This article was downloaded by: [University of Guelph]On: 14 November 2014, At: 14:52Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Journal of Nuclear Science andTechnologyPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/tnst20

Compactibility of Graphite Powder forHigh Temperature Reactor FuelsTakashi OIKAWA aa Central Research Laboratory , Showa Denko, K.K. , Ohta-ku, TokyoPublished online: 15 Mar 2012.

To cite this article: Takashi OIKAWA (1968) Compactibility of Graphite Powder for High TemperatureReactor Fuels, Journal of Nuclear Science and Technology, 5:4, 168-178

To link to this article: http://dx.doi.org/10.1080/18811248.1968.9732430

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis, ouragents, and our licensors make no representations or warranties whatsoever as to theaccuracy, completeness, or suitability for any purpose of the Content. Any opinions andviews expressed in this publication are the opinions and views of the authors, and arenot the views of or endorsed by Taylor & Francis. The accuracy of the Content should notbe relied upon and should be independently verified with primary sources of information.Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands,costs, expenses, damages, and other liabilities whatsoever or howsoever caused arisingdirectly or indirectly in connection with, in relation to or arising out of the use of theContent.

This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Journal of NUCLEAR SCIENCE and TECHNOLOGY, 5l4), p. 168-178 lApril 1968). 168

Compactibility of Graphite Powder for

High Temperature Reactor Fuels

Takashi OIKA W A*

Received September 19, 1967

The compactibility of three kinds of graphite powder WaR investigated, and fmmd to be good except in the case of artificial graphite. The compactibility was found impaired by purification of the powder, but restored completely or even improved above the original value by through ball milling. And it was found that the porosity V of the compacts is related to the compaction pressure P by the function log V=A-B logP, where A and Bare constants

The cause of these changes observed in compactibility was investigated. It was attributed to the shape of the powder particles. The graphite particles had acquired complex shapes during the puri­fication. The high temperatures to which the powder was exposed and purified had caused recrystal­lization through the action of free carbon particles precipitated from the thermodecomposition of freon gas (CC12F2) used for purification. These complex shaped particles disintegrated again into thin flaky particles by:_ thorough milling.

I. INTRODUCTION

Graphite matrix fuel compacts have good high-temperature physical, mechanical, ther­mal and nuclear properties.

Since graphite powder, fuel particles and binder are mixed, compacted and heated to high temperature in the process of producing these fuels, the properties of the fuel particles and binder as well as the heating all affect the characteristic of the final product.

Nevertheless, it may be justified to estimate the properties of the product fuel on the basis of the compactibility of the graphite powders, since this characteristic of the material may rightly be considered to largely determine the final properties of such fuels.

Properties such as apparent density, hard­ness and electrical resistance of the graphite powder compacts were measured for correla­tion with compactibility. Compactibility was found to deteriorate with purification. Various means were tried for regaining the original compactibility possessed prior to purification, and the most effective treatment was found to be thorough milling. This is to be recom­mended when purified graphite powder is to be used as a raw material for nuclear fuels.

Phenomena similar to these have been reported by Watanabe, etal.(l), but the cause was not clearly determined. Livey, et at/'>

have studied the compaction of artificial graphite powders and reported that com­pactibility was improved by milling and worsened by heat treatment, and further provided some discussion on the cause of these phenomena. Bacon<a> had previously suggested that the presence of rhombohedral graphite might have some bearing on the matter, for the reason that the fraction occu­pied by this form of graphite increased by milling, and restored upon annealing - which is analogous to the high-temperature purifica­tion dealt with in this report. But Livey did not find any change in the proportion of rhombohedral graphite brought about by milling. The acceptability of Bacon's sugges­tion, therefore, is doubtful, although it must be borne in mind that reliable detection of such crystalline forms of graphite is an inherently difficult task. Bacon<4> has also reported that during grinding of graphite powder on a commercial production scale, the size of crystallite as well as that of the particle size is reduced. But here again Lively found no appreciable alteration in crystallite size during grinding, done in experimental scale. He suggested that these differences in observed results might well be attributed

* Central Research Laboratory, Showa Denko, K.K., Ohta-ku, Tokyo.

-24-

Dow

nloa

ded

by [

Uni

vers

ity o

f G

uelp

h] a

t 14:

52 1

4 N

ovem

ber

2014

Vol. 5, No.4 (Apr. 1968)

purely to the difference in milling procedure. Based on the foregoing knowledge, the

present study was made to cover particle size distribution, true density, X-ray diffraction analysis, as well as microphotographic and electron micrographic observations of various graphite powders to assemble a wide scope of information that might possibly throw light on the cause of changes in compactibility due to purification and milling.

169

II . ExPERIMENTS ON CoMP ACTIBILITY

1. Graphite Used

Three varieties of -graphite - extremely fine flaked natural graphite powder, thermodecomposed coarse graphite powder and artificial graphite pow­

der- were selected among a number of avail­able varieties, and purified at temperatures above 1,860°C in freon CCCJ,F2) gas flow. Ana­lytical data on these powders before and after purification are given in Table 1.

Table 1 Results of analysis of various graphite powders

Kind of graphite I

Treatment

Natural graphite Unpurified

(extremely small particles) 1-

After purification ·-·· --- ··-------

Thermodecompo3ed Unpurified graphite

(coarse particles) After purification

Artificial graphite Purified

The conditions of milling performed to regain the compactibility impaired by purifi­cation were as follows: Stainless steel mil Graphite powder charge Milling medium

25 em dia., 30 em long 1.0~1.5 kg. 22 balls 20 mm diam. and 88 balls 25 mm diam. (stainless steel)

Milling time 40 hr

Iron pick-up from the balls during the milling could be minimized to a negligible amount (i.e. 30 ppm, after 40 hr milling) by using pre-conditioned balls that had acquired a coat­ing of graphite<2>.

2. Measurement of Density, Hardness, Electrical Resistance of Compacts

Compacts 10 mm square were prepared under high pressure (0.6~7 t/cm2), and their properties were examined, Micro- Vickers hardness measured under a 25 g load; electri­cal resistance by drop voltage method; poro­sity calculated from apparent density, and true density obtained by the buthanol immer­sion method. The results are presented graphically in Fig. 1 (a), (b) and Fig. 2.

With natural graphite powder, the compac­tibility was good before purification, i.e. high

Water Volatile Ash Fixed

(%) matter (%) carbon (%) (%)

0.14 0.90 0.85 98.11

0.05 0.82 0.004 99.13 ------ ---------

0,07 0.23 0.60 99.10

0.02 0.40 0.002 99.58

0.06 I 0.50 0.002 99.44

density and hardness, low porosity. The compactibility impaired by purification was fully restored by subsequent milling, to the original level or even higher.

Thermodecomposed graphite powder show­ed a compacting behavior somewhat similar to that of natural graphite, but hardness was low before and after purification and high after milling.

The purified artificial graphite powder failed to compact at all before being thorough­ly milled, but even then the properties of these compacts were inferior to those of compacts prepared from natural graphite powders. (Fig. 3)

As a whole, density and hardness increased while porosity decreased progressively with compacting pressure. Electrical resistance was found highly anisotropic. This may be due to preferential orientation acquired by the flaky particles, which would tend to arrange themselves with their surfaces per­pendicular to the axis of compaction.

3. Observation of the Side Surfaces of Compacts

The side surfaces of the compacts were smooth and flat, which facilitated observation

-25-

Dow

nloa

ded

by [

Uni

vers

ity o

f G

uelp

h] a

t 14:

52 1

4 N

ovem

ber

2014

170

o'£~~·--a.o ,...---:: .,...--•

:. /~.--· \\f/·~-i 1.1 \~ ': ',.

f(')t, ,, 'x.. Po· . y " ----~os,t.r ,. ,, "---

i l.ll '~::::-o--:~:=~~-: ~ -·----------

6

COMPACTING PR£SSURE TD'}"cM 2

"' .. .. N

c

a .J

10

I ~ zr

J. Nucl. Sci. Techno!.,

i Ll ----~----~-----L-• • 4 I

COMPAITING PRESSURE TOtycM2 COII,.ACTINI PRUIURl TO'Yc.l

(a) Obtained from natural graphite powder

a,o

ul.l

~ " ,.. ~ 1.8 .. z .. a

2 • 6

COMPACTIN8 PRESSURE TOtycM2

•o

20

,.. .. 10 ..

0

"' 0 ..

"' u

c: ... u z c .. .. ii ... .. u .: u ... .. ..

X

0.011

\ 0,012

. -~ ~~·-·--0.001

~. ............... .....___,

0.004 ~

t~~==~~~~~~;~;~-=t==-==" • 6

COMPACTING PRES SUitE TO"'CM2

/; 0:

1/ .. ..

6 N

a c 0 .J /• /' "_,o"'

I 1 I 1

~ /t3/ .· ,.:..-- -~rr' .. .,.---- -· .. ... ~~~~.-----x

z • y ·-----·-c .. c X

"

• • COMPACTING PRESSUftE TON/CM2

(b) Obtained from thermodecomp03-ed graphit-e powder

30

(a)

0-0- Before purification x-x- After purification

e-e- After milling

--- Parallel to pre-ssing direction -------- Perpendicular to pressing direction

Fig. 1 Density, porosity, hardne3s and electrical resistance of com pacts

COMPACTING PRESSURE P TON CM z COMPACTif'«i PRESS~E P TON~ ·eM C

0-0- Before purification x-x~ After purification e-e- After milling

Natural graphite (b) Thermodecomposed powder graphite powder

Fig. 2 Porosity-pressure relationship of graphite powder

of the surface microstructure. (Photo. 1 (a)~(c)) The natural graphite compacts revealed

side surfaces embodying small particles ap­proximately lOfl' in size and finer, which corresponded to the original particle size distribution (described later). Also observed were longitudinal lines which were transfers of the side walls of the mold, and corres­ponded to the direction of pressing. But numerous cracks were observed on the sur­faces of compacts made from purified powders, the direction of these cracks being perpendi­cular to that of compaction. But no such

26-

Dow

nloa

ded

by [

Uni

vers

ity o

f G

uelp

h] a

t 14:

52 1

4 N

ovem

ber

2014

Vol. 5, No.4 (Apr. 1968)

1.1

'·' u

~ ,:

1.4 1-iii z .. 0

" •~x u

./ ~ o.o .. ·\ 0: .. u

" 0,.--- z N

"' 1- 0 Ul c iii 0,04 0 4 .. -' ..

'~ -' ~ c ~0 u Ul

/' ~ (102 Ul /' " • .. 2

u z 0

I. / ·/' .. ' .... 0 ;.l

"' -' oo.--..=-::::=::-::=:- ~ "' ~~#· /" .. :z: ,. --:::-. 1.!'"'

.-------~.--. ~ . 4 6 2 • 2 4

COMFIACTtN"l P~ESSURE TO%M2 COMPACTING PRESSURE TO'j/ch!2 COh!PACTING PRESSURE

0-0·- Milled for A:) hr with 88 balls --- Parallel to pr£>ssing direction X-X- MillPd for 40hr with 40 balls --------Perpendicular to ptessing dirPction

Fig. 3 Apparent density, electrical resistance and hardness of compacts obtained from artificial graphite powder

Before purification After purification After milling (a) Obtained from natural graphite powder

Before purification After purificatiori After milling

(b) Obtained from thermodecomposed graphite powder

(c) Obtained from artificial graphite powder

Photo. l(a)~(c) Side surface of compact cxzso)

-27-

I rorych!2

171

Dow

nloa

ded

by [

Uni

vers

ity o

f G

uelp

h] a

t 14:

52 1

4 N

ovem

ber

2014

172

cracks were observed on compacts made from powders milled after purification. The variations observed in density and hardness of compact accompanying purification or milling would appear to be well correlated to these surface conditions.

The thermodecomposed graphite powder compacts similarly revealed relations between compactibility and precompacting treatment correlated by surface conditions.

Compacts made from artificial graphite purified and milled had extremely rough surfaces, even when compacted under pressure as high as 6 t/cm2•

4. Porosity Change in Compacting

The compacting behavior of powders has been investigated by a number of workers<6J<sl.

To express the , tendency of the porosity v of metallic powder compacts to decrease with increasing compacting pressure P, Kono­picky<7l has proposed the equation

V=A-BlogP, ( 1)

where A and B are constants. The above relationship between V and P

was found to fit the present results better when replaced by

log V=A-Blog P (2)

as shown in Fig. 2. The values of B noted in Fig. 2 have been

derived by least square fitting to the plots in the ranges of pressure above the point of change in direction.

This value B has a close relationship with powder compactibility i.e., the larger the absolute value of B, the better the compacti­bility. The B value of natural and thermo­decomposed graphite was lowered by purifica­tion and restored by milling.

If the movement of the ram during a single compaction operation is continuously observed, the "at pressure" density dv of the powder compact under compaction pressure, and the "zero-pressure" density dpo upon removal of the pressure can both be obtained<sl.

The relation between ram movement and pressure exerted on the specimen is shown schematically by the curves AB and BC in Fig.4.

The "zero-pressure" density dvo may be

A~

I

J. Nucl. Sci. Technol.,

:t~~-~-g' ~~ocf~ ~ Up~-r---·--- N

i D tL_,-------Li_ ____________ JB ·; ------ • _e.p_ , .s::. ---------~ E E I ~ I

0: --- l_ 0 p

Pressure

Fig. 4 Ram movement following application and removal of pressure during compaction

calculated for any pressure P up to Pmax,

assuming that, if the pressure is removed after having reached a point M (pressure P), the ram would return along the curve MM', which is of similar configuration to the portion NC of the curve BC. (MN=M'C) The value of dvo may be derived from lv and uv through the relation

dpo= W Ctv-uP+to)A,

where the weight w, sectional area A and height to of the compact are all measured after removal from die.

The ram movement is the sum of the change in powder height in the die and the elastic strain ep of the ram, which latter may be determined from a blank run without powder (DE in Fig. 4). The "at-pressure" density dv may be obtained in terms of the notation described above, and uo in Fig. 4:

d- w p- (lp+ep-uo+to)A

The results obtained are shown in Fig. 5. The "zero-pressure" density dvo corresponds to the density represented in Fig. 1, and there is fairly good agreement between the two values. The relationship log V=A-B log P also applies, and the changes in B value by purification and by milling are similar to that seen in Fig. 2.

Elastic behavior (dp>dvo) was apparent in compacts from powders of natural graphite

-28-

Dow

nloa

ded

by [

Uni

vers

ity o

f G

uelp

h] a

t 14:

52 1

4 N

ovem

ber

2014

Vol. 5. No.4 (Apr. 1968)

THERMODECOMPOSED

8 2.0~---+----~~~~~~~~~~~

"' "' > ,... UJ z w 0 1.8 ,... z w "' "' ll. ll.

"' I

before purification

·after purification

after milling

I. 6

__ _ __ at-press. densHy

--~-- -- rco·pcess. density

20

~10

2 4

PRESSURE TON.'CM 2

-~ I

f--

THERMODECOMPOSED

GRAPHITE POWDER

I

~ ~ ~

6

I

- ---> 8 ,... ....... ~ .....

~ 6

"' 0 ll.

4

lOgY•

l : !'"--, ['-.,

A- Blog P r--.....

'

I 2 3 4 5 6

COMPACTING PRESSURE TON CM 2

a value

Zero-press. At-press.

Before purification 0.35 0.58 After purification 0.36 0.50 After milling 0.39 0.60

0~0- Before purification X-X- After purification

·-·-- After milling

u u

"' "' > ,... iii z w 0 ,... z w "' "' ll. ll.

"'

2.0

1.8

30

20

> 8

~ 6 iii 0

"' ri 4

NATURAL POWDER

--

2 4

PRESSURE TON ·"CM 2

NATURAL

GRAPHITE POWDER

------

8-J '

•--.. I --~

~ ~ I --I ~ ' r-.....::.:- ~-

--~

. I

I OgV- A-BiogP

:Hl---l I II -- -- ·-T-= -t-2 3 4 5 6

COMPACTING PRESSURE P TON CM 2

li value

Zero-press. At-press.

Before purification After purification After milling

0.43 0.26 0.43

0.47 0.33 0.59

At-press. porosity Zero-prE'ss. porosity

173

6

Fig. 5 At-pressure and Zero-pressure relationships between density or porosity and compacting pressure of powder

and of thermodecomposed graphite either unpurified or milled, but this property was found lowered by purification and restored by milling. These changes in property cor­relate with the variations in compactibility.

Furthermore, with unpurified or milled graphite powders, a subsidence in density of about 2% was observed upon releasing the applied pressure of 6 t/cm2

• This is almost twice the change in density that should occur

29

from the elastic behavior of the graphite particles if a modulus of 700 kg/mm2 is assumed to be applicable under these conditions. The foregoing behavior would appear to indicate that the volume increase of the specimen occurring upon release of the compacting pressure is due not only to elastic expansion of the graphite particles but also to some form of relative movement among the particles.

Dow

nloa

ded

by [

Uni

vers

ity o

f G

uelp

h] a

t 14:

52 1

4 N

ovem

ber

2014

174

m. ExPERIMENTS OF PowDER

CHARACTERISTICS

In order to throw some light on the cause of the changes thus observed in compactibility by purification and by milling, a series of experiments were undertaken on the three kinds of graphite previously selected.

1. X-ray Diffraction Analysis and Other Studies

An X ·ray diffraction analysis of the gra-

J. Nucl. Sci. Technol.,

phite powder was made, and the lattice constant and crystallite size were thereby determined to determine the changes brought to the internal structure of the powder, and to judge between the results obtained by Livey and by Bacon. Next, the true density d.. was calculated from the lattice constant, and the microporosity 1/d, -1/dx. calculated in turn from d. and true density d, determined by the buthanol immersion method. The results are presented in Table 2.

The lattice constants ao and co were not

Table 2 Lattice constant ao, co, crystallite size La, Lc, true density dx. d, and micropores of various graphite powders

Kind of graphite Nat'.lral graphite Thermodecomposed graphite Artificial graphite ----------

Treatment Unpurified Purified Milled Unpurified Purified Milled Purified Milled ---~----

Crystallite size La (A) I 1,000 730 1,000 1,000 1,000 1,000 1,000 1,000

II Lc(A) 1,000 1,000 1,000 1,000 1,000 1,000 250 250 >, Lattice const. ao(A) 2,460 2,461 2,462 2,462 2,462 2,462 2,460 2,461 ..... ....

coCA) (IJ Lattice const. 6,709 6,707 6,708 6,707 6,707 6,708 6,720 6,721 0. 0 .... True density dx (g/cc) 2,252 2,252 2,249 2,250 2,250 2,251 2,248 2,247 0..

True density d,(g/cc) 2,241 2,209 2,240 2,242 2,212 2,231 2,191 2,217

Microporet (cc/g) 0.00218 0.00864 0.00194 0.00162 0.00767 0.00408 0.01169 0.00610

t Micropore (1/db-1/d.l:) means volume (rc) existing in unit weight of powdN.

altered by either purification or milling, and the values obtained of ao=2.46 A and co=6.70 A

agree with established values for graphite. But in artificial graphite, the ao was slightly

smaller and the co slightly larger than those of the natural and thermodecomposed forms. This would indicate insufficient graphitization in the case of artificial graphite.

Crystallite sizes La and Lc were larger than 1,000 A, except for artificial graphite. Studied on the basis of X -ray diffraction analysis, it is thought that the crystallites of these graphites are large and are not affected by either purification or milling.

Similarly unaffected was the value d.(true density based on lattice constant) and d,(true dersity based on buthanol immersion) was changed by these treatments, which also altered the micropores in unit volume. It can be judged from this that purification and milling does not change the crystallographic character of graphite (as reported by Livey, ei al.), but changes d, and microporosity. Furthermore, these

changes correspond to the changes in com­pactibility.

2. Particle Size Distribution and Microscopic Observation

It can be inferred that the improvement of powder compactibility obtained by milling is due to reduction of the particle size, since alteration of particle size distribution and of particle shape often influencies compactibility.

In order to confirm these facts, the changes brought to particle size distribution were observed. The results of experiments n.ade with the Andreasen sedimentmetric method using an ethyl alcohol solution medium are shown in Fig. 6.

The natural graphite powders before puri­fication were mostly composed of particles finer than 30 JL, but upon purification they appeared to shift their size distribution to a coarser range of 20 to 60 JLcf>.

It is thought that the coarser particles detected by the Andreasen method were really

-30-

Dow

nloa

ded

by [

Uni

vers

ity o

f G

uelp

h] a

t 14:

52 1

4 N

ovem

ber

2014

Vol. 5, No.4 (Apr. 1968)

NATURAL GRJ.PHIT£ ]IIIIlllllliiJUJlllil BEFOfiE PURIFICATION

24 ~~~; AFTER PIIRIACATION

8 12 16 20 24 28 JZ 34 40 44

PARTICl-E: RADIUS fL

16 •• 32 40

PARTICLE RADIUS fL

II ARTIFICIAL GRAPHITE

AFTER MILLING

PARTICLE UOIUS fL

Fig. 6 Particle size distribution of graphite powder measured by Andreasen method

175

coagulations of finer particles, since there was no appreciable change in particle size by purification according to microscopic observa­tion. (Photo. 2) Thus from the microphoto­graphs, the size distribution of natural graphite was not changed by purification but shifted toward a finer range by milling.

The thermodecomposed graphite showed a behavior similar to natural graphite. The artificial graphite powder, purified and tho­roughly milled had particle sizes distributed over a wide range, extending from several f.L'P to 40 f.Lcf>.

From the results thus obtained, the cause of compactibility increase by milling could be attributed to the resulting finer particle size. But a reason had yet to be found for the impairment of compactibility brought by purification, when no change was discerned in particle size in the cases of natural and thermo-decomposed graphite, as well as for the inferior compactibility observed in artificial graphite powder even upon thorough milling.

Photograph 2, below, was taken with a microscope of the light transmission type which mainly shows the external shape of the particles. Therefore, for the purpose of ascertaining changes brought to particle shape and surface, the particles were placed between two leaves of thin glass, and the microphot.o-

tietore punhcation After purification After milling subsequent to purification

Photo. 2 Light transmission micrograph of natural graphite powder (X40l

graphs taken by back reflection light. The natural graphite powder was found too fine to yield clear information by this method.

The coarse thermodecomposed graphite grains (Photo. 3(a)) revealed themselves to be flat and smooth. The flat surfaces might be presumed to coincide with the cleavage planes parallel to the layers of graphite cry­stal. Upon purification, grains appeared to

-- 31

lose the flat surfaces, and to acquire compli­cated shapes Photo. 3(b). Then, upon milling the flat planes again made their appearance (Photo. 3(c)).

3. Transmission Electron Microscopic Observation

Observation by transmission electron micro­scope was made to further pursue the changes in the shape of the fine natural graphite

Dow

nloa

ded

by [

Uni

vers

ity o

f G

uelp

h] a

t 14:

52 1

4 N

ovem

ber

2014

176 J. Nucl. Sci. Techno!.,

(a) Before purification (b) After purification (c) After milling subsequent to purification

Photo. 3 Micrograph of coarse thermocomposed graphite powder (145---250 mesh) by back reflection light c x IOO)

grains. Other graphite powders also were examined for comparison, the results being as shown in Photos. 4(a)-(c).

The natural graphite grains appeared to have a flaky shape, but their thickness could not be determined very precisely, since only semitransparent micrographs were obtained. Their surfaces are presumably flat, as in the case of the thermodecomposed graphite. Upon purification, the powder no longer permitted the electron beam to pass through, indicating apparently that the particles had

considerable thickness. Penetration of the electron beam was again observed after sub­sequent milling, which pointed toward a reversion to the thin flaky state.

The coarse thermodecomposed graphite showed a behavior similar to that of natural graphite. (Photo. 4(d))

The purified and milled artificial graphite grains did not permit electron beam penetra­tion, indicating that they were either platelets of considerable thickness or else solid three dimensional bodies. (Photo. 4(e))

(a) Natural graphite before purification

(b) Natural graphite after purification

(c) Natural graphite after milling subsequent to purification

(d) Themodecomposed graphite (e) Artificial graphite after before purification purification and milling

Photo. 4 Transmission electron micrograph of graphite powder (xi,600)

IV. DISCUSSION

As described above, neither purification or milling of graphite powder changes the crystallographic character but affects the

particle shape. The cause of shape alteration by purifica­

tion might be attributed to the occurrence of recrystallization during high temperature purification. It is reasonable to think that

-32-

Dow

nloa

ded

by [

Uni

vers

ity o

f G

uelp

h] a

t 14:

52 1

4 N

ovem

ber

2014

Vol. 5, No.4 (Apr. 1968)

the thin flaky particles become adhered to one another, which is none other than the phenomenon commonly known as recrystalliza­tion. This is provoked by free carbon parti­cles precipitated from the thermodecomposition of the freon gas (CCbFz) used to purify the graphite powder (about so~so kg/t).

Then when the purified particles are milled, these particles of complex shape disintegrate again into thin flaky particles and finer grains, the force of adhesion in the recrystallized particles not being apparently very strong.

Considered from this point of view, the various phenomena reported in the foregoing sections can be well illustrated as follows:

(1) True density d, and micropores: Many defects or pores should be created by purifi­cation (recrystallization). Hence purification should lower the true density d, and increase the quantity of micropores. Milling would reverse the effect by reducing grain shape to simpler form.

(2) Apparent density of powder compact:

It is reasonable to consider that complex shaped particles lend themselves less easily to compaction into dense solids as compared to flaky particles of simple shape: Hence the low density of artificial graphite, the changes seen in the compactibility of natural and thermodecomposed graphite could also be interpreted from alterations in grain shape.

(3) Hardness: It is generally considered that hard compacts are obtained when particle shape is simple and flaky, and of fine size distribution.

Natural graphite powder is of fine particle size, which explains the high hardness before purification and after milling, while the low value after purification can be attributed to the sole action of particle shape. With ther­modecomposed graphite powder, hardness is low before and after purification due to the coarse size of particles, and high after milling because the two conditions mentioned above are satisfied. Artificial graphite powder, can only produce compacts of poor hardness, whether before or after milling, due to its three dimensional particle shape.

( 4) Electrical resistance: Since electrical resistance is markedly more influenced by

177

crystallographic anisotropy and contact resis­tance between the compacted particles than by particle size or shape, changes in electrical resistance could not be significantly discerned in the present series of experiments. The compacts obtained with the three kinds of graphite studied showed low resistance in the transverse direction (perpendicular to the C-axis of the graphite crystal and direction of compaction),

and high resistance in the longitudinal direction and this tendency was found unaffected by either purification or milling. Furthermore, resistance was highest after milling, due to the increased total contact resistance between the particles.

(5) Compaction behavior: It can be under­stood that thin flakes of graphite show an elastic behavior. Accordingly, in natural and thermodecomposed graphite powders, elastic behavior is observed before purification and after milling, but it is less marked after purification. Furthermore, the change in B

values in Eq.( 2) have also been verified. (6) Effect of carbon fluoride: Carbon fluo­

rides (CF, CF. and CFa, etc.) are thought to be produced at high temperature during the purification of graphite powder with freon gas<9>, and that fluorine would then be liberated from the graphite powder upon subsequent milling. This fluorine might exert some in­fluence over the compaction behavior.

Although the free energy of formation of carbon fluoride is as shown in Fig. 7oo, a US patent01

> reports that fluorine reacts with graphite, producing CF4, CFb, CF, etc. at a temperature below 1,860°C, which then would dissociate at a temperature above 1,860°C.

To avoid the formation of carbon fluoride, it is recommended that the impurities should be removed by Ch gas treatment at a tem­perature below 1,860°C, and boron which still remains eliminated by fluorine gas treatment above 1,860°C, after which the purified gra­phite powders are cooled in Ar flow.

A similar purifying process was adopted in the present case, therefore, an analysis was made of the fluorine content in the SiC thermodecomposed graphite powders both before and after purification, as well as after subsequent milling. Fluorine was extracted

-33-

Dow

nloa

ded

by [

Uni

vers

ity o

f G

uelp

h] a

t 14:

52 1

4 N

ovem

ber

2014

178

~ -40

\i ,. 0: 0 .. !!; ,. "' "'

-60

"' -80 z "' "' "' "' ..

8~~-----ti-

500 1000 1500 2000 TEMPERATURE 'K

Fig. 7 Free energy of formation of CF, and BFa

from the samples in the form of hydrogen fluoride by combustion in water, then its presence was determined by alizarin complex­one photometry. These analyses showed a fluorine content of about 3 ppm and 4 ppm respectively before and after purification, and 4 ppm after milling subsequent to purification. Accordingly, it may be stated that the effect of fluorine on compactibility can be ignored owing to its low content.

V. CoNcLusiON

It has been found that the compactibility of graphite powder varies extensively accord­ing to the kind of graphite used as material as well as according to the treatment applied to it, i.e., purification or milling.

Natural graphite powder has good com­pactibility, but which is impaired by purifica­tion, and which can subsequently be completely restored or even raised above the original value by thorough milling. For instance, with purified and milled natural graphite, a com­paction pressure of 5 t/cm2 has produced com-

J. Nucl. Set. Techno!.,

pacts possessing a density of 2.07 g/cm3,

Vickers hardness of 8.0 kg/mm2, and electrical resistance of 0.008 Ocm(the last value applicable

to direction of compaction). The porosity V was found to vary with the compacting pressure P according to the relation

log V=A- B log P.

Natural graphite powder thoroughly milled subsequent to purification thus appeared to be among the best materials for use as fuel matrix.

It was found that a major factor that influences compactibility is neither particle size distribution nor crystallographic character, but the shape of the particles.

ACKNOWLEDGMENTS

Acknowledgements are due to Prof. S. Takeuchi of Tohoku University for his un­failing guidance, and to Dr. S. Sonoda of the Showa Denko K.K. for permission to publish this reports. Also the assistance and discus­sions offered by· other members of the Labo­ratory staff are gratefully acknowledged.

--REFERENCES--

(1) WATANABE, et al.: Lecture at Nippon Kinzoku Gakkai 1961, April., Tokyo.

(2) LIVEY, D. T.: Powder Met., 5, 130 (1960). (3) BACON, G.E: Acta. Crystallogr., 3, 320 (1950). (4) BACON, G.E.: ibid., 5, 392 (1952). (5) SUGIYAMA, M.: "Powder Metallurgy", (1959),

OHM Co., Tokyo. (6) BEAVER, W.W., et al.: J. Metals, 8, 445 (1956). (7) KONOPICKY, K.: "Parallelitat der Gasetzmassig­

keiten in Keramik und Pulvermetallurgie", Radex·Rdsch., (1948), 141.

(8) HECKEL, R. W.: Trans. Met. Soc. AIME, 221, 671 (1961).

(9) Y AJIMA, S.: Tanso, 37, 21 (1964). (IQ GLASSNER, A.: ANL-5750. (1.1) BROOKS, L.: U.S. Patent-2, 734,800, (1956).

-34-

Dow

nloa

ded

by [

Uni

vers

ity o

f G

uelp

h] a

t 14:

52 1

4 N

ovem

ber

2014