The behavior of insulating materials under high direct voltage (DC) stresses depends on many factors compared to alternating current (AC) stresses and is currently not fully predictable according to the current state of research. The main reasons for this are, on the one hand, the electrical conductivity of the insulating materials used and, on the other hand, the accumulation of space charges inside of the insulating materials. Both factors lead to a complex electric field distribution and might cause accelerated aging. To overcome this issue two approaches are currently being pursued: the use of high-purity insulating materials to reduce the DC conductivity and the use of nanofillers for a defined distribution of charges through so called deep traps. Both approaches are currently used for cross-linked polyethylene (XLPE) in extruded underground power cables up to a DC voltage level of 525 kV. The weakest points in a cable system, however, are the high number of installed cable joints. Due to the outstanding mechanical and electrical advantages, liquid silicone rubber (LSR) is dominantly used as an insulating material for high-voltage alternating current (HVAC) cable joints. Thus, regarding the use for cable joints in high-voltage direct current (HVDC) transmission LSR has not yet been the subject of intensive research. For the reasons mentioned, a standard AC-LSR (for Alternating Current applications) will be examined and optimized with regard to its properties for application in DC systems. However, due to its chemically amorphous structure, the approach of a high-purity LSR is not possible and the approach with nanofillers is pursued. In this work a carbon black (CB) is used as the filler, which is already used in high concentrations for field control elements (e.g. for deflectors in cable joints and terminations). At the beginning of this research work, the influence of the CB concentration on basic electric properties is examined. In addition to the DC conductivity, this includes the permittivity, the loss factor and the DC dielectric strength. Based on these results, a range for the CB concentration is determined, which is suitable for use as an insulating material in HVDC cable joints. For a better understanding of the charge transport mechanisms and the effect of the used CB nanofiller, the method of thermally stimulated currents (TSC) is used. In order to investigate the space charge behavior, a test setup was designed according to the Pulsed Electro-Acoustic (PEA) method and optimized to investigate silicone elastomers. In addition to the technical optimizations, the signal processing of the PEA system is also discussed in detail. The investigations at different CB concentrations, electric field strengths and temperatures show that the use of nanoscale CB has positive influence on the space charge behavior, and the accumulated space charge is effectively reduced. Using the presented characterization methods, a range for the CB concentration could be determined that is suitable for insulating materials in HVDC application. This has already been taken up by the manufacturers of the LSR material and is to be tested in real cable joints.