Ultralow-Temperature Field Activated Sintering of Amorphous B₄C under Millimeter Wave Irradiation and Pressure

Abstract

The millimeter wave pressure sintering featuring power output (W_m) control of amorphous B₄C powders is proposed without the need for any additive, in order to reveal definitely divided densification far below a glass transition. With the constant-rate heating technique, amorphous B₄C containing nanocrystalline rhombohedra can be consolidated without sub-μm sized pore at 50 MPa. The heating rate used is found to be a dominating variable; with an increase from 10 to 20 K·min^-1, it leads to a decrease from 510 to 444 K in full-density temperature. Its second-stage densification is expressed by the constitutive equation of consolidating strain rate ε˙_n via non-Newtonian flow: ε˙_n=ε˙_osinh(Cσ_eff/RT)exp(-Q/RT) with a decreasing apparent activation energy Q from 42.5 to 35.4 kJ·mol^-1 under an increased W_m. The constant-power output technique with a maximum W_m of 2 kW enables one to obtain a considerable initial abrupt shrinkage and subsequently full consolidation at the lowest temperature of 386 K under 98 MPa. The process control of the first-stage densification is then characterized by a neck formation law based on the first order chemical reaction d(A_f-A)/dt = -k_p(A_f-A) where A is the fraction of a contacted area between particles, having an increasing k_p with W_m.