E.3.4 The ASTM method for measurement of surface resistivity and its adaptation for space used materials

A standard ASTM (D257-93) method exists for surface resistivity, although it is not adapted to materials in space, under vacuum and particles. It is known that insulators resistivity is enhanced by the space radiation and we know that the charging process occurs under electron bombardment. In other words, measuring surface resistivity is probably worthwhile if space environment effects are reproduced. Consequently, we envision adapting the ASTM method under an electron beam. The standard ASTM method requires the measurement of a current I under the application of a voltage V across a gap. Instead, the voltage V under the application of a current I delivered by an electron gun is measured. See [29]. The electron beam charges up the central floating electrode while the external electrode is kept grounded. In the same time the electron beam charges the central electrode and irradiates the gap where the surface resistivity is to be measured, between the central and the external electrode. Another deviation from the standard ASTM method is that samples have no backside conductive electrode. This guarantees that the voltage V stabilizes due to only the surface current.

The surface resistivity R_ is computed from the knowledge of I_t (total current on inner electrode); D1, D2 (see Figure E-14) ; and the measured voltage V_m.

R_ = π (D2 + D1)V_m/ (D2- D1) I_t

The external metal coating is grounded and the internal metal coating is charged by the electrons to a measured level Vm, depending on the surface resistivity and sample geometry. With a total current It (It = S*Ii/cm2), if the measured voltage is Vm, there is a relationship between the surface resistivity and the measured voltage Vm. The relation ship is: R = π(D2 + D1)Vm/ (D2- D1) It . With D2 = 2.1 cm and D1= 2 cm, R = 128.8 (Vm/It)

A slightly different method consists in the same geometry, but without inner coating. In that case, the measured voltage Vm = Ii*R*(x2 – R2)/4 is variable along the axis. (R is the surface resistivity [W/square], x is the distance to the centre [cm] , R is the radius (D2/2) [cm], and Ii the incident electron current [A/cm2]).

An inner coated disk Φ1= 2 cm, and a concentric
external coating define an angular gap of 0,5 mm.

Figure E-14: Basic experimental set up for surface conductivity