Unfilled epoxy polymers are naturally electrically insulating materials. However, when epoxy resins are filled with metal particles and then cured, the resulting composite material will be substantially less insulating than a neat epoxy resin. The particle material, shape, size, and percent loading will determine how much the electrical resistance is reduced compared to an unfilled alternative. Neat epoxy polymers have resistance of 1.0x10^12 ohm ⋅ cm, and silver filled epoxies can have resistance as low as 1x10^-4 ohm ⋅cm.
Although silver is expensive (its a precious metal!), silver is still the most common filler used for conductive epoxy adhesive formulating . Silver particles are highly electrically conductive, are available in useful shapes and sizes, and have a conductive oxide state.
Many engineers will be tempted to try adhesives filled with other metals such as nickel and copper to reduce costs. Unfortunately, these materials are far less effective than silver filled alternatives and tend to come with a long list of issues.
Pure nickel is about as conductive as silver (7x10^-8 ohm ⋅m for nickel compared to 1.6x10^-8 ohm ⋅m for silver). However, nickel particles do not disperse well in an epoxy resin. Loading an epoxy with a high percentage of nickel will result in a dry, clumpy material which makes for an impractical adhesive resin. Silver particles on the other hand result in a smoother resin, can be loaded at a higher volume, and ultimately make for far superior adhesives.