Materials which melt

Spectral radiance of the hot samples was measured in situ , showing that the optical emissivity of these compounds plays a fundamental role in their heat balance. Independently, the results show that the melting point for HfC 0. The design of next generation hypersonic flight vehicles has raised interest in the discovery and development of materials that can operate in extreme environments. Hypersonic vehicles are equipped with sharp nose tips and leading edges to maximize flight performance.

The extreme conditions required for hypersonic applications have introduced the motivation for research and development of high temperature materials, including a group of ceramics commonly known as ultra-high temperature ceramics UHTCs. From a wide selection of materials that an engineer can choose only a limited number have melting temperatures above this criterion 3.

They are known to form a continuous solid solution over the whole range of compositions. The measurement of thermophysical properties at such high temperatures is difficult due to increased reactivity of the materials, heat losses, volatility, and loss of mechanical strength, particularly in the carbides of interest in this work.

Several experimental investigations of the high temperature behaviour of pure and mixed tantalum and hafnium carbides were carried out mostly between and , including the early work by Agte and Althertum 4 and the comprehensive experimental campaign provided by Rudy 5.

In all these cases, high temperature melting of these carbides was studied with the help of the so-called Pirani-Alterthum approach 4. This experimental method consists in observing the disappearance, in a sample heated by Joule effect, of a blackbody hole when it is filled by a newly formed liquid. Such an approach has been shown to be effective in many cases. Agte and Altherthum 4 reported a maximum melting temperature T m for Ta 0. The temperature trend with a maximum melting point at an intermediate composition was not confirmed in the work by Rudy 7.

Rudy reported that the highest melting temperature compound is TaC 0. More recently, Hong and van de Walle 9 predicted the melting temperatures using density functional theory DFT and their calculations suggest that the melting temperature of HfC 0.

Also these calculations were consistent with a local maximum within the solid solutions, for a composition close to Ta 0.

The work of Hong and de Walle was particularly useful, in that it identified precise physical mechanisms behind the effects of carbon hypostoichiometry and alloying on the melting temperature of a given composition. In addition, HfC 0. In summary, the uncertain and contradictory results on the melting points reported suggest that: i thanks to the entropic contribution of lattice defects to reducing the free energy of the solid phase, slightly hypostoichiometric monocarbides might have melting temperatures higher than their solid solutions 7 , 8 , 9 ; ii because of the Fermi energy position in the mixed carbides, a maximum melting temperature within the solid solutions TaC-HfC might exist 4 , 6 , 9.

This calls for further experimental work perhaps with other techniques than were previously available. The laser melting technique used in this work constitutes an alternative to the Pirani-Alterthum method for studying the melting temperatures of the Ta-Hf-C compounds.

It allows precise control of the time for which the sample is kept at extreme temperatures, which can be reached in the order of milliseconds if need be. Experiments of subsecond duration address several of the challenges associated with measurements at very high temperatures.

The laser pulse shape and duration can be optimised in order to produce the desired heating and cooling rates while avoiding or at least minimising undesired effects such as sample vaporisation and reaction with the container. A pressure cell filled with an inert buffer gas at a moderate pressure of about 0. Millimetre long graphite screws hold the sample, so that interaction with the container is kept to a minimum.

In addition, only the central region of the sample surface is typically melted. This is then surrounded by solid material in both the radial and axial directions. The laser melting technique has been successfully used to study several refractory systems such as uranium carbides 10 , uranium oxides 11 , plutonium oxides 12 , 13 , uranium nitrides 14 and zirconium carbides 15 , The results obtained on compounds with already well-assessed phase transition temperatures are in good agreement with the literature data.

The current experiments were carried out using a laser-heating technique, as described in detail by Manara et al. A schematic of the experimental laser-heating set-up is shown in Fig. They were used in the present work and in the previous investigations on the same materials 4 , 6 , 7. They measure at one or more wavelengths the sample radiance, which is the electromagnetic radiation power density per unit surface, wavelength and solid angle thermally emitted by the sample at a given temperature.

A blackbody is a surface that absorbs all radiant flux of all wavelengths and polarisations incident upon it from all possible directions. For a prescribed temperature and wavelength, no surface can emit more thermal radiation than a blackbody, which in addition is an isotropically diffuse lambertian emitter. A blackbody is an ideal surface that can only be approximated in practice. Therefore, it assumes values between zero and unity, the latter value corresponding to a blackbody. For the particular case of emission in the direction normal to the surface it is called normal spectral emissivity NSE.

The value of this parameter is related to the electronic properties and the surface morphology of the investigated material. The latter can however be neglected, in a first approximation, when one assumes as ideally flat the surface of a melting solid, or a freezing liquid like those studied here. The value of the NSE determines how bright a surface appears compared to a blackbody at the same temperature and wavelength.

The spectral radiance temperature of a surface is the temperature at which a blackbody emits the same amount of spectral radiance as the surface. As the NSE is smaller than one, non-blackbodies always appear less bright than a blackbody would at the same temperature and, therefore, the radiance temperature is always lower than the true temperature of the non-blackbody. This reflected light signal RLS was used to facilitate the detection of phase transitions by observing sudden changes and oscillations in the RLS, in parallel to the classical thermal arrest analysis directly carried out on the temperature vs.

The latter NSE data were preferred to those measured by other authors 20 , 21 , 22 because the samples investigated by Deadmore were produced with a more similar technique and had more similar carbon content to the current ones. For example, Danilyants et al. However, their data cannot be used as a reference for the current investigation, as they were obtained on samples with significantly less carbon, which obviously resulted in a more metallic behaviour of the NSE. It should be noted, as highlighted by Danilyants et al.

In order to shed some light on this complex matter, a test exercise was performed. In a first approximation, recommended by Neuer 23 , the fitting procedure was performed within the grey-body hypothesis, i. Subsequently, more tests were performed assuming a decreasing NSE with increasing wavelength.

The pre-heating stage and the successive pulses were conceived to minimise thermal stresses and the risk of mechanical failure of the samples. The output signals of a laser experiment for TaC 1. In order to produce a pronounced freezing arrest it was not enough to exceed the melting temperature momentarily.

A sufficiently large mass had to be melted, for instance by using a longer laser pulse. Oscillations in the RLS and a thermal arrest in the cooling section of the thermogram confirmed that the sample had melted and re-solidified.

Melting of the sample might be difficult to identify using only the thermogram, since, as predicted by numerical simulations of the experiments 11 , due to the highly non-uniform surface character of laser heating no thermal arrest is observed during fast heating. Only a weak change in the surface heating rate may be visible in these cases, caused by the progression of the melting front into the sample that consumes a part of the imparted laser energy as more material is melted.

Therefore, the melting initiation is often easier to identify with the RLS signal. The noise observed after melting at the highest temperatures may in some cases be caused by boiling of the sample surface Since no further inflections were observed during heating or cooling it can be inferred that, within the current experimental uncertainties, TaC melts congruently and does not show further phase changes below the melting temperature.

Further analysis of the clear solidification thermal arrest showed that the T m of TaC 1. Here and in the rest of this work, uncertainty bands are calculated using the error propagation law [11], considering as independent the uncertainties due to NSE, data dispersion and pyrometer calibration.

During melting a significant volume of vapour was observed visually, which is related to the disturbed shape of the thermogram at the highest temperatures The experiment geometry was actually conceived in such a way that the vapour plume evolving parallel to the surface towards the upper and cooler parts of the disk would not interfere with the optical measurement of the temperature, performed on the hottest zone and perpendicularly to it.

Sequences of four successive pulses were performed. This helped to reduce thermal stresses and preserve the mechanical integrity of the material throughout the tests. In the current tests, durations of the successive laser pulses were varied by a factor up to ten, and their powers by a factor up to two. The observed phase transition temperatures were indeed independent, within the experimental uncertainty, of the laser pulse power and duration, whereas, of course, longer and more powerful laser pulses implied higher peak temperature, longer dwelling time beyond melting, larger vapour production etc.

In order to carry out additional tests of possible vapour plume interference with the pyrometric temperature measurements, more laser shots were performed with higher peak power. Nonetheless, even in these extreme cases, the solidification arrests were observed at the same temperatures as those recorded in more standard experiments, within uncertainty. The red signal is the power output profile of the laser.

Similarly, Ta 0. Comparing the profiles of TaC 1. At higher temperatures there was less disruption in the RLS signal compared to that of TaC, and subsequently less vaporisation during melting. After the end of the laser pulse the sample was cooled naturally and a slight undercooling effect was produced during the thermal arrest.

This effect is common during fast freezing conditions and is related to the solidification kinetics nucleation and growth It should be noted here, that the observed thermal arrests correspond to the solidus temperature, which is the most relevant for technological applications as it marks the onset of melting and the thermal stability limit of these materials. Unfortunately it has been impossible, with the current set-up and uncertainty, to distinguish a liquidus transition in the present thermograms.

Similar experiments were conducted on Ta 0. During the initial pulse of the HfC 0. Melting of HfC 0. The lower thermal diffusivity 24 of HfC compared to TaC produces higher heat concentration in the sample surface, producing higher temperatures at lower applied power. Furthermore, much less vaporisation was observed in pure hafnium carbide compared with all of the other compositions, judging from the size of the vapour plume traces visible on the sample holder above the sample after the melting experiments.

Most probably the excess power needed to heat the other compositions was largely dissipated in the sample vaporisation. However, higher temperatures were necessary to melt the HfC samples. A change in slope in the thermogram was observed as the sample started to melt Fig. With the Deadmore NSE value of 0. The results are summarised in Fig.

They reveal that, within the uncertainty of the Deadmore NSE values, there could indeed exist a local maximum melting point of the solid solutions near the Ta 0. However, this maximum melting point is only local, because the maximum observed melting point in the Ta-Hf-C system is for HfC 0.

Intermediate compositions between Ta 0. The highest melting temperature previously reported for HfC 0. Gusev et al. The trend obtained in our work is similar to the one predicted by Hong and van de Walle 9 with HfC 0. Interestingly, the solidification-temperature radiance spectra follow only partially the trend reported in Fig.

Therefore, the existence of a local maximum melting temperature for Ta 0. In addition, the monotonic trend of spectral radiance vs. Aluminum-Nickel Alloy. Aluminum-Scandium Alloy. Aluminum-Silicon Alloy. Beryllium-Copper Alloy. Magnesium-Antimony Alloy. Magnesium-Nickel Alloy. Nickel-Tungsten Alloy. Wood's Metal. The team analyzed the Hf-Ta-C material for which the melting point had already been experimentally determined.

The simulation was able to elucidate some of the factors that contribute to the material's remarkable heat tolerance. The work shows that Hf-Ta-C combined a high heat of fusion the energy released or absorbed when it transitions from solid to liquid with a small difference between the entropies disorder of the solid and liquid phases. Researchers then used those findings to look for compounds that might maximize those properties.

They found that a compound with hafnium, nitrogen, and carbon would have a similarly high heat of fusion but a smaller difference between the entropies of the solid and the liquid.

When they calculated the melting point using their computational approach, it came out K higher than the experimental record. The group is now collaborating with Alexandra Navrotsky's lab at the University of California, Davis, to synthesize the compound and perform the melting point experiments.

Navrotksy's lab is equipped for such high-temperature experiments. The work could ultimately point toward new high-performance materials for a variety of uses, from plating for gas turbines to heat shields on high-speed aircraft.