Laser ablation synthesis of tantalum carbide particles with specific phase assemblage and special interface

Abstract

Pulsed laser ablation of bulk TaC in vacuum under a high power density was used to fabricate fine-sized tantalum carbide particles, i.e., γ-TaC_1−x with varied extent of carbon deficiency and α-Ta₂C surrounded by an amorphous phase of Ta-doped carbon clusters or lamellae according to X-ray and electron diffraction. The predominant γ-TaC_1−x has a high x value (~0.4) and almost spherical shape when rapidly solidified as submicron-sized particulates, whereas x ~ 0.2 and facetted with occasional {111} coalescence twin when condensed as nanoparticles. The minor α-Ta₂C occurred either as nanocondensates with hexagonal crystal form or as a stable epitaxial intergrowth with the γ-TaC_1−x particulate having close-packed planes in parallel with the precipitation process. The γ-TaC_1−x and α-Ta₂C nanocondensates were also coalesced approaching a secondary relationship, i.e., [011]_TaC1−x //[01$\overline{ 1}$0]_Ta2C and (100)_TaC1−x //(0001)_Ta2C having a fair coincidence site lattice at the interface. The refractory materials have a bimodal minimum band gap (ca. 3.8 and 2.3 eV) for potential optocatalytic and tribology applications at high temperatures.

Appendices

Appendix 1

See Fig. 14.

Fig. 14

Ta–C phase diagram [1]

Appendix 2

See Fig. 15.

Fig. 15

TEM lattice image magnified from the inset in Fig. 10a showing that the γ-TaC_1−x core has the faceted surface parallel to the adjoined turbostratic graphene layers developing from the amorphous carbon (a-C) shell

Appendix 3: Gaseous phase and liquid droplet formation by nanosecond (ns) laser pulses

For the ablation of the ns laser, the material ejection is likely to be dominated by thermal processes [37]. The laser pulse duration, in these applications, is typically shorter than the time of dissipation of the absorbed laser energy by the thermal conduction, the condition that is commonly referred to as thermal confinement [38, 39]. As a result, the absorbing material can be over-heated much beyond the boiling temperature, turning a normal surface evaporation at low laser fluences into an explosive vaporization, or phase explosion, at higher fluences. Theoretical predicts [38] and laser ablation experiments [40] indicated that phase explosion results in a spontaneous decomposition of the ejected plume into a two-phase system of gaseous phase and liquid droplets.

During the ns laser ablating, photons can couple with both electronic and vibrational modes of the target material, and furthermore, the electron–electron coupling results in an immediate rise in the electron temperature and eventual vaporization of the transiently heated target [30]. When the laser power density becomes sufficiently high (>10⁵–10⁸ W/cm²), the vaporization starts, and the evaporated material (vapor atoms) will expand [41]. Then, the vapor plume and background gas interact each other, yielding the confinement of the plume, whereas the background gas is pushed further away from the solid target. Since the temperature in the vapor plume can rise to much high values, a plasma plume will be generated during the front part of the incident laser pulse irradiating the solid target [30]. The plasma plume excitation and ionization are mainly a result of the multiphoton absorption, ionization, and inverse-Brehmsstrahlung absorption in the gaseous phase induced by the laser pulse [39]. Therefore, the plasma plume consists of clusters, molecules, atoms, ions, and electrons from the target solid [30, 41, 42].

Appendix 4

See Fig. 16.

Fig. 16

Standard Gibbs free energy (ΔG ^o, kJ/mol) of carbide-formation reactions as a function of temperature up to ca. 1600 K for TiC, TaC, and Ta₂C (solid lines after Ref. [48]) having extrapolation (dash line) intersection at ca. 3000 K for TiC and TaC (cf. text)