C.Q. Hu

Ar plasma treatment on few layer graphene sheets for enhancing their field emission properties

Plasma treatment in Ar gas on few layer graphene sheets (FLGSs), synthesized by plasma enhanced chemical vapour deposition, has been performed for enhancing their field emission properties. The plasma etching treatment for 3?min on the FLGSs, forming an extremely sharp edge, decreases the turn-on electric field from 3.91 to 2.23?V??m?1, and increases the maximum emission current density, drawn at a field of 4.4?V??m?1, from 33 to 1330??A?cm?2. It is expected that plasma treatment provides an efficient way to improve the field emission properties of FLGSs.

Effects of substrate bias on the preferred orientation, phase transition and mechanical properties for NbN films grown by direct current reactive magnetron sputtering

NbN films are deposited using direct current reactive magnetron sputtering in discharge of a mixture of N2 and Ar gas, and the effects of substrate bias (Vb) on the preferred orientation, phase transition, and mechanical properties for NbN films are explored by x-ray diffraction, selective area electron diffraction, and nanoindentation measurements. It is found that Vb has a significant influence on the stress in NbN films, leading to a pronounced change in the preferred orientation, phase structure, and hardness. As the substrate is at voltage floating, the stress is tensile. In contrast, as negative Vb is applied, the stress becomes compressive, and increases with increasing the absolute value of negative Vb. It is observed that a phase transition from δ (face-centered cubic) to δ′ (hexagonal) for NbN films occurs as Vb is in the range of −80to−120V, which can be attributed to a decrease in the strain energy for NbN films. In order to explore the relationship between the stress and phase transition as w...

Increasing sp3 hybridized carbon atoms in germanium carbide films by increasing the argon ion energy and germanium content

We have prepared germanium carbide (Ge1−xCx) films on Si(0 0 1) by radio frequency (RF) reactive sputtering a pure Ge(1 1 1) target in a CH4/Ar mixture discharge, and found that the sp3 hybridized carbon atoms in the Ge1−xCx film can be significantly increased in two ways. One is by increasing the Ge content via increasing the RF power during the film deposition, which can lead to a transition from sp2 C–C to sp3 C–Ge bonding in the film. Another is by increasing the Ar ion energy in a discharge Ar/CH4 gas by applying the negative bias voltage, which plays an important role in inducing the compressive stress in film. We find that when the compressive stress increases above a critical value of 2.2 GPa, an abrupt transition from sp2 C–C to sp3 C–C bonding occurs in the Ge1−xCx film, which is a consequence of energy minimization.

Modulation periodicity dependent structure, stress, and hardness in NbN/W2N nanostructured multilayer films

NbN/W2N nano-multilayer films with a modulation periodicity, Λ, ranging from 5.1 to 157.4 nm have been deposited on a Si(100) substrate by reactive magnetron sputtering in Ar/N2 mixtures. The Λ dependent structural and mechanical properties for the resulting NbN/W2N multilayers have been evaluated by means of low-angle x-ray reflectivity, x-ray diffraction, high-resolution transmission electron microscope, and nanoindentation measurements. The finding is that for films with Λ ≤ 10.6 nm, fcc NbN layers are coherent with cubic W2N layers, resulting in NbN layers and W2N layers that are in the compressive and tensile states, respectively. In contrast, as Λ is larger than 10.6 nm, a phase transition from W2N to W occurs in the W2N layer, which is a result of the coherent interface strain relaxation. For this case, all layers are in the compressive state, and the coherent interface disappears. The intrinsic compressive stress evolution with Λ can be interpreted in terms of interface stress. The formation of co...

Field electron emission enhancement of amorphous carbon through a niobium carbide buffer layer

We investigate the field electron emission for amorphous carbon (a-C) films deposited on Si (100) substrates through a niobium carbide buffer layer with different structures and find that the niobium carbide buffer layer can substantially improve the electron field emission properties of a-C films, which can be attributed to an increase in the enhancement factor β on the surface of a-C films after the insertion of the niobium carbide layer in between a-C film and substrate. Moreover, a phase transition for niobium carbide layer from hexagonal (Nb2C) to cubic (NbC) structure, revealed by x-ray diffraction, further enhances the electron field emission. The first-principles calculated results show that the work function of NbC is lower than that of Nb2C, which is the reason why the electron emission of a-C is further enhanced.

Thermal Stability of Microstructure and Mechanical Properties of NbNhard Films

Cubic δ-NbNfilm with (200) texture, hexagonalδ′-NbN films with a mixed (100)+(110) texture and (110) texture have been deposited on Si (100) substrate at-40, -160 and-200Vsubstrate bias, respectively. Vacuum heat treatments were performed to investigate the effects of annealing temperature on structural stability and hardness of δ-NbN and δ′-NbN films. The results show that for δ-NbN film and δ′-NbN films with a strong (110) texture,no phase transition occuredafter heat treatments.But for δ′-NbN films with a mixed (100)+(110) texture, phase transition from δ′-NbN to δ-NbNtook place, which can be ascribed to small lattice mismatch between δ′-NbN (100) and δ-NbN (111) and low phase transition barrier. In addition, the high substrate bias can improve the interface adhesion due to interface mixing resulting from high energy ions bombardment. Even after annealing at 900°C, the hardness for δ′-NbN deposited at-200V still remains 32GPa, which shows a potential application at the field of protect coatings.

Preparation and Microstructure, Mechanical, Tribological Properties of Niobium Carbide Films

Niobium carbide films was deposited by direct current reactive magnetron sputtering on Si (001) substrates in discharging a mixture of CH4/Ar gas. The effects of growth temperature (Ts) and methane flow rate (FCH4) on the phase structure, composition, mechanical and tribological properties for NbCx films were explored. For the film grown at FCH4=6 sccm, a phase transition from cubic-NbC phase to hexagonal-Nb2C phases occurred with increasing the Ts; In contrast, when the film deposited at FCH4=16 sccm, only the cubic-NbC phase was observed at different Ts. The surface of all the films became rough with increasing the Ts. In addition, when the Ts increased from RT to 600 °C, the films exhibited the compressive stress and kept rising. While as the Ts > 600 °C, the stress partially relaxed both at FCH4=6 sccm and FCH4=16 sccm. The hardness (H) for sample grown at FCH4=6 sccm first increased up to a maximum value, and then decreased with increasing the Ts. And the films grown at FCH4=16 sccm kept decreasing with the maximum super-hard value of the filmsof 40.5 GPa at FCH4=6 sccm and 600 °C. The friction coefficient for the film obtained at FCH4=16 sccm was lower than that at FCH4=6 sccm, which might be due to the presence more carbon in the film grown at FCH4=16 sccm.

Comparison of Residual Stress Measured by Diffraction Method and Matrix Etching Method in SiC Fiber Reinforced Titanium Matrix Composites

Measurement and modeling of residual stresses in SiC fiber reinforced titanium matrix composites (SiCf/Ti) are still a challenge. Diffraction method and matrix etching method can characterize the strains of matrix and fiber, respectively. A In this work, SiCf/Ti samples with perpendicular and parallel cross-sections to the fibers have been fabricated, and the residual stresses were measured by X-ray diffraction and matrix etching. The results showed that the axial matrix stress in sample with perpendicular cross-sections to the fibers was larger than that in sample with parallel cross-sections to the fibers, while the axial matrix stress calculated from etching was consistent with that from X-ray diffractions based on triaxle stress model, confirming the reliability of triaxle stress model.