Open Access

Amol Karandikar

• A new ‘laser flash-heating’ method has been developed for measuring melting temperatures above 2000 K in a diamond anvil cell at gigapascals of pressure. It overcomes the general difficulties in detecting an onset of melting in a diamond anvil cell. It also circumvents the notorious experimental difficulties associated with the long heating durations of the CW laser-heating and the short timescales in the pulsed laser-heating and shock-compression experiments. In this method, the duration of heating a sample is tuned to avoid chemical reactions of the sample with the diamond anvils and the surrounding pressure medium, while maintaining the accuracy of the temperature measurements. The absence of chemical reactions is confirmed by the EDS technique. Melt detection is now unambiguous from the analysis of textures on the surface and in depth of the recovered samples using the SEM and FIBM techniques, respectively. Using this method, the following has been achieved. 1. The melting curve of hcp-Re has been measured to 48 GPa, 4200 K for the first time. It has a significantly steeper slope than those observed for other transition metals like W and Mo with bcc structures. Above 20 GPa, Re becomes the most refractory metal surpassing W. 2. The melting curve of bcc-Mo has been measured to 45 GPa, 3100 K. It agrees with previous melt-slopes approaching zero value with pressure as reported in the LHDAC experiments using ADXRD and visual observation techniques for inferring the onset of melting. Flash-heating experiments at pressures higher than 50 GPa are required to further corroborate the flat melt-slope and resolve the long standing controversy about melting of Mo. 3. The melting curve of bcc-Ta has been measured to 85 GPa, 4300 K. Unlike in previous experiments using ADXRD and visual observation as probes, it has been tightly bracketed with an unambiguous detection of the onset of melting, without any chemical reaction. The present melting curve cannot be reconciled with shock measurements and theoretical predictions, and the precision of measurements calls for a reevaluation of theoretical, shock compression, and other DAC approaches to determine melting at high pressures. A further analysis with TEM technique for investigating the structure of the heated portion below and above melting temperatures of Ta may benefit in resolving various phase transitions predicted to explain the vast discrepancies in the reported melt-slopes. When extrapolated to one atmosphere pressure, all the measured flashmelting curves agree with the known melting points.
• Die Änderung der Schmelztemperatur einer Substanz unter hohen Drücken ist insbesondere in den Erd- und Materialwissenschaften von Interesse und deren Messungen sind für die Entwicklung bisher unzureichender theoretischer Modelle von grosser Wichtigkeit. In den Erdwissenschaften spielen Schmelzeigenschaften bei hohen Drücken eine wichtige Rolle bei der Interpretation seismischer Daten und der Modellierung dynamischer Prozesse. Insbesondere sind die Schmelztemperaturen von Eisen (Fe) und seiner Verbindungen mit Elementen kleiner Ordnungszahl an der Grenze des inneren Kerns (einem Druck von 330 GPa entsprechend) ein Schlüssel zur Abschätzung der Kerntemperatur. Da eine Messung von Schmelztemperaturen mit der notwendigen Genauigkeit bei solch extremen Bedingungen (über 300 GPa und 4000 K) nicht möglich ist, müssen bei niedrigeren Dr¨ucken gemessene Daten angemessen extrapoliert werden. Dies erfordert die genaue Messung von Schmelzkurven bei P -T -Bedingungen, die mit den verfügbaren experimentellen Techniken erreichbar sind...