In the academic world, there has been a surge of interest into the melting studies of molybdenum (Mo) due to there being major discrepancies in the literature regarding high-pressure melting temperature determination. Now, a group of researchers from the Carnegie Institution of Washington in Illinois have deduced that the melting temperature is defined by microstructural changes and an unexpected transition at high pressures and temperatures.

There is currently a large discrepancy in the literature surrounding molybdenum melting temperatures. Some theoretical reports have stipulated that the melting temperature of molybdenum should undergo a steep increase when subjected to high pressures (100 GPa), whilst some experimental results have stated an almost consistent melting temperature, regardless of the pressure applied. As such, there is currently a resurgence in molybdenum-based research to form a unified decision surrounding its ambiguity.

There have been many methods attempted to produce an accurate understanding into the melting characteristics of molybdenum, but for one reason or another, have failed to do so. Of these, notable methods are a laser heated diamond anvil cell and melt detection through synchrotron X-ray diffraction measurements.

Until now, these two methods have failed to produce accurate measurements, but show great promise. These are the two methods employed by the US-based researchers in their most recent study.

Laser heated diamond anvil cells are used to simultaneously generate high heat and pressure- up to several hundred gigapascals of pressure and several thousand degrees Celsius of temperature. Experiments utilising this method have been found to possess inaccurate results through spectoradiometry, but the errors are not large enough to account for the major discrepancies, suggesting that something else is to blame.

Synchrotron X-ray diffraction is considered to be a more consistent approach which usually depends on the in-situ observation of a diffuse scattering-halo, which is generally present as a liquid. However, the diffusion scattering produces a weak signal with laser heated diamond anvil cells, due to a minute amount of liquid being produced during the heating process.

Determining the Melting Characteristics

The researchers employed a solid-liquid-solid reaction that involved the encapsulation of molybdenum in magnesium oxide. The magnesium oxide acted as an insulating layer and the samples were heated to 1000 °C in a glove box (Innovative Technology PL-2GB-IL-GP1) with less than 0.1 ppm of water. Symmetrical diamond anvil cells, alongside cubic-BN seats and diamonds with diameters of 150-300 µm were used to introduce a high pressure. Raman spectroscopy, X-ray diffraction and spectoradiometry were used to characterise the melting phase transition, measure temperatures and thermal pressures, and image the system in two dimensions.

Through repeatedly heating and rapidly quenching the molybdenum samples, the researchers found it to possess a high-slope melting curve- a drastic difference compared to previous linear extrapolation results. The results also show temperatures theorised by computational predictions. Previous reports only tested up to 119 GPa, with this research determining the temperature changes up to 400 GPa. This method does also provide the potential for several hundred GPa of pressure, so there is room to investigate molybdenum at even higher pressures.

The melting curve, unlike other results, showed a low melting temperature for molybdenum at high pressures. There are distinct phenomena that could have attributed to the low melting temperature. The molybdenum undergoes a microstructural change at high pressures- a previously unknown, and unexpected, transition. The micro structural change produced a highly-textured body-centred cubic nanograin structure due to unstable grain boundaries and high atomic mobility. However, there is also the possibility that the low melting temperature is due carbon contamination- in the form of molybdenum carbide.

Microstructural changes were confirmed to have occurred and previous reports of d-electron band mediated melting causing the low-melting temperature has been disproved. The rate of nucleation and grain growth was also disbanded as a theory to why the melting temperature was low.

The results produced are significantly more consistent than any previous studies have provided. However, there is still one concern- whether carbon contamination was involved in lowering the melting temperature. This research has provided a very accurate basis to finally understand molybdenum’s interesting melting phase-transition temperature, but further work is yet required to investigate the presence of carbon impurities and to test the melting transition at extremely high pressures (up to several hundred GPa).

Source:

Hrubiak R., Meng Y., Shen G., Microstructures define melting of molybdenum at high pressures, Nature Communications, 2017, 8, 14562

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