Joanna R. Groza

Sintering activation by external electrical field

Field assisted sintering technique (FAST) is a non-conventional powder consolidation method in which densification is enhanced by the application of an electrical discharge combined with resistance heating and pressure. Interest in FAST is motivated by its ability to consolidate a large variety of powder materials to high densities in short times. Full densification of metal and ceramic powders has been achieved within minutes, with a reduced number of processing steps, no need for sintering aids and more flexibility in powder handling. Although the electrical discharge effects have not been completely elucidated, distinct surface effects created by micro-discharges have been noticed in FAST consolidated specimens such as atomically clean grain boundaries and new resistivity peaks in superconductors. On-going experimental and theoretical studies to provide more quantitative insight into the relevant FAST mechanisms are presented.

Principles of particle selection for dispersion-strengthened copper

Abstract A new fundamental approach to the design of high strength, high thermal conductivity dispersion-strengthened copper alloys for applications in actively cooled structures is developed. This concept is based on a consideration of the basic principles of thermodynamics, kinetics and mechanical properties. The design requirements for these materials include a uniform distribution of fine particles for creep and fatigue resistance, a high thermal conductivity, thermodynamic and chemical stability at temperatures up to 1300 K, a small difference in the coefficients of thermal expansion between the particle and matrix, and low particle coarsening rates at the processing and service temperatures. The theory for creep of dispersion-strengthened metals developed by Rosler and Arzt is used to predict the optimum particle size for a given service temperature and to illustrate the need for a high interfacial energy. Resistance to coarsening leads to a requirement for low diffusivity and solubility of particle constituent elements in the matrix. Based on the needs for a low difference in the coefficients of thermal expansion to minimize thermal-mechanical fatigue damage and low diffusivity and solubility of the constituent elements, several candidate ceramic phases are compared using a weighted property index scheme. The results of this quantitative comparison suggest that CeO2, MgO, CaO and possibly Y2O3 may be good candidates for the dispersed phase in a copper matrix.

Plasma activated sintering of additive-free AlN powders to near-theoretical density in 5 minutes

AlN powders (particle size = 0.44 ± 0.08 μm) containing no deliberate sintering additives were consolidated to near theoretical density in 5 min at 2003 K (1730 °C) using a Plasma Activated Sintering (PAS) process. PAS is a novel consolidation method that combines a very short time at high temperature with pressure application in a plasma environment. The in situ cleaning ability of powder particle during plasma activated densification leads to enhanced particle sinterability. The densities of undoped AlN specimens that were PAS consolidated at 2003 K for 5 min under 50 MPa pressure ranged from 97.5 to 99.3% of theoretical. The initial submicron particle size of AlN powders was retained in the final microstructure that consisted of polycrystalline grains with an average size of ≍0.77 ± 0.1 μm.

Surface effects in field-assisted sintering

The more stringent requirements for densification of new out-of-equilibrium powders have created a growing demand for non-conventional rapid sintering processes. Among those, field assisted sintering techniques (FAST) have seen a recent renewed interest motivated by their ability to consolidate a large variety of powder materials to high densities in short times. Characterization of a range of FAST consolidated materials display relevant associated surface effects, such as grain boundary cleaning with direct grain-to-grain contact and advanced densification without sintering aids. These effects may be attributed to phenomena ranging from dielectric breakdown to a possible non-conventional plasma generation. Such surface effects provide a better intergranular bonding of powder particles during subsequent sintering.

In-situ TEM observations of abnormal grain growth, coarsening, and substrate de-wetting in nanocrystalline Ag thin films

Abstract Abnormal grain growth is studied in nanocrystalline sputtered Ag films. Eighty nanometer thick Ag films are DC sputter deposited onto back-etched amorphous silicon nitride membranes. Specimens are annealed in a heating stage in an in-situ TEM for various temperatures and hold times. With the same specimen, we proceed to higher temperatures after the apparent halt of growth for sufficiently long hold times. The grain size distribution of the as-deposited films is bi-modal, with large abnormal grains with 100 nm diameters, embedded in a matrix of smaller grains of 15 nm diameters. Coarsening begins at temperatures of approximately 100°C, and quickly reaches a plateau. The growth process restarts only after sufficient temperature increases, and plateaus at each succeeding temperature. Using a variation of the Mullins–Von Neumann law, the activation energy for the abnormal growth is found to be 0.274 eV, consistent with the value reported for pore formation during electromigration via surface diffusion in Ag. Grain growth appears to stop above temperatures of 350°C, eventually leading to triple junction pore formation at 350°C and de-wetting of the film from the substrate at 600°C. The de-wetting is the high temperature limit of the thermal grooving which cancels the driving force for grain growth at the lower temperatures. TEM images as evidence of this effect are presented, along with observations on the pore formation that support surface diffusion as the mass transport mechanism for grooving, pore fomation, and as the limiting mass transport mechanism for the grain growth.

Surface characterization of metal nanoparticles

Air exposed aluminum and nickel nanoparticles were investigated using high-resolution and analytical transmission electron microscopy (HREM/AEM), X-ray diffraction analysis (XRD) and X-ray photoelectron spectroscopy (XPS). The diameter of the Al particles ranged from 20 to 260 nm with an average size of 100±50 nm. Individual Ni particles were spherical, ranging from 7 to 230 nm in size with an average size of 70±45 nm. The Al nanoparticles were covered by a 3 nm thick amorphous and compact surface oxide layer. Ni nanoparticles were surrounded by a non-uniform, crystalline surface oxide layer. Elemental analysis indicated that the Al nanoparticles contained O and Si and that the oxygen is associated with the particle surface layer, while no oxygen confinement in the surface layer was found in Ni nanoparticles. XPS confirmed the NiO presence in the surface layer of Ni particles. These results suggest that the sintering behavior of these metal nanoparticles may be inhibited, though to a different degree, due to the surface oxides.

TEM annealing study of normal grain growth in silver thin films

Normal grain growth in 80-nm-thick sputter-deposited Ag films was studied via in situ heating stage transmission electron microscopy. The as-deposited films with an initial grain size of 40–50 nm were held at a series of temperatures (one per specimen) below 250°C. A grain growth exponent n=3 from the law Dn−Don=k(T)t was calculated by minimizing the deviation in the fitting function to the experimental data. An activation energy for grain growth of 0.53 eV (53 kJ/mol) is found, which is close to surface diffusion. These findings are consistent with our previous work on abnormal grain growth in Ag: that grain growth in thin film nanocrystalline silver is dominated by surface diffusion mass transport.

Heat-Resistant Dispersion-Strengthened Copper Alloys

Processing methods for producing dispersion-strengthened (DS) copper alloys with high strength, high conductivity, and good long-term stability at elevated temperature are reviewed. Particle size and stability are related to material characteristics and processing route. Physical and mechanical properties of DS copper alloys are directly associated with microstructural features such as particle volume fraction, stability, size, solubility in the matrix, and interfacial properties. New avenues for DS copper alloys design are suggested based on thermal conductivity concept and recent Rosier- Artz theory of high-temperature strength.[1]

5 – Nanocrystalline Powder Consolidation Methods

Publisher Summary This chapter discusses nanocrystalline powder consolidation methods and covers some thermodynamic and kinetic aspects of nanopowder densification, which are driving force, surface energy, sintering mechanisms, activation energies, and scaling laws. Due to the critical effect of surface contamination on small particles, the role of impurities in sintering is described separately. The chapter addresses the cold compaction (CC), with resultant pore size and size distribution, and their effects on sintering and grain coarsening. The presentation of the sintering process of nanoparticles is divided into pressureless and pressure-assisted sintering. Furthermore, the chapter presents the methods for full densification of nanopowders and their ability to maintain nanosize features. The most distinctive features of the sintering process of nanosize powders are the high driving force and enhanced kinetics due to large curvature effects. Densification of nanopowders takes place at temperatures consistently below those of larger-grained powders by up to several hundreds of degrees. The numerous benefits from using lower sintering temperatures are small final grain sizes, elimination of sintering aids, avoiding undesirable phase transformations, and deleterious decomposition or interfacial reactions.

Surface oxide debonding in field assisted powder sintering

Field activated sintering techniques (FAST) have been applied to two high-temperature powder materials: tungsten and NiAl. High and atomic resolution electron microscopy (HREM/ARM) of tungsten powder sintered via FAST showed essentially clean boundaries. Analytical transmission electron microscopy (TEM) of FAST sintered NiAl also showed boundaries free of surface oxide layer(s). However, small alumina precipitates were found at and near prior particle (grain) boundaries. The boundary cleaning and precipitation phenomena are manifestations of an applied pulsed electric field.