results

which are presented more completely in graphical form in the existence diagrams (Fig 1-11).

copper

An inspection of Fig 11 shows that Cu2O is the only oxide observed over the tem-

copper ions reach the surface. This is evi- denced by an inspection of the K B values given with Eq 1-4 inclusive. CuO may occur only on the surface providing copper ions have rates of formation and diffusion which are too low to permit reaction (I) to be operable.

Since the experimental conditions are such that an excess of oxygen is always

TABLE 2--Oxides Found on the Metals agzd Alloys

t perature range 1o04oo"C. The patterns become more diffuse as the temperature is elevated. Thermodynamic data for the copper oxides are given in an earlier paper.' The following reactions may occur in the oxidation of copper:

(I) 2 Cu (s) + 0.5 0 2 (g)* CulO (s) where K B = 10+11.8 a t 3m°C and 10+6.6 a t 7m°C - (2) CU (s) + 0.5 0 2 (g) * CuO (s) where K B = 10+0.4 a t 3m°C and IO+S.~ a t 700°C

(3) cuno (s) + 0.5 0 s (g) * 2 CuO (s) where KB = 10+7.0 a t 3m°C and 1o+2J a t 7o0°C

(4) Cu (s) + CuO (s) + Cu20 (s) where K B = I O + ~ - 4 a t 3m°C and 10+1.7 a t 7o0°C.

Reaction (I) probably occurs initially. At least electron diffraction investigations of very thin flms show the presence of CulO. The oxide f lm may continue to grow in thickness by copper ions diffusing out- ward, by oxygen diffusing inward or by a combination of these mechanisms. In the temperature range 30~-700"C C U ~ O should be the only oxide observed if sufficient

Metal or Alloy

Nickel ............ 10.00 Cu-Ni.. ..... 19.94 Cu-Ni.. ..... 19.30 Cu-NI.. ..... 38.90 Cu-NI.. ..... 49.37 Cu-Ni.. ..... 59.56 Cu-Ni.. ..... 68.78 Cu-Ni.. ..... 8o.ja Cu-Ni.. ..... 88.74 Cu-Ni.. ..... Capper ...........

available to take part in the gas-metal reaction, the reaction products which are observed will be dependent upon the rates of formation and diffusion of copper ions and the rate of diffusion of oxygen through the oxide flm. The experimental results indicate that the oxidation of copper proceeds primarily according to reaction (I). If any CuO does form, a solid phase reaction according to reaction (4) imme- diately occurs to form Cu20. This would indicate that the oxidation of copper is determined by the formation and diffusion of copper ions and that the diffusion of oxygen inward a t best plays only a minor role in the reaction mechanism.

Nickel

Fig

I a

3 4 5 6 7 8 9

10 11

The existence diagram of the oxides occurring on nickel (Fig I) shows only the presence of NiO over the complete time- temperature range. The diffraction patterns undergo a change from diffuse to sharp in the temperature range 400 to,z:5m°C. Although the structures of the substrates

Oxid. Temp OC Time Oxide Structure

--- 300-800 300-800

300-800 300-800 300-loo 300-800 300-800 30-800 300-800

300-800 300-600

(Min.)

1-60 1-60

1-60 1-60 1-60 1-60 1-60 1-60 1-60

1-60 1-60

NiO CuzO. CuO at 300-4o0°C; CurO. NiO at 5ooaC. NiO at 60& 800°C

Cur0 at 300- ooaC; CuzO. CuO at 5o0°C; NiO at 600-8o0°C Cur0 at 300°& Cur0 CuO at 400-5o0°C. NiO at 600-8o0°C Cu10 at 3 0 0 " ~ ' CUSO' CuO at 4 0 0 - 5 0 0 ~ ~ : NiO at 600-8oo°C Cur0 at 300-4000~. dur0 CuO at 5 0 0 ~ ~ ' NiO at 600-8o0°C CurO. CuO at 3 o o - j o o o ~ ; ' ~ ~ 0 at 600-8060~ Cur0 CuO at 00-5oo°C. N10 at 600-8o0°C CUSO' at 300'2; Cua0 CuzO + Cu0 at 400-5o0°C; CuaO -D

CuO at 600°C NIO at 700-8ooDC Cua0 at ~OO-SO'O~C; Cus0 -r CuO at 600-7oo°C, NiO at 8o0°C Cu-0

I0 AN ELECTRON DD?PRACTION STUDY OF OXIDE FILMS

are the same in all cases, the oxide films oxides on all of the alloys after 5 and 60 show a tendency to be oriented in the min. respectively. An inspection of Fig 12

higher temperature range. This cannot be and 13 shows that oxides of copper are attributed solely to substrate influence observed on all of the alloys a t 5o0°C and

TYPE OF PATTERN

)( = NiO 0 ORIENTED 8 = SHARP M = MEDIUM D = DIFFUSE

I I I I 20 4 0 60 80 00

PERCENT NICKEL

FIG 12-OXIDES ON ALLOYS AFTER 5 MINUTES OXIDATION.

since a t 700°C the iXms are not oriented in the thin region while they become oriented as the iXm becomes thicker. I t appears that orientation is occurring as a result of growth.

The existence diagrams of .the oxides occurring on the alloys (Fig 2-10) show that oxides of copper and nickel are ob- sewed on all of the alloys somewhere in the time-temperature range. Fig 12 and 13 are included to show the occurrence of the

4

below while nickel oxide is observed on all the alloys about 7o0°C. There appears to be a mass action effect occurring in these reactions since copper oxides are observed up to 7m°C on 88.74 pct copper-nickel and up to 6m°C on 80.32 pct copper-nickel while nickel oxide is observed alone on the I

remaining alloys a t 6m°C and above. In addition to the reactions which may

occur on the metals separately, the following I

':I solid phase reactions may occur: I ,

(5) Ni (s) + CuzO (s) $ NiO (s) + 2 I 1

cu (s) where 1 1

J. W. HICKMAN AND E. A. GULBRANSEN-TP 2391 I I

Kg = 10+7.1 at 3o0°C and 1o+8.2 at 7o0°C. nickel oxide in contact with the substrate (6) Ni (s) + CuO (s) S NiO (s) + Cu does have an effect on the diffusion of

(s) where copper ions to the surface. This is evidenced Kg = 1o+9J at 3o0°C and 10+4.9 at 7o0°C. by the appearance of CuO on the alloys in Reactions ( 5 ) and (6) show that NiO should contrast with its absence on copper.

TYPE STRUCTURE 0 ORIENTED 9 * SHARP n 0 MEDIUM o 0 DIFFUSE

I I I I I 20 40 60 80 100

PERCENT NICKEL FIG IS---OXIDES ON ALLOYS AFTEB 60 biINUTES OXIDATION.

- - be observed over the complete time-tem- perature range if nickel ions are formed and reach the surface.

Since the results show that NiO is not observed over the complete temperature range, sufficient nickel ions have not formed and diffused to the surface to make reactions (5) and (6) operable. The results indicate that copper ions have greater rates of formation and diffusion than nickel ions in the temperature range below 600°C.

Below 600°C the presence of nickel in the alloy lattice or the presence of a thin film of

Probably .reaction (3) which gives the formation of CuO from Cud3 is operable. I t appears that nickel hinders the diffusion of copper and would therefore reduce the oxidation rate of copper. In an earlier paper1 the authors studied the oxidation of 50 pct Ti-Ni, 70 pct Ti-Ni and a 8 pct Ti-Cu. The results on these alloys show only the presence of Ti02 (rutile) on the nickel alloys while Cu& is observed at 300-4o0°C, and Ti02 (rutile) a t 500- 7o0°C on the copper alloy. Table 3 shows a comparison of the sum of the relative

:I2 AN ELECTRON DIFFRACTION STUDY OF OXIDE FILMS

rates of formation and dffusion,of the ions of titanium, copper and nickel as obtained from this study and the earlier report.'

An inspection of Table 3 shows that Cu > Ti > Ni a t 300-400~C; Ti > Cu > Ni a t po°C, and Ti > Ni > Cu a t 600- 7o0°C. One would predict from Table 3 that a ternary alloy composed of these three metals would have oxides of copper on the surface a t 300-400°C; titanium oxide a t 5oo-po°C and that nickel oxide would not be observed in the outer surface of the oxide layer.

of the oxides which are present in the outer surface of the oxide layer may not be correlated with the decreased rate. Since it is known that nickel oxidizes a t .a much lower rate than copper, our results indicate that one might expect to observe a gradual decrease in the rate of oxidation as the percentage of nickel is increased.

Our results are not in agreement with those reported by Miyakel0 who found only a CuO film on the surfaces of the following alloys: 70 Ni 30 Cu, 40 Ni 60 Cu and 7 Ni 93 Cu, with the suggestion that

TABLE 3-Sum of Rates of Formation and DiJwion of th.e Ions

Although alloys of copper and nickel are reportedZ to be passive to corrosion when the at. pct of nickel reaches 40 pct, the results of this study show no indication that there is a structural change in the oxide coating formed a t elevated temperatures as the result of a gas-metal reaction. If the appearance of passivity to corrosion is connected with a filling up of the d band of electronic energy states of the alloy by donations from copper atoms, then this filled d band does not appear to affect the structures of the oxides which form on the alloy as the result of gas-metal reactions. The methods used in this study cannot determine the rates of the reactions which occur. In order to determine whether the composition which corresponds to passivity to corrosion also corresponds to a di- minution in oxidation rate, it is necessary to make rate studies using the quartz microbalance.14

If there exists a composition in the copper-nickel alloy system where a sudden change in oxidation rate occurs, the results of this study indicate that the structures

Alloy

a8 Ti-Cu. ................................... &Ti-NI.. .................................. &o TI-NI.. ..................................

p to 68.78 Cu-Ni.. .........................

NiO may be present beneath the uppermost layer. This study shows the presence of CupO, CuO and NiO on the surface of 70 Ni 30 Cu and 40 Ni 60 Cu depending upon the temperature of oxidation (Fig 13) while 10 Ni 90 Cu shows the presence of Cup0 and NiO.

The results of this investigation of thin oxide films formed on alloys of copper and nickel are presented as existence diagrams of the oxides on a time-temperature scale. The study shows that:

I. The oxide structures of thin films are identical with those of bulk oxides as determined by X ray diffraction.

2. Only simple oxides of the metals are found; no complex oxides containing both metals are observed.

3. In general oxides of copper are observed on all the alloys a t low tem- peratures while nickel oxide occurs a t higher temperatures.

4. The temperature a t which nickel oxide occurs on the surface does not appear

joo°C

Cu > Ti Tj > Ni Ti > NI Cu > Ni

4oo°C

Cu > Ti Ti > Ni Ti > Ni

Cu > Ni

5oo°C

Ti > Cu Ti > Ni Ti > N/

Cu > NI

6oo°C

Ti > Cu Ti > Ni Ti > Ni Ni > Cu

7o0°C

Ti > Cu Ti > Ni TI > Ni Ni > Cu

J. W. HICKMAN AND E. A. GULBRANSEN-TP 2391 .I 3

to be very dependent upon composition although it does occur a t lower tempera- tures for higher nickel concentrations.

5. The occurrence of nickel oxide on the surface is more strongly dependent upon temperature than upon compositibn.

6. There does not appear to be any unique structural change a t that composi- tion where the copper-nickel alloy system approximates zero in magnetic suscepti- bility and becomes passive to corrosion. This may indicate that the mechanisms of corrosion and oxidation are different.

7. The experimental results indicate that the rates of formation and diffusion of ions are important factors in determining the oxide which will be observed.* In general, copper ions form and diffuse more readily a t low temperatures while nickel ions reach the surface more quickly a t high temperatures.

*The authors consider this statement expresses a valid conclusion from their experi- mental results. I t is evident that the thermo- dynamic stabilities of the oxides are not the controlling factors since nickel oxide is not observed over the complete time, temperature and composition range. The important factors determining the oxide that will be observed must, therefore. be the formation and diffusion of metallic ions.

8. The fact that NiO does not occur over the complete time-temperature range for all alloy compositions indicates that, although thermodynamic equilibria have undoubtedly been attained, the. rates of formation and diffusion of nickel ions are not suficiently high for the solid phase reactions to occur.

I. Hickman and Gulbransen: Anal. Chem. (in press).

a. Uhlig: Trans. Electrochem. Soc.. 85, 307- 333 (1945).

3. Mott and Jones: The Theory of the Prop- erties of Metals and Allovs. 106. Oxford . . < . Press. (1936).

4. Grew: Proc. Roy. Soc. A145, 509 (1934). 5. Preston:Phil. Mag. 17 (7). 466470 (1934). 6. Darbyshire: Trans. Faraday Soc. 27. 675

~ . -

(1931). 7. Gulbransen and Hickman: Trans. AIME.

Metals Tech. Oct. 1946. .TP 2.068. 8. Germer: Phys. Rev. ga, 959967 (1939). 9. Smith: Jnl. Amer. Soc. 58, 173 (1936).

10. Miyake: Sci Papers Inst. Phys. Chem. Research (Tokyo) 31, 161-73 (1937). C.A. 31s 4244 (1 37).

11. Frohlich: Ztsch. detallkunde 28. 368-375 (1936).

Ia. Gulbransen: Jnl. Appl. Phys. 16, 718 (1945).

13. Gulbransen: Rev. Sci. Instruments 18, 546 (1947).

14. Gulbransen:.Rev. Sci. Instruments 15, aor (1944).