When I tested semi-conductive rubber composite, I received negative delta values. Then how can I calculate dielectric loss of the composite?

 

Go back to basics. Construct a test capacitor from a pair of metal plates of known dimensions (maybe 100 mm x 100mm). Place a sample of your rubber composite of known thickness (preferably quite thin) between the plates. Place the capacitor in a test circuit and apply a signal from a signal generator. The test circuit could be an RC circuit with a known resistance R. Use an oscilloscope to measure the results. You haven't said why you want to know the dialectic loss. You may want to know loss versus frequency or you may be interested in DC loss. In any event, you can find the relevant equations here: http://academy.cba.mit.edu/classes/input_devices/meas.pdf I did a similar experiment once but the dialectic material was a liquid and I was looking for dialectic constant. I purchased a test capacitor with known dimensions for the experiment because I was working for a research center with very deep pockets.