Comparing Dielectic Materials

[EUnderwood]

I'm trying to compare two materials, one is macor ceramic and the other is called pyroceram (a glass-ceramic). I know the Volume Resistivity of Macor which is >10^14 ohm/cm but the spec sheets for the pyroceram shows Resistivity: Log
it seems to be in p(ohm x cm) = 6.8 @ 250C. Is there an easy way to compare the two materials. Any info would be great. Thanks

 

[DougMSOE]

You have hit on a problem that has driven me nuts for the last 30+ years.
The p (assume you mean rho) actually has the units of
(ohm * cm^2)/cm Hence the 'ohm * cm' units.
With this information you should be able to sort things out.

 

[jghrist]

The once common misapplication of ohm/cm units to resistivity was usually actually ohm·cm. I don't understand the meaning of "Log" in "...shows Resistivity: Log it seems to be in p(ohm x cm) = 6.8 @ 250C..."

6.8 ohm·cm is much too low for a ceramic glass. The resistivity of ordinary glass is 9x10^13.

 

[EUnderwood]

Thanks for replying and trying to Help me. I agree with you jghrist that 6.8 is too low, I am assuming that the number is low because it is some other form of Resistivity. That is what log must mean. I'm glad to know that ordinary glass is 9x10^13.

 

[jbartos]

Suggestion: Visit

http://www.coorstek.com/coorstek/pdf/8510-1072PlasmaPure.pdf

for: Log (ohm-cm) units

http://www.mindrum.com/tech.html

for: Log 10 (ohm-cm) units.
Specifically in your case:
10^14 ohm/cm
log 10 of 10^14 = 14
where log 10 is the decadic logarithm or the logarithm of base 10.
Therefore, to be able to compare those two, express 6.8 as
10^6.8=6309573.445 in ohm-cm
versus
10^14 ohm/cm
log 10 of 10^14 = 14
However, the units should correspond, i.e. ohm-cm in both cases.

 

[cuky2000]

Volume resistivity is a temperature dependant electrical characteristic. So it is specified at particular temperatures (usually 25°C).

For ceramic materials the resistivity is very high and is not a lineal function either as is for pure metals. Often the resistivity is expressed for convenienceas as logarithmic value.

The volume resistivity could be expressed as p = (LxW/T).R[sub]ref [/sub]

[sup] Where:
p= volume resistivity (ohm-cm)
L = length of chip element (cm)
W = width of chip element (cm)
T = thickness of chip element (cm)
R[sub]ref [/sub]= measured resistance (Ohm) @ reference temperature (Typ. 25°C)[/sup]

Here are few values for comparison purposes:

Macor Resistivity @ 25 oC > 1016 ohm-cm.

Log p @ 170oC > 11.5 p>10^11.5 ohm-cm
Log p @ 250oC > 10, p>10^10 ohm-cm
Log p @ 350oC > 8.3 p>10^8.3 ohm-cm

PyroCeram:
Log p @ 170oC > 8.0 p>10^8.0 Ohm-cm
Log p @ 250oC > 6.7; p>10^6.7 Ohm-cm
Log p @ 350 oC >5.3 p>10^5.3 Ohm-cm

For additional reference see the enclose sites

http://www.corning.com/lightingmaterials/images/macor.pdf

http://www.abrisa.com/downloads/dark_robax.pdf

 

[cuky2000]

Correction:

Macor Resistivity @ 25 oC > 10^16 ohm-cm.

 

[jbartos]

Comment: It is better to indicate the decadic logarithm log10 since there is then no confusion with the natural logarithm loge.

 

[EUnderwood]

You guys are awesome! Thanks,
-Eric