Initiation of Self‐Propagating Combustion Waves in Dense Mo + 2Si Reactants through Field‐Activation

Abstract

Because of kinetic limitations, self-sustaining combustion synthesis reactions cannot be initiated in dense powder compacts. In compacts of Mo + 2Si, self-propagating waves can be initiated in samples with less than 78% relative density. At this and higher densities, no waves could be initiated without field-activation. In the presence of an electric field (at values of 7 and 13 V⅐cm −1 ), reactant compacts with densities up to 95% could sustain a combustion wave to produce MoSi 2 . In the absence of a field (for lowerdensity samples) the wave propagated in a non-steady-state (pulsating) mode, while under the influence of a field the wave propagated in a steady-state mode. The dependences of wave velocity and combustion temperature on the relative density of the reactants were qualitatively similar, showing maxima at a relative density of about 65%. These observations are explained in terms of the contribution of a liquid phase in the MoSi 2 -Mo 5 Si 3 binary to the synthesis kinetics. Although not detected by X-ray diffraction analysis, small amounts of Mo 5 Si 3 were discerned at the grain boundaries of the MoSi 2 product. The particle size of the silicide synthesized from 95% dense reactants was significantly smaller than those synthesized from reactants with lower densities, but the reason for this observation is not well understood at this time.

Historical perspective and contribution of US researchers into the field of self-propagating high-temperature synthesis (SHS)/combustion synthesis (CS): Personal reflections

“…A significant contribution into the area of shock densification of combustion synthesized intermetallics and ceramic materials was also made by Professor Thadani [212][213][214][215][216][217].…”

Combustion Synthesis: An Update

“…Initiation of self -propagating combustion waves in dense Mo + 2Si reactants was achieved through fi eld activation [31] . Such reactions were obtained in MoSi 2 B -Si composites in the FACS mode [32] .…”

Combustion features in the Mo–Si–B system Part 2. The effect of mechanical activation

“…Механическое активирование (МА) широко ис-пользуется для повышения реакционной способ-ности шихтовых смесей за счет увеличения вну-тренней энергии, снятия кинетических затрудне-ний, что позволяет осуществлять СВС даже в низ-коэкзотермических системах [2][3][4][5][6][7][8][9][10][11][12][13]. Ранее [2,5,6,[8][9][10]14] МА успешно применялось в качестве предварительной стадии подготовки реакцион-ной смеси к последующему синтезу материалов в системах Mo-Si [2,5,[8][9][10] и Mo-B [6,8,14].…”

“…Ранее [2,5,6,[8][9][10]14] МА успешно применялось в качестве предварительной стадии подготовки реакцион-ной смеси к последующему синтезу материалов в системах Mo-Si [2,5,[8][9][10] и Mo-B [6,8,14]. Так, было показано, что МА в системе Mo-Si за счет структурных изменений, заключающихся в повы-шении как удельной поверхности исходной смеси, так и концентрации дефектов, приводит к сниже-нию (на 400 К) температуры начала взаимодей-ствия и росту скорости горения.…”