1997 TMS Annual Meeting: Tuesday Session

CHEMISTRY AND PHYSICS OF NANOSTRUCTURES AND RELATED NONEQUILIBRIUM MATERIALS: Session IV: Synthesis and Microstructure

Session Chairperson: En Ma, Louisiana State Univ., Dept. of Mechanical Eng., Baton Rouge, LA 70803

2:00 pm INVITED

COMBUSTION FLAME SYNTHESIS OF NANOPHASE MATERIALS: B.H. Kear*, G. Skandan***, N. Glumac**, and Y. Che*, *Ceramics, **Mechanical & Aerospace Engineering, Rutgers University, NJ, ***Nanopowder Enterprises, Inc. Piscataway, NJ

Non-agglomerated nanopowders are becoming increasingly important for a number of commercial applications including UV-scattering, chemical mechanical polishing (CMP), displays and catalysis, among others. We have developed a continuous nanopowder production process, called Chemical Vapor Condensation (CVC), which involves precursor pyrolysis and condensation in a reduced pressure environment. We have introduced a flat-flame combustor unit, operable at low pressures, as a heat source in place of the original hot wall reactor. The temperature profile is uniform across the entire face of the burner, therefore, the reactants experience the same processing history and the powder has a uniform particle size distribution. The modified process, called Combustion Flame-Chemical Vapor Condensation (CF-CVC), has been used to produce a range of non-agglomerated nanoparticles (3-50 nm average particle size) of single phase, multiphase, and multicomponent materials. Examples include Al2O3, SiO2, TiO2, Al2O3/SiO2 and Eu:Y2O3. The as-synthesized powder is fully pyrolyzed (characterized by TGA), has a high surface area (SiO2 > 300 m2/g; TiO2 > 80 m2/g), and is non-agglomerated (TEM and BET pore size distribution). In addition, when the superheated particles leaving the combustion flame impinge on a heated substrate, in situ sintering can occur. Nanoporous or dense films or multilayered thin film structures can be synthesized. We have demonstrated the scalability of the process by increasing the burner diameter. Design consideration, processing parameters, powder characteristics and the commercial potential for the powders will be discussed. This work is supported in part by the Office of Naval Research contract #N00014-95-C-0283.

2:30 pm

EVIDENCE OF SURFACE ROUGHNESS IN NANOSTRUCTURES: M. José Yacamán, Institute de Física, Universidad Nacional Autóma de México, Apdo, Postal 20-364, 01000 México, D.F., México

Nanostructured Materials present very unique properties which has allow several technological applications. In the case of chemical reactions nanostructured materials have been used as catalyst. It is know that reduction of the particle size to a few nanometers produce an increased catalytic activity. The origin of this activity has been attributed to the increase surface area. We will show in this paper that nanoparticles present an increased surface roughness. This roughness increases the number of kink which provide an excellent site for promoting chemical reactions. In order to characterized the roughness we have used a new technique which combines High Resolution Electron Microscopy with computer length, It will be also shown that in several cases, the particle size in the image do Dot correspond with the true particle size. A method to correct this problem will be discussed.

3:00 pm INVITED

NANOSTRUCTURED MATERIALS VIA CHEMICAL ROUTES: Kenneth E. Gonsalves, Department of Chemistry & Institute of Materials Science, University of Connecticut, Storrs, CT 06269

This talk will focus on the chemical synthesis and processing of nanostructured materials. The precursor chemistry for the synthesis of nanostructured metals, ceramics, polymers, biomaterials, semiconductors and nanocomposites will be outlined and selected examples presented. Issues such as material purity, homogeneity, agglomeration and scale-up will be addressed.

3:30 pm BREAK

3:45 pm

TEM AND HRTEM OF NANOSTRUCTURED M50 TYPE STEEL PREPARED BY HOT PRESSING OF CHEMICALLY SYNTHESIZED POWDERS: G. M. Chow1, C.R. Feng2, Naval Research Laboratory, Washington, DC 20375; S.P. Rangarajan, X. Chen, K.E. Gonsalves, Institute of Materials Science, University of Connecticut, Storrs, CT 06269; C.C. Law, Pratt & Whitney, United Technologies Corporation, East Hartford, CT 06108. 1Laboratory for Molecular Interfacial Interactions, 2Materials Science Division

Nanostructured M50 type steel materials were prepared by hot pressing the precursor powders chemically synthesized using two different techniques, namely, thermal decomposition and co-reduction. During the hot press process, the precursor powders were transformed to nanocrystalline phases with the precipitation of carbides. Simultaneously the crystalline powders were densified. The densified samples were studied using both conventional and high resolution transmission electron microscopy. The effects of hot pressing temperature, time and pressure on the evolution of nanostructures, grain growth and defects formation are discussed.

4:05 pm

SYNTHESIS OF NANOSTRUCTURE W/Cu/Co ALLOY BY THERMOCHEMICAL METHOD: Gil-Geun Lee, Gook-Hyun Ha, Dong-Won Lee, Byoung-Kee Kim, Korea Institute of Machinery & Materials, 66 Sangnam-Dong, Changwon, Kyungnam 641-010, Korea

Nanostructure W/Cu/Co alloy was developed by thermochemical processing method using metallic salt precursors as the starting material for improving thermal, electrical and mechanical properties. Nanostructure W/Cu/Co powder have loosely agglomerated homogeneous clusters of nanoscale size W( 50 nm), Cu. and Co particles. The density and microhardness of the sintered nanostructure W/Cu/Co increased with increasing of Co content from 0.1 to 0.7wt.%, but electrical conductivity drastically decreased with addition of Co. Full density for nanostructure W/Cu/Co was achieved when sintered at 1473K for 20 minutes in H2 atmosphere with addition of 0.5wt.%Co. It was proved that optimum Co content is under 0.5wt.% when based on the relations between electrical conductivity and density. It is shown that nanostructure W/Cu/Co have higher sinterability and better electrical conductivity than conventional W/Cu/Co.

4:25 pm

MÖSSBAUER EFFECT STUDY OF MECHANICALLY ALLOYED -Fe3Zn10 and 1-Fe5Zn21 CUBIC INTERMEDIATE PHASES: Oswald N.C. Uwakweh, Zhentong Liu, Materials Science & Engineering, University of Cincinnati, Cincinnati, OH 45221-0012

The Mössbauer effect measurements of as-ball milled mechanically alloyed Fe-Zn intermediate phases of -Fe3Zn10 and -Fe5Zn21 compositions exhibit characteristic spectra consisting of triplets. Each is characterized with an Fe-site with a high quadrupole splitting measuring 0.94 mm/s, together with three other doublets. In the aged states, both compositions show spectra free of the Fe-site with the large quadrupole splitting. This suggests that both have similar metastable states, while their separate transformation paths to stable equilibrium states yield distinct crystal structures as found in the literature.

4:45 pm

NEUTRON DIFFRACTION & PHASE EVOLUTION OF MECHANICALLY ALLOYED -FeZn13 INTERMETALLIC: Oswald N.C. Uwakweh, Zhentong Liu, Materials Science & Engineering, University of Cincinnati, Cincinnati, OH 45221-0012; Brian Chakoumakos, Stephen Spooner, Oak Ridge National Laboratory, Solid State Division, Oak Ridge, TN 37831-6393

High Energy Ball -Milling is used to synthesize -FeZn13 intermetallic. The mechanically alloyed phase in the as-millet state is determined to be metastable, while the characteristic stages associated with the stable equilibrium transformation are identified based on DSC measurements. The as-milled material is described in terms of mechanical mixture of the elemental constituents, while the equilibrium state is confirmed to have a C2/m space group, with lattice parameters of a=13.40995 Å, b=7.60586 Å, c=5.07629 Å, and =127° 18'. The atomic positions of Fe and Zn are compared with reported values.