Reza Abbaschian

Growth of large-area graphene films from metal-carbon melts

We have demonstrated a new method for the large-area graphene growth, which can lead to a scalable low-cost high-throughput production technology. The method is based on growing single layer or few-layer graphene films from a molten phase. The process involves dissolving carbon inside a molten metal at a specified temperature and then allowing the dissolved carbon to nucleate and grow on top of the melt at a lower temperature. The examined metals for the metal-carbon melt included copper and nickel. For the latter, the high-quality single layer graphene was grown successfully. The resulting graphene layers were subjected to detailed microscopic and Raman spectroscopic characterization. The deconvolution of the Raman 2D band was used to accurately determine the number of atomic planes in the resulting graphene layers and access their quality. The results indicate that our technology can provide bulk graphite films, few-layer graphene as well as high-quality single layer graphene on metals. Our approach can also be used for producing graphene-metal thermal interface materials for thermal management applications.

Growth Kinetics of Solid-Liquid Ga Interfaces: Part II. Theoretical

The faceted (111) and (001) Ga interfaces grow at low supercoolings with either of the lateral growth mechanisms, two-dimensional nucleation growth (2DNG) or screw dislocation-assisted growth (SDG), depending on the perfection of the interface. The classical theories regarding the growth kinetics of smooth interfaces describe the results qualitatively but not quantitatively. The latter is due to the inadequacy of the assumptions made in the classical theories, which treat the interfacial atomic migration the same as the liquid bulk diffusion process and the step edge energy as independent of the supercooling. Beyond a threshold supercooling, the results show that the faceted interfaces gradually become kinetically rough as the supercooling increases. The step energy, treated as a function of the supercooling, is shown to diverge exponentially with the supercooling at the faceted/nonfaceted transition. At supercoolings exceeding the transition value, dislocations do not affect the growth rate. Furthermore, beyond the transition, the growth rates are linearly dependent on the supercooling, which implies that the growth mode changes from lateral to normal. A generalized lateral growth equation, which includes the interfacial diffusivity and supercooling-dependent step edge free energy, is given to describe the growth kinetics of both interfaces up to supercoolings marking the kinetic roughening transition.

Growth kinetics of solid-liquid Ga interfaces: Part I. Experimental

A technique based on the Seebeck effect was used to determine directly the solid-liquid (S/L) interface supercooling and toin situ monitor the interfacial conditions during growth of high-purity Ga single crystals from a supercooled melt. Using this nonintrusive technique, the growth kinetics of faceted (111) and (001) interfaces were studied as a function of the interface supercooling in the range of 0.2 to 4.6 K, corresponding to bulk supercoolings of about 0.2 to 53 K. In addition, the growth kinetics have been determined as a function of crystal perfection related to the emergence of dislocations at the S/L interface. The results show that at low super-coolings, the faceted interfaces grow with either of the lateral growth mechanisms: two-dimensional nucleation-assisted (2DNG) or screw dislocation-assisted (SDG), depending on the perfection of the interface. At increased interfacial supercoolings, however, both growth rates (2DNG and SDG) become a linear function of the supercooling. Application of the existing growth theories to the experimental results gives only qualitative agreement and fails to predict the observed deviation in the kinetics at high supercoolings. A theoretical treatment of the growth of faceted interfaces will be given in Part II of this series.1

Microstructure of Cu-Co alloys solidified at various supercoolings

The effects of supercooling on the microstructure of Cu-Co alloys containing 10 to 65 wt pct Co were investigated. Supercooling of the alloys below a characteristic temperature,tSEP, resulted in a metastable phase separation into two liquids: one Co rich (L1) and the other Cu rich (L2). The microstructure of the phase-separated alloys consisted of spherulites of one phase embedded in a matrix of the other. The spherulites in alloys containing less than 40 wt pct Co were solidified from the L1 melt and from L2 in alloys containing more than 55 wt pct Co. Supercooling of copper alloys containing around 50 wt pct Co resulted in a duplex structure of fine and coarse dendrites. Microstructural evidence was presented for the formation of aε-Cu metastable phase in alloys containing less than 30 wt pct Co.

Supercooling effects in Cu-10 Wt Pct Co alloys solidified at different cooling rates

Electromagnetic levitation and electron beam surface melting were employed to study the effects of supercooling and cooling rate on the solidification of Cu-10 wt pct Co alloys. Two major effects were observed in the supercooled alloys: the nucleation of a metastable copper-rich phase which contains 13 wt pct to 20 wt pct Co in samples supercooled between 105 and 150 K and liquid phase separation which occurs in samples supercooled 150 K or more. The microstructure of the electron beam melted surfaces consisted of very fine spheres which were similar to those of the sample supercooled more than 150 K but with a refined microstructure. The results indicate that a dynamic bulk supercooling of 150 K may exist in the molten pool during the solidification of electron beam melted surfaces.

Liquid separation in Cu–Co and Cu–Co–Fe alloys solidified at high cooling rates

The impact of cooling rates on the microstructure of Cu–Co and Cu–Fe–Co alloys was investigated by scanning electron microscopy. The high cooling rates entailed in the electron beam surface melting of the alloys result in bulk supercooling of at least 150 K, which in turn causes three microstructural effects: (i) melt separation into two liquids, namely copper poor, L1, and copper rich, L2; (ii) microstructural refinement; (iii) enhanced solute trapping of Cu in the Fe- or Co-rich phases. No evidence of metastable liquid separation was found for Cu–50 wt% Co. There are indications that similar dynamic supercooling exists in copper-quenched or arc-melted samples near the splat contact.

In-situ processing of NiAl-alumina composites by thermite reaction

Abstract NiAl–Alumina composites have been fabricated by reactive sintering of ball-milled and compacted powder mixtures containing NiO and Al. Dense NiAl–Al 2 O 3 composite with an interpenetrating Al 2 O 3 network was fabricated after 2 h of vacuum hot pressing at 1200°C and 50 MPa. The thermite reaction mechanism, as well as phase and microstructural development of the composites, were investigated by quenching the compacts from different temperatures during vacuum hot press. It was found that different intermediate phases are formed depending on the temperatures; Ni formed first at around 600°C, followed by NiAl 3 , Ni 2 Al 3 and NiAl as temperature increased. The formation of Al 2 O 3 phases during reactive hot press is believed to be a three-stage process. Initially, a small amount of Al 2 O 3 was produced by the reduction of NiO with solid Al. As temperature increases, Al 2 O 3 particles forms by the reduction of NiO with liquid Al. Finally, a NiO/Ni/Al 2 O 3 /Ni m Al n layer structure forms via solid state displacement reaction between NiO and nickel-aluminides (NiAl 3 , Ni 2 Al 3 , NiAl) leading to the formation of interpenetrating Al 2 O 3 network in the final product.

The metastable liquid miscibility gap in Cu-Co-Fe alloys

The metastable liquid-phase separation (MLPS) in the Cu-Co-Fe system was investigated using an electromagnetic levitation melting and solidification technique. It was found that when ternary alloys containing more than 10wt.% (12 at.%)Co and 10wt.% (11at.%)Fe were undercooled below a certain temperature, T sep, the homogeneous melt separated into two liquid phases. In alloys containing more than 54 to 57wt.% (49 to 54at.%)Cu (depending on the Co and Fe content), the phase separation generally appeared as dispersed (Fe, Co)-rich droplets (L1) in a Cu-rich matrix, whereas for alloys containing less copper, the separation resulted in Cu-rich droplets (L2) in a (Fe, Co)-rich matrix. The metastable liquid miscibility gap boundary of the Cu-Co-Fe ternary was determined using the measured T sep and the composition of the separated phases. The ternary liquid-phase separated boundaries were found to be consistent with a cross-sectioned phase diagram in which one axis represents pure copper and the other Fe + Co.

Interfacial modification of Nb/MoSi2 composites and its effects on fracture toughness

Niobium-reinforced MoSi2 composites have shown a large improvement in fracture toughness over the MoSi2 matrix. However, the chemical incompatibility of niobium with MoSi2 has to be solved for high-temperature structural applications of the composite. In addition, the effects of interfacial coatings on the fracture toughness of ductile-phase-reinforced composites need to be investigated to find the optimum toughening effect of niobium reinforcement. In the present study different oxide coatings, Al2O3 and ZrO2, were applied to niobium reinforcement and effectiveness of the coatings as diffusion barriers was evaluated. The mechanical behavior and the fractographic characteristics of constrained niobium were also examined. Finally, the effect of the coatings on fracture toughness of the composites was studied and compared with predictions based upon tensile tests on a single constrained niobium reinforcement. The results are discussed in terms of the interfacial fracture energy and micromechanical models of ductile-phase-reinforced composites.

Eutectic solidification processing via bulk melt undercooling

The control of eutectic microstructures is generally achieved via controlling the growth velocity, thermal gradients and the alloy composition. While current efforts in producing in-situ nanocomposites are centered on conventional rapid solidification processing (RSP), we describe here a more non-traditional processing of eutectics by RSP via bulk melt undercooling of near-eutectic compositions. It will be shown that a wide variety of microstructures can be produced in alloys with the same composition. These include the formation of metastable phases, growth of coupled eutectic compositions and the formation of mixed metastable/stable eutectics. These effects are discussed using examples of rapidly solidified Nb-Si alloys processed in a containerless environment.