Non-equilibrium Spark Plasma Reactive Doping Enables Highly Adjustable Metal-to-Insulator Transitions and Improved Mechanical Stability for VO₂

Abstract

Although vanadium dioxide (VO 2 ) exhibits the most abrupt metal-to-insulator transition (MIT) properties near room temperature, the present regulation of their MIT functionalities is insufficient owing to the high complexity and susceptibility associated with V 4+ . Herein, we demonstrate a spark plasma-assisted reactive sintering approach to simultaneously achieve in situ doping and sintering of VO 2 within a largely short period (∼10 min). This enables high convenience and flexibility in regulating the electronic structure of VO 2 via dopant elements covering Ti, W, Nb, Mo, Cr, and Fe, leading to a wide adjustment in their MIT temperature (T MIT ) and basic resistivity (ρ). Furthermore, the mechanical strength of the doped VO 2 is meanwhile largely improved via the compositing effect of the highmelting-point dopant oxide. The high adjustability in MIT properties and improved mechanical properties further pave the way toward practical applications of VO 2 in power electronics, thermochromism, and infrared camouflage.

Manipulating the metal-to-insulator transitions of VO2 by combining compositing and doping strategies

“…This would further improve the transition abruption of the MIT properties of the VO 2 bulk pellet, in addition to strengthening their mechanical properties due to compositing, as demonstrated recently. 18 To introduce the compositing phase within the VO 2 pellets, a spark plasma-assisted reactive sintering (SPARS) strategy was used to co-sinter the powders of VO 2 and the dopant and/or compositing phase, according to our previous work. 18 On the one hand, the non-equilibrium process related to SPARS is expected to achieve elemental substitution when low-meltingpoint doping oxides are used (e.g., TiO 2 , WO 3 , MoO 3 and Nb 2 O 5 ).…”

“…18 To introduce the compositing phase within the VO 2 pellets, a spark plasma-assisted reactive sintering (SPARS) strategy was used to co-sinter the powders of VO 2 and the dopant and/or compositing phase, according to our previous work. 18 On the one hand, the non-equilibrium process related to SPARS is expected to achieve elemental substitution when low-meltingpoint doping oxides are used (e.g., TiO 2 , WO 3 , MoO 3 and Nb 2 O 5 ). On the other hand, since the compositing oxides with relatively high melting points (e.g., HfO 2 , Al 2 O 3 and CoO) cannot be effectively incorporated into the lattice of VO 2 , the unreacted precursors are expected to composite with the VO 2 matrix in this situation.…”

“…A spark plasma-assisted reactive sintering (SPARS) strategy was utilized for synthesizing doped VO 2 bulk materials, as reported in our previous work. 18 In brief, the powders of the precursor oxides, such as the VO 2 matrix and transition metal oxides (TM m O n ), were homogenously mixed at a desired stoichiometric ratio and pressed inside a graphite die. Afterwards, the graphite die was subjected to a large electric current (300 A cm À2 ), together with high pressure (20 MPa), to rapidly elevate the temperature to 900 1C and thereby trigger the solid-state reaction for 10 min in a coarse vacuum atmosphere.…”

“…Introducing a secondary compositing phase within VO 2 has previously been demonstrated to be an effective approach to suppress the propagation of inner cracks along the grain boundary and therefore improves mechanical properties. 18 For example, compositing VO 2 with highly resistive Al 2 O 3 largely enhanced the mechanical strength of VO 2 bulk pellets by 3.5 times (e.g., from 267.2 to 950 kgf mm À2 ). 18 Nevertheless, the impact of the presence of these compositing phases on the electrical transport properties is yet unclear.…”

“…18 For example, compositing VO 2 with highly resistive Al 2 O 3 largely enhanced the mechanical strength of VO 2 bulk pellets by 3.5 times (e.g., from 267.2 to 950 kgf mm À2 ). 18 Nevertheless, the impact of the presence of these compositing phases on the electrical transport properties is yet unclear. It has been previously recognized that the carrier transport and resulting MIT property of VO 2 are exceptionally susceptible to defects (e.g., grain boundary), as revealed by the polycrystalline VO 2 /quartz heterostructure.…”