# Material with fairly high resistivity, but allows flow of charge

Is there a material with fairly high resistivity (at least semi-conductor level), but also allows the flow of charge through it (and subsequently to the ground)? The flow of charge does not need to be fast, it can be very slow if necessary. The higher resistivity, the better.

So ideally, if that material is left alone on the ground, its steady-state should have very little charge, and thus have negligible or zero electric field (even if you initially applied some charge to it). Basically, the material is able to be discharged in finite-time, regardless of its fairly high resistivity.

The speed of discharge is the property I'm particularly interested in, but based on what I've seen, this property might be independent of resistivity.

I am not sure if such properties are documented, so if you know what the property is called, please tell.

Thanks

Edit: My wording is apparently confusing, so let me try to put it in other words. Here is a phenomena I have observed. There is a piece of rubber and a piece of glass on the ground, both equal in size and resistivity. I apply a static charge to both, and the rubber ends up discharging (to ground presumably) much faster than glass. What is that material property called?

• It is all relative so you could say that rubber has a smaller resistivity than glass or that rubber has a larger conductivity than glass. May 23, 2018 at 13:56

The two main properties that determine the speed of discharge of a conductive object are capacitance and resistance.

The product of the capacitance and the resistance, a time constant, could be used to ballpark the discharge rate (which follows exponential decay curve) and discharge time.

It is fairly straightforward to determine, when you have a discrete capacitor discharging though a discrete resistor.

It is more complicated, if you have a distributed capacitance and a distributed resistance, like in a human body or an anti-static mat (an example of a material you are asking about at the beginning of your post).

If you have a particular object grounded in a particular way, you can measure its time constant by charging it to a particular voltage, then grounding it (and the way you ground it may affect the result) and then monitor the voltage on the object as it discharges using a field meter or a voltmeter (if the resistance of the voltmeter is much higher than the effective resistance of the discharge path), minding that, for high resistivity objects, the voltage will be different at different points.

The time constant of your setup will be the time it takes for the voltage to drop by about 63%.

• Thanks. So an anti-static mat has high resistivity? What are they made of? May 24, 2018 at 3:18
• does discharge time depend on contact surface area/geometry at all? May 24, 2018 at 5:14
• Anti-static mats are made out of rubber with some conductive additives, possibly, carbon powder. They usually have the second layer at the bottom with much higher conductivity, which makes grounding and dissipation more consistent. The discharge time, in general, depends on the contact surface area and geometry, but I would not claim that it is always the case: it is better to know what particular setup is being considered.
– V.F.
May 24, 2018 at 18:13
• Would that mat have high resistance? (I'm guessing not). How would I choose/construct materials which maximise 'resistance' (not necessarily resistivity), but minimizes discharge time? May 26, 2018 at 11:38
• The top, dissipative, layer of the mat has very high resistance (Gohms), the bottom is less resistive (MOhms). As for a material which maximizes resistance and minimizes discharge time, I am not sure how to "choose/construct" it, at least without knowing more details about specific application or setup you have in mind.
– V.F.
May 26, 2018 at 14:16