Preparation of Epitaxial Uranium Dicarbide Thin Films by Polymer-Assisted Deposition

Abstract

High quality epitaxial thin films of cubic UC 2 were synthesized using a solution based technique. The films were characterized using XRD, UPS, Raman, and resistivity. The substrate lattice is yttrium stabilized zirconia and serves to stabilize the high temperature cubic phaseof UC 2 (>1765°C) at room temperature. The resistivity and UPS data indicate that UC 2 has relatively low electrical conductivity consistent with HSE hybrid DFT calculations showing a narrow band gap. In situ XRD measurements show that the UC 2 films oxidize to U 3 O 8 above 200°C. KEYWORDS: actinide carbide, epitaxial thin film, nuclear energy ■ INTRODUCTIONThe carbides of uranium have received much attention in recent years as potential fuel sources for Generation IV nuclear reactors. While at least six different Generation IV reactor concepts are currently being explored, the consensus is that new nuclear fuels must be developed for these advanced systems to maintain reasonable operating temperatures. 1 Two such possibilities are uranium carbide (UC) and uranium dicarbide (UC 2 ), which have much higher thermal conductivities than conventional nuclear fuels such as UO 2 , mixed-metal oxides, and ThO 2 . This would have the effect of lowering fuel centerline temperatures, which would increase operational safety, fuel lifetime, and efficiency. In addition to their potential as nuclear fuels, uranium carbide has been used for many years as a target material in the production of neutron-rich isotopes. 2 In order to gain a more in-depth understanding of the properties of uranium carbide materials, we have synthesized and characterized epitaxial thin films of UC 2 . These thin films are of high purity and are nearly single crystal in quality and thus facilitate high quality measurements to validate computational models. Herein, we report the synthesis and characterization of these thin films including valence band photoemission, Raman spectroscopy, electrical conductivity, and in situ XRD studies of oxidation.Uranium carbides have been known for quite some time, with U 2 C 3 and UC 2 first mentioned over a century ago. 3 These bulk materials were originally prepared by treating U 3 O 8 with graphite in an electric arc furnace. 3a,c,4 While carbothermic methods are still used to prepare uranium carbides, 5 other strategies such as the reaction of uranium metal with methane have been used to prepare UC at much lower temperature. 3b In spite of the wealth of knowledge on these materials, little is known about uranium carbide thin films. In 2004, researchers used sputter codeposition to generate films that were found to have compositions UC 1.2 and UC 1.6 . X-ray diffraction studies revealed the presence of polycrystalline UC and UC 2 in these films; however, the diffraction peaks were broadened possibly due to stress, inhomogeneities, or the presence of amorphous material. 6 Herein, we report the first preparation of singlecrystal quality UC 2 by polymer-assisted deposition (PAD). In this process, metal polymer solutions are used as film precursor...

Change of Composition in Metallic Fuel Slug of U–Zr Alloy from High‐Temperature Annealing

“…The profile of the Raman spectrum in Figure (b) is close to that reported for amorphous carbon (a‐C), in which the peaks of D and G bands appear at 1350–1360 cm −1 and 1560–1580 cm −1 , respectively . It is difficult to distinguish that the peaks at 1337 and 1575 cm −1 for uranium carbide occur in Figure (b) . Thus, our Raman data indicate that the U–Zr sample after annealing contained a‐C, which could originate from adventitious carbon.…”

“…After annealing at 610 C, it contained uranium metal, uranium carbide, uranium oxide with the U(V) valence state, and zirconium metal. 22 Thus, our Raman data indicate that the U-Zr sample after annealing contained a-C, which could originate from adventitious carbon. While no distinct peaks are observed in Figure 3(a), after annealing two peaks occur at 1330 and 1580 cm −1 (Figure 3(b)).…”

Recent Progress in Emerging Two-Dimensional Transition Metal Carbides

“…The TiC film shows a semiconducting behavior, where its resistivity at room temperature is about 372 μΩ cm. By the similar method, the uranium dicarbide films have also been epitaxially grown on yttria-stabilized zirconia substrate [110]. The investigation exhibits that by controlling the precursor and the annealing progress, it was able to prepare TMCs film with high quality.…”

Polymer assisted deposition of epitaxial oxide thin films

“…By following this simple procedure many different compounds have been grown: single [68][69][70][71][72][73][74][75][76][77][78][79][80][81] and multicationic oxides [38][39][40]60,[82][83][84][85][86][87][88][89][90][91][92] , nitrides 35,36,[93][94][95][96][97] , carbides [98][99][100] and pure element semiconductors, like Germanium 101 . Multicationic oxides are probably the more interesting category due to their large variety of compositions, crystal structures and functionalities.…”