Preparation of nitrogen-rich CNx films with inductively coupled plasma CVD and pulsed laser deposition

Nitrogen-rich amorphous carbon nitride films with N/(N+C)≥0.5 have been deposited with three different methods, namely: (i) inductively coupled plasma CVD utilizing chemical transport reactions (ICP–CTR); (ii) inductively coupled plasma CVD with gaseous precursors (ICP–GP) and (iii) pulsed laser deposition (PLD) with additional r.f. plasma discharge. By means of plasma diagnostic measurements it is shown that in each case high concentrations of active radical species (e.g. CN and N) are necessary to obtain high nitrogen concentrations. On the other hand, these nitrogen-rich films turned out to be mainly sp² bonded having rather low densities of 1.8–2.0 g cm⁻³ only, irrespective of the method. From a comparison of the three techniques, and of further literature data, conclusions are drawn regarding the conditions necessary to obtain high N/(N+C) ratios, and regarding the deposition of superhard, crystalline sp³ bonded carbon nitride modifications.     

Unusual electron transport in Heusler-type Fe2VAl compound

The electrical resistivity of D0₃-type (Fe_1−xV_x)₃Al shows an anomalous temperature dependence characterized by a resistance maximum near the Curie point and a negative resistivity slope at higher temperatures up to 1000 K and above. In particular, the Heusler-type Fe₂VAl compound is found to be in a marginally magnetic state and to exhibit a semiconductor-like behavior with the resistivity reaching 3000 μΩ cm at 2 K, in spite of the presence of a clear Fermi edge as revealed in valence-band photoemission spectra. A substantial mass enhancement deduced from electronic specific-heat measurements suggests that Fe₂VAl is a possible candidate for a 3d heavy-fermion system. According to electronic structure calculations, Fe₂VAl is a nonmagnetic semimetal with a narrow pseudogap at the Fermi level. The unusual electron transport is mainly interpreted in terms of the effect of strong spin fluctuations, in addition to the existence of very low carrier concentrations.     

Thermal expansion of some pre-stressed nuclear grade graphites

Thermal expansion coefficients (CTE) of some nuclear grade polycrystalline graphites were measured in the range from room temperature to 1173 K after pre-stressing to levels comparable with their fracture strength. The CTEs has been found to increase with residual strain independently of the samples as well as their manufacturing processes. Analytical discussions are given on the effects of both pre-stressing and/or thermal annealing. Phenomenological formulae appropriately expressing CTEs in heating and cooling processes have been derived.     

Effect of melting rate on microsegregation and primary MC carbides in M2 high-speed steel during electroslag remelting

Large-size primary MC carbides can significantly reduce the performance of M2 high-speed steel. To better control the morphology and size of primary MC carbides, the effect of melting rate on microsegregation and primary MC carbides of M2 steel during electroslag remelting was investigated. When the melting rate is decreased from 2 kg·min⁻¹ to 0.8 kg·min⁻¹, the columnar dendrites are gradually coarsened, and the extent of segregation of Mo and V is alleviated, while the segregation of Cr becomes severe. At 2 kg·min⁻¹, the number of primary MC carbides per unit area with the sizes in the range of 2 µm to 6 µm accounts for about 75% of all MC carbides, while the carbides are mainly concentrated on the size larger than 8 µm at 0.8 kg·min⁻¹. Thermodynamic calculations based on the Clyne-Kurz (simplified to C-K) model shows that MC carbide can be precipitated in the final solidification stage and a smaller secondary dendrite arm spacing caused by higher melting rate (2 kg·min⁻¹ in this experiment) facilitates the refinement of primary MC carbides.

     

Microstructural features and thermal stability of alumina-bonded nano-polycrystalline diamond synthesized by detonation sintering

In this work, alumina-bonded nano-polycrystalline diamond (NPD) was synthesized by detonation sintering in the temperature range of 3000–3500 K and pressure range of 15–25 GPa. The microstructures and thermal stability of the NPD detonation sintered at 3255.05 K and 24.51 GPa were studied, and are described herein. Transmission electron microscopy and electron diffraction revealed that the polycrystalline diamond has a unique formation process and no graphitization. Scanning electron microscopy indicated that the size of polycrystalline particles increased in samples 2, 3, and 4. And thermogravimetric analysis indicated that the thermal stability of the diamond particles was enhanced. The 18% mass loss of specimen corresponded to the oxidation and decomposition of the amorphous carbon and carbon-containing compounds synthesized by detonation. Finally, graphitization calculations showed that the graphitization probability of polycrystalline diamond produced at the temperature of 3255.05 K and pressure of 24.51 GPa was 15.04%.     

Simulation on vertical wicking behaviors of liquid hydrogen within metallic weaves in terrestrial and microgravity environments

To predict the wicking performance of liquid hydrogen (LH₂) in porous screens, we introduced an evaporative wicking model which has been verified by experiments in liquid nitrogen (LN₂). The effects of the fluid properties, the gravity level, the superheated degree and the screen structure parameters on the vertical wicking of saturated LH₂ were systematically investigated. The wicking in LH₂ performs better than that in LN₂, presenting higher maximal wicking height and wicking velocity. At lower gravity, the wicking performance is better in whole but much more sensitive to the changes of superheated condition and structure parameters of the screen. The thermal effect of superheated gas region plays a decisive role in wicking behaviors, significantly reducing the wicking velocity and equilibrium height. The effects of permeability and thickness of the screen are obviously different at superheated condition compared to the isothermal cases.