We study collisions between dust aggregates to construct a model of their structural evolution in protoplanetary disks. We carry out three-dimensional simulations of aggregate collisions and examine their compression and disruption processes following our previous two-dimensional simulations. We take clusters of ballistic cluster-cluster aggregation (BCCA) formed by a hit-and-stick process as initial structures and study their head-on collisions with the use of realistic binding forces. Our numerical results indicate that the energy criteria for compression and disruption of BCCA clusters are consistent with previous two-dimensional simulations. For aggregate compression at a collision, we succeed in obtaining a scaling law in which the gyration radius of the resultant aggregate is proportional to Eimp-0.10, where Eimp is the impact energy. Furthermore, we derive an "equation of state" of aggregates which reproduces the scaling law for compression. The equation of state is useful for describing the density evolution of dust aggregates during their growth.
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