A vector-vector effect in the electrical conductivity of pyrolytic graphite

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l.E’l‘-I‘ERS 1’0 .l‘tiE E1~I~I‘OR REFERENCES I. ‘l‘hrower l’. A., Rrit. J. Appl. Phy. 15, 1153 (1!164). 2. Heerschap M., Dislocation loops in naturztl gt-aphite single crystals reactor-it.racti~Ite(l at 1200”~~.Rep. No. EUR 3289e (1966).

A Vector-Vector Effect in the Electrical Conductivity of Pyrolytic Graphite

3. (;rovcs (1961).

G. \V. ;md Kelly :I.. Phi/. :Llq.

Material Ke,~ecmh Laboratq, EC’KATOM E,dabli.thment, Petten. Thu Nethdonds

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6, IFiY7

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I‘ltc purpose of’ this repot3 is to illu\lt.;ttc. ;t vector-wctor efrect irt the ek.c.tt-ic;tl c~oitcluc-tivil\ of I’(;.

1 hc

l)li\ sic,ktl properties of single cr) st;tls d epcticl ott the direction Aong \vhic-h thq XC me;tsrtred rel;ttive CO the crvst:tl :txes. In ltex;~gonal c-r-)-stat, such as graphite: the electrxc:tl resistivitv dcpcnds on the angle 0 twtwcert the direction ‘in which it is measured zttid lhr ltcxagonal ;ixis (c,-axis), according to ;t vector vector effect [ I]. Powell [2] measured [jH l’or S;tllium sliowrd 3 \ ectot’(&./\I,! = 5.5), and \‘rcIor effect in its electrical conducti\ it\. t’yrol)tic graphite (PC;) is composdd 01 c‘r\stallites kiligned p;tr;tllel to the surfwe of’ tleposition. Ir is :t pol!,ct-! stalline solid with ;I high dc~~rce ot preferred 0rient;ltion 13, I]. A ntarhed ;tntsotrop?; in ph~sic:tl properties ot I’(; is c~~usctl I,\ its unlquc structtirztl features[.?-91. For l’( ;. the ~ttiisotrop~ 01’ the resistivity (~+/p,,), in tltc directiotts perpendicular (c-axis) t0 kuid I);it~;tllel (cl-axis) to the surt’aw of’ deposition, depcn(la OII dcpositioti temper-attires and ittdicittes ;t tttasitnttm v~iluc 01’ ahout 10:‘. Many studies 01 p,. :rttd p,, hwc Iwet~ made, hut no mention has bwn n~adc of tlir. resistivit? (PO) in ;I directiotr ;tt ;I*I gcwl-all~

;1ngle

pyrolytic__-graphite

8 =0”

e=90" c-axis

lli?fc 9

a-axis

H to rhc c-axk.

I. Structural gr;tphite used

characterislics

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iii the present

ot pyi olytic experitncttt _

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(g/c-m”) spacing

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THE

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measurement. 90 to the c-axis. For the resistivity determinations a current of about 0+05-O+ A was passed through the specimen and a standard resistance connected in series. A potentiometric method of measurement was used. The measurements of resistivity were carried out at 25°C and at liquid nitrogen temperature. The results of room temperature resistivity (pet,J are shown in Fig. 2: pefrf decreases with the increasing 8. in the @ range of 60”~90”, perk decreases rapidly with 8. Figure 3 shows the ratio of the resistivity at liquid nitrogen temperature @ec,,,) to the room temperature resistivity. In the These

prepared

TO

resistivity

0 = 0, 30, 45, 60, 75 and

angles

I 30

84 I

60

90

e Fig. 3. The ratio of the resistivity at liquid nitrogen temperature peCI,) to the room temperature resistivitypeCl.) at an angle 13.

l

-,

, experimental relationship

B range of O”-84”, ~~~~~~~~~~~~ is independent of B. Above 84”, however, it increases rapidly with 8. In Figs. 2 and 3, the closed circles represent the experimentally measured values of P@(,.)and p,& PO(~),and the open circles show the mean values; PCtr2(= pan,,) = 5.0 X 1OV’ tkm, pntr~(= p80d = 5.0 X If)-” R-cm, ~~~~,~~~~~~, = 1,C35, and P<~c~_>/Pnir, = I ,7-t. Electrical conductivity is a vector--vector effect in the sense that a vector I (electrical current density) is evoked by another vector E (voltage gradient). l’hus the resistivity values measured at an angle N should conform to the ellipse which represents the intersectiotl of the plane inclining at an angle 8 with the conductivity ellipsoidthe ellipsoid for which the square of the radius vector is equal to the electrical conductivity[Z]. If 2c and “Leebe the c- and u-axes of this elhpse, and ~e(,., be the resistivity at ‘25°C in the direction at an angle H to the c-axis then,

data (21

(1) where pcC,.,= l/2 and P,,~,.)= I/C?. Thus 0 Angie

30 to the

60 c-axis,6fdeg)

Fig. 2. The relation between electrical and the angle (0) to the c-axis at room ture.

90

resistivit, tempera-

~~,.~(~-cm)

Similarly,

= 5-O X It)-” - (5.0 X 10-I - 5.0 X IOP) sin’@.

at liquid nitrogen

(2)

temperature,

pot,,, = prc,,, co? 0 +plto,., sin2 0.

(3)

2. 3.

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10.

Acoustic Emission From Graphite Under Stress

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