I think the main problem associated with the measurement of  the conductivity using powder specimen is its compactness, which determines the volumetric fraction of the current carrying conducting network compared to the voids present between individual grains. One   can't handle this specific problem  by sintering because that might deteriorate the sample. The best thing to get the particle size and its distribution function through XRD and then use it in a special network analysis software which converts the conductivity of porous substance into the bulk one. Even though due to cuırrent crowing at the contact surfaces of the grains becomes major problem but still you get some feeling about the effects of the particle size and the compactness. Good Luck. 

Note: we have the similar problem with the heat  conductivity of the green sands used in the molding for Foundry Casting practices. 

Dear Dr. Rubí Aglaé Hernández Carrillo 

Have a nice time and good day

I think that there is no ways to measures the conductivity of materials in their powder form. But,we can press the powder by using a hydraulic piston or any compressor of known pressure to get cylindrical-shaped pellets. These pellets of course have well know diameter and thickness.

Then the two opposite faces of the obtained pellets will be coated with conducting material, e.g. silver paste, paint or any other one. It should be noting that the two opposite faces must be gently polished and smooth before coating. Then each face after its coating will connected by a fine copper wires. The pair of these Cu wires will connected to the designed electric circuit. There are ways of connections, they are two point probe and four point probe methods.

For more detail, please go to this link. It is a research work  devoted to study the electrical properties of semiconducting  CdSSe bulk glasses. It may help you in your study.

https://www.researchgate.net/publication/283421231_Electrical_transport_properties_and_Mott%27s_parameters_of_chalcogenide_cadmium_sulphoselenide_bulk_glasses

Thanks and good luck

Your, 

Ahmed Saeed Hassanien

Article Electrical transport properties and Mott's parameters of cha...

Yes, four probe technique will be helpful for measuring the conductivity. You canmake paste of your material on steel electrodes or any surface which can be sandwiched easily fro measuring in sample holder.