Point defect scattering via the formation of solid solutions to reduce the lattice thermal conductivity has been an effective method for increasing ZT in state-of-the-art thermoelectric materials such as Si-Ge, Bi2Te3-Sb2Te3 and PbTe-SnTe. However, increases in ZT are limited by a concurrent decrease in charge carrier mobility values. The search for effective methods for decoupling electronic and thermal transport led to the study of low dimensional thin film and wire structures, in particular because scattering rates for phonons and electrons can be better independently controlled. While promising results have been achieved on several material systems, integration of low dimensional structures into practical power generation devices that need to operate across large temperature differential is extremely challenging. We present achieving similar effects on the bulk scale via high pressure sintering of doped and undoped Si and Si-Ge nanoparticles. The nanoparticles are prepared via techniques that include high energy ball milling of the pure elements. The nanostructure of the materials is confirmed by powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and dynamic light scattering. Thermal conductivity measurements on the densified pellets show a drastic 90% reduction in the lattice contribution at room temperature when compared to doped single crystal Si. Additionally, Hall effect measurements show a much more limited degradation in the carrier mobility. The combination of low thermal conductivity and high power factor in heavily doped n-type nanostructured bulk Si leads to an unprecedented increase in ZT at 1275 K by a factor of 3.5 over that of single crystalline samples. Experimental results on both n-type and p-type Si are discussed in terms of the impact of the size distribution of the nanoparticles, doping impurities and nanoparticle synthesis processes.