Graphite/diamond transformation mechanism under the action of an iron-based catalyst

Abstract

Although it has been near 70 years since iron-based catalyst was used to synthesize diamond from graphite at high pressure and high temperature (HPHT), the graphite to diamond transformation mechanism...

New Insights into the Mechanism for Growing Cubic Boron Nitride Crystals under High Pressure

“…To obtain a temperature gradient, the sample is slightly off center as shown in Figure a because there is a temperature decrease from the cell center to the two ends of the cylindrical sample chamber with a temperature difference of 70 °C. The thus-determined high- P – T formation diagrams are plotted in Figure c–f, showing a similar valley shape of formation regions for different nitrides, distinct from previous reports with a narrow valley shape that might be tentatively depicted based on limited experiment data …”

“…The thus-determined high-P− T formation diagrams are plotted in Figure 1c−f, showing a similar valley shape of formation regions for different nitrides, distinct from previous reports with a narrow valley shape that might be tentatively depicted based on limited experiment data. 35 Using Ca 3 N 2 as a catalyst, the required minimum pressure for the formation of cBN is as high as 4.8 GPa, which is by contrast to that of Mg 3 N 2 and Sr 3 N 2 with a nearly same minimum pressure of ∼3.8 GPa. An intermediate pressure value of ∼4.5 GPa is observed for Li 3 N. According to these P− T diagrams, the lower limit of formation temperature for Li 3 N, Mg 3 N 2 , and Ca 3 N 2 is in the 1300−1350 °C range, slightly higher than that of Sr 3 N 2 (i.e., 1200 °C).…”

“…Further dissolving hBN into those molten intermediate phases would lead to supersaturated melts for precipitating cBN crystals because cBN is the most stable form above the associated minimum pressure. This is reminiscent of the concept for synthesizing diamond through a similar dissolution–precipitation method from Fe–Co–Ni alloy melts with a supersaturated carbon content at high- P – T conditions. − …”

“…Although the equilibrium phase boundary between cBN and its hexagonal precursor (hBN) is close to 5 GPa, , the direct conversion of hBN → cBN can only be realized above 11 GPa because there exists a large kinetic barrier between them, which is practically unfavorable for massive production of cBN as the required pressure is beyond the pressure capability of most large-volume presses (i.e., 5.5 GPa). In fact, such a stringent requirement for high pressure can be greatly relaxed by involving catalyst solvents such as alkali and alkali-earth metal nitrides [e.g., Li 3 N and M 3 N 2 (M = Mg, Ca, and Sr)] because the presence of those solvents profoundly reduces the kinetic barrier for transforming hBN to cBN, ,− similar to that of diamond with Fe–Co–Ni alloys as catalysts. − In contrast to diamond with a well-studied dissolution–precipitation formation process, , knowledge on the case of cBN is relatively fragmented, in spite of intensive experimental investigations over the past decades; thus, its formation mechanism is not clearly understood.…”

Mechanism of NO removal in selective catalytic reduction based on γ‐Fe2O3 catalyst doped with Mg element

“…[ 7 ] It is found that the catalytic effect of iron oxide is very impressive in the middle‐ and high‐temperature ranges. [ 11 ] It is found that loading Mn‐, Ce‐, and Fe‐based oxides on the catalyst can improve the de‐NO x effect of SCR. [ 9,12,13 ] Liang and Gui [ 14 ] verified the de‐NO x activity of the γ‐Fe 2 O 3 catalyst.…”