Junze Jin

Preparation of diamond nanocrystals from catalysed carbon black in a high magnetic field

Under a static high magnetic field of 10 T, diamond-like carbon (DLC) nanocrystals and graphite-coated n-diamond nanoparticles have been synthesized after a pyrogenation of carbon black and a nanometre-sized iron catalyst at atmospheric pressure and a temperature of 1100 °C. The product is analysed by x-ray diffraction, Raman spectroscopy, transmission electron microscopy and electron-probe microanalysis. The average size of the DLC nanopowders is about 20 nm, and that of the graphite-coated n-diamond particles is about 100 nm. The yield of diamond is as high as 30%.

Formation mechanism of diamond nanocrystal from catalysed carbon black

Recently, our group has synthesized nanocrystal n-diamond and diamond-like carbon (DLC) from catalysed carbon black. Based on the results of XRD, TGA and DTA, a formation mechanism has been proposed to explain the phase transformation from carbon black to diamond nanocrystal. With the increase of temperature and hence the carbon diffusion in iron, the phase sequence is from Fe(OH)3 into Fe2O3, α-Fe, γ-Fe, then liquid iron. When the carbon in the liquid iron is saturated, DLC or graphite separates out of the liquid iron. With decrease of temperature, the carbon in γ-Fe is separated out, and n-diamond nuclei form and grow.

Study of the stability of n-diamond

Powders of n-diamond can be synthesized by pyrogenation of carbon black and nanometre-sized iron catalyst at atmospheric pressure and at a temperature of 1100 °C. The stability of n-diamond was investigated with x-ray diffraction, thermal gravimetric analysis and differential thermal analysis. The results indicated that n-diamond was a metastable phase: it can decompose at room temperature slowly. Thermal decomposition of n-diamond begins at 150 °C and is complete at 400 °C, and the decomposition of n-diamond was an exothermic reaction.

Separation efficiency of alumina particles in Al melt under high frequency magnetic field

The effects of separation time and magnetic induction intensity on the separation efficiency of alumina particles with diameters varying from 30 to 200 μm in aluminum melt were investigated. The experimental results show that the particle-accumulated layer is formed in the periphery of the solidified specimen when the diameter of the separated molten metal, the magnetic induction intensity and the separation time are 10 mm, 0.04 T and 1 s, respectively. When the separation time is 2 s, the particle-accumulated layer can be observed obviously and the separation efficiency is about 80%. There are few alumina particles in the inner of the solidified specimen when the separation time is 3 s. The separation efficiency higher than 85% can be achieved when the separation time is longer than 3 s. When the magnetic induction intensity is 0.06 T, the visible particle-accumulated layer can be formed in 1 s and the separation efficiency is higher than 95%. The experimental results were compared with the calculated results at last.

Transformation Mechanism from Carbon Nanotubes to n-diamond

Nanocrystal n-diamond particles were synthesized after a pyrogenation of carbon nanotubes and colloidal Fe(OH) 3 at atmospheric pressure. The product was investigated with x-ray diffraction, transmission electron microscopy, thermal gravimetric analysis, and differential thermal analysis. The results indicate that the n-diamond can be synthesized with the carbon nanotubes as carbon source. The formation mechanism of the n-diamond is suggested in this paper. With the increase of temperature and hence the carbon diffusion in iron, the phase sequence is from Fe(OH) 3 into Fe 2 O 3 , α–Fe, γ–Fe, and then liquid iron. When carbon in the liquid iron is saturated, graphite separated out of the liquid iron. With the decrease of temperature, the carbon in γ–Fe is separated out, and the n-diamond nuclei form and grow.

Effects of electromagnetic field on structure and heat treatment behavior of Mg-Li-Al alloys

The effects of external fields such as electromagnetic field on the structure and heat treatment behavior of Mg-Li-Al alloys were studied. Mg-8Li-3Al alloys cast with and without electromagnetic stirring were used for solution treatment and aging treatment. Experimental results show that the dendritic arms are broken and a large quantity of equiaxed grains appear in the microstructure of specimens with electromagnetic stirring (EMS). With the increase of the quenching temperature (150-350℃), the solution of Mg and Al in solid β phase increases, and the Brinell hardness of the alloy increase as well. Aging peak and over aging happen because θ(MgLi2Al) which precipitates in the β matrix and strengthens the alloy is unstable and transforms to stable AlLi phase. Aging curves of EMS specimens change in a smaller amplitude.

Fabrication of ZrB2 particles reinforced AZ31 magnesium matrix composite by powder metallurgy and subsequent hot extrusion

Abstract AZ31 magnesium alloy metal matrix composites (MMCs) reinforced with 5, 10, 15 and 20 wt-%ZrB2 were successfully fabricated by powder metallurgy and subsequent hot extrusion. The structural evolution of AZ31magnesium matrix composite was reinforced by ZrB2. Particles were characterised by optical microscope, XRD and SEM. The reasonably uniform distribution of ZrB2 reinforcement with good interfacial is present in the AZ31/ZrB2 composite. The hardness and wear behaviour were evaluated. The hardness of composites reinforced with 5, 10, 15 and 20 wt-%ZrB2 particulates were increased by 26·8, 41·5, 97·5 and 107% compared with as cast AZ31 magnesium alloy. Correspondingly, the wear resistance of AZ31 magnesium alloy MMCs increases with increasing reinforcement weight fraction.

A 3-D Mathematical Model of Thermal Field Evolution in the Direct Chill Casting of Superlight Magnesium Alloy Slabs

Abstract A mathematical model of the direct chill (DC) casting process for superlight Mg-Li alloy (LA141) slab has been developed using the finite differential method (FDM). Thermal boundary conditions including primary and secondary cooling conditions have been selected based on knowledge of the physical process and the literatures. The variations of heat transfer coefficient between slab and dummy block interface resulting from the deformation of the slab have been considered. By calculating the temperature distribution, solid-liquid interface shape and position, the influence of casting parameters on DC casting process, such as the pour temperature, cooling water flow rate and casting speed have been analyzed. Moreover the profiles of the solidification fronts during the casting processes for the alloys of LA141 and AZ31 are compared.

Study on electromagnetic suspension casting of wrought magnesium alloy

Abstract A new grain refinement method, electromagnetic suspension casting, has been proposed, which is based on the virtues of the electromagnetic stirring and the suspension casting. The experiments on AZ61 wrought magnesium alloy by electromagnetic suspension casting were investigated. The results show that the grains were refined, and thin β-Mg17Al12 phase was observed. Moreover, the distribution uniformity of β-Mg17Al12 phase was improved. The mechanism of grain refinement by electromagnetic suspension casting and the solidification of electromagnetic suspension casting of AZ61 magnesium alloy were studied. The experiment results indicate that electromagnetic suspension casting process raises the solidification rate of billet especially in the central region, and the solidification of electromagnetic suspension casting was close to mushy freezing.

EFFECT OF Al-Ti-B ON MAGNESIUM ALLOY MICRO-ALLOYED WITH Ca

Effect of Al-Ti-B master alloy on the microstructure and mechanical properties was investigated in AZ31 magnesium alloys micro-alloyed with Ca. During the casting process, electromagnetic field was also introduced. The results suggest that the micro addition of Ca to magnesium alloy retards the oxidation rate during melting process, improves casting qualities of magnesium alloy ingots. The grain size of AZ31 magnesium alloy has been effectively reduced by optimum addition of 1 wt.% (designed composition) Al-Ti-B master alloy. In this process, the addition level of Ti is the key factor to affect grain size of magnesium alloy, in which two grain refinement mechanisms are built by both TiB2 and residual Ti. Moreover, the electromagnetic field leads to uniform distribution of temperature field and solute field in the molten pool, increases casting qualities and refines grain size further.