The melting curve of cobalt under high pressure
Introduction
In the early 1950s, high-pressure phenomena began to attract widespread interest [1]. A longstanding problem in the study of materials is the determination of their melting curves and phase diagrams. The melting of transition metal at high pressures has wide scientific implications, particularly for earth and planetary sciences [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]]. Cobalt, as one of the typical representatives of transition metal, which is adjacent to iron and nickel in the periodic table, is potentially necessary for understanding the properties of the Earth's core, which is believed to be composed of iron dominated alloys, with possibly Co or Ni as minor components. The melting behaviour of Co at high pressure and high temperature may shed light on the phase diagram of iron [9,13]. In addition, cobalt, as a binder material or solvent, is widely used in polycrystalline diamond (PCD) synthesis and the growth of large diamond single crystals in industrial applications [[14], [15], [16]]. Therefore, the study of the melting curve of cobalt under high pressure may have considerable significance for the synthesis of PCD and large diamond single crystal growth.
The melting point of metals is primarily studied using laser-heated diamond anvil cell (DAC). However, there are considerable discrepancies in the melting curves of metals among laser-heated DAC measurements, especially for transition metals [17,18]. Furthermore, there is virtually no experimental data on the melting point of Co under high pressure ranging from 5 to approximately 10 GPa, and this pressure range is just the synthetic pressure range for industrial PCD and diamond large crystals [11,19]. However, although differential thermal analysis (DTA) can be used to study the processes that a substance undergoes when heated, it has lower assembly integrity than the HPTM that can reduce the temperature and pressure accuracy [[20], [21], [22], [23]]. Therefore, we surmised that the HPTM was an effective and simple method of measuring the melting point at high pressure. However, the melting curve of Co cannot be determined using a large volume cubic press. This motivated us to investigate the high-pressure melting curve of Co using the HPTM in a large volume cubic press.
We calibrated the cubic press pressures using the well-known pressure-induced phase transitions of metals (Bi: 2.55 GPa, Tl: 3.68 GPa, and Ba: 5.5 GPa) and semiconductors (ZnTe: 5 GPa, 9.5 GPa and 12 GPa, and ZnS: 15.5 GPa) [[23], [24], [25], [26], [27]]. The high-pressure melting curve of aluminium and lead were measured to assess our calibration results using the HPTM, which agreed with those of previous studies [19,28,29]. The melting points of cobalt are measured at 3, 4, 5, 5.5, 8, 10, and 12 GPa using the same method. The scanning electron microscopy (SEM) and energy spectrum analysis (EDS) of the quenched samples obtained the extra evidence of the melting behaviour under different pressures. Additionally, the melting curve was compared with theoretical calculations, DAC measurements that agreed with Lazor's experimental data, and Wen-Jin Zhang's calculations [30,31]. Our results indicate that cobalt has a similar trend with the melting curve of Fe and Ni under high pressure [[32], [33], [34], [35]].
Section snippets
Experimental
The high-pressure high-temperature (HP-HT) experiments up to 5.5 GPa were conducted in a DS6 × 14 MN cubic press. A schematic diagram of the cell assembly is shown in Fig. 1(a). HP-HT experiments at 8, 10, and 12 GPa were carried out in a two-stage multi-anvil apparatus based on the DS6 × 8 MN cubic press (Zhangjiakou Exploration Machinery Factory, China) developed at Sichuan University using the same experimental technique. The cell assembly details have been described elsewhere [36,37]. The
Pressure assessment
The actual pressure during heating may differ from the value determined before/after due to the thermal pressure effects [47]. Thus, the known fusion curves of aluminium (purity: 99.95 wt%, pre-compressed density: ~1.8 g/cm3, and granularity: 1 μm, Aladdin, Shanghai, China) and lead (purity: 99.999 wt%, pre-compressed density: ~9 g/cm3, and granularity: 200 mesh, Sinopharm Chemical Reagent Co., Ltd., Beijing, China) were used to assess the HPTM. The results were compared with the previous
Conclusions
Cobalt's melting curve was determined under high pressures up to 12 GPa in a large volume cubic press via the in situ high-pressure temperature measurement method. Two melting criteria were used to confirm the high-pressure melting behaviour. The slope of the new melting point with increasing pressure was approximately 33 K/GPa in our experimental pressure range and progressively decreased under higher pressure. In addition, our new melting curve under high pressure was reasonably consistent
Acknowledgements
This study was supported by the National Key R&D Program of China (no. 2018YFA0305900) and the National Natural Science Foundation of China (grant nos. 11427810 and 51472171).
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