It is not due to aggregation.NPs and nanoceramics sintering are example of Ostwald ripening. It happen because chemical potential of small particle is higher than the bigger particle. So atoms leave from high potential smaller particle and attaching to bigger particle. Since atoms leaving leads to increase in surface energy thereby chemical potential. So bigger particle growing bigger and bigger at the expense of smaller particle.
Low temperature compacting is very difficult with nanoparticles since they have kind of fractal structure due to agglomeration. Whereas the high temperature compacting results in the particle growth. Severe plastic deformation methods could be used during sintering to retain the nanostructured state.
If we calculate, number of atoms for a particular area of nanoparticle & bulk: in nanoparticles, number of atoms is less than their bulk matter where millions of atoms together in (for example - number of atoms calculation for Pb nanoparticles - Res.Appl.Mat.07/2013; 1(4):36. DOI:10.12966/ram.07.02.2013);
Will the same millions of atoms occupy particular area, while we transform the nano into bulk? If yes, from where the atoms will come; If no, the newly made bulk matter, will maintain the number of atoms (as equal to nano) for a particular area;
If the new bulk matter has number of atoms equal to nano: will it maintain all characters of nano? or will it maintain all characters of origin bulk? Or will show new characters with new bulk?
Article Lead Nanopowder as Advanced Semi-Conductor, An Insight
In generaI is possible, but which type of particles are you working?, also I agree with Sergei, SPD could be a technique for retain nanostructure. The characteristics of the bulk depends of several factors.