**3.3 HPHT and high energy ball milling**

HPHT is a really appealing method of preserving ND fluorescent Nanodiamonds through electron radiation and subsequent high pressure high temperature microdiamond integration. The incorporation of irradiation and removal of mature HPHT diamonds for the creation of NV center in the diamond lattice are the two primary processes in this approach. The N2 melts the molten metal in the HPHT reactor and is then injected into the diamond for subsequent crystal formation. For thermos-kinetic reasons, the changed atoms of N2 remain isolated inside the diamond crystal and are bonded to neighboring atoms in a diamond sequence.

Strong particles drive carbon atoms out of their regular lattice positions upon irradiation by elevated fragments such as neutrons, ions, photons or electrons, creating voids (V) in the diamond line. Annealing, which is usually done at 800°C below vacuum, aids in the transferring of spaces to N2 atoms, resulting in the formation of N-vacancy centers. By choosing the right time and acceleration separator, separated nanodiamond sizes created for the HPHT technique via ultracentrifugation can be modified (from 4 to 25 nm). Using nitrogen-rich type Ib diamond powders, it is able to enhance the percentage of N-void centers in Nanodiamonds. As a result of decreased spin contamination within and surrounding the nanocrystal, the HPHT technique can create NDs of ultrasmall fluorescent NDs with substantially longer durations of infidelity [14].
