D. Lynn Johnson

New Method of Obtaining Volume, Grain‐Boundary, and Surface Diffusion Coefficients from Sintering Data

A sintering model is proposed by which all of the significant mechanisms of material transport may be identified, even though more than one mechanism may be operating simultaneously. For diffusion‐controlled sintering it is possible to calculate both the volume and the grain‐boundary diffusion coefficients from measurements of neck size, shrinkage, and shrinkage rate. Furthermore, the surface diffusion coefficient may be estimated through computer synthesis of the sintering curves. Preliminary results on iron and copper have yielded values of the diffusion coefficients which are in good agreement with those measured by other techniques.

Self-assembled (SA) bilayer molecular coating on magnetic nanoparticles

Abstract Protectively and functionally coated magnetic nanoparticles are of special interest because of their important technological applications in diverse fields, ranging from biotechnology to transportation. In this paper, monolayer and bilayer surfactant coating on magnetite (Fe3O4) nanoparticles has been obtained using the self-assembly method. Magnetic properties such as magnetization, ZFC and FC curves, blocking temperature, hysteresis loop, coercivity and remanent magnetization of the SAM and bilayer coated magnetite nanoparticles have been investigated. The results show the efficacy of our synthesis approach not only to protect magnetic nanoparticles via surfactant-mediated self-assembly but also their stable suspension in a variety of appropriate liquid media. The superparamagnetic nature of the magnetic nanoparticles remains unchanged with self-assembled coatings and paves the way for their use in colloid suspension for device applications.

Grain boundary and volume diffusion in the sintering of silver

Abstract Models are proposed in which both grain boundary and lattice or volume diffusion of vacancies contribute to the initial sintering shrinkage of compacts of spherical particles, or the neck growth between spheres or between a sphere and a plate. It is assumed that all the diffusing vacancies are annihilated at the grain boundaries between particles, whether they move along the grain boundary or through the lattice. This assumption is justified on the basis of the vacancy concentration gradient required to give the observed diffusion sintering behavior of crystalline materials. Compacts of various size fractions of high-purity silver spheres were prepared and the shrinkage of these compacts in purified argon was measured with a recording dilatometer. The data were analyzed according to the model and found to yield values of both the volume and grain boundary self-diffusion coefficients that are in good agreement with the values measured by radioactive tracer self-diffusion techniques.

Graphite encapsulated nanocrystals produced using a low carbon : metal ratio

Graphite encapsulated nanocrystals produced by a low carbon tungsten arc were analyzed to determine their chemistry, crystallography, and nanostructural morphology. Metallic nanocrystals of Fe, Co, and Ni are in the face-centered cubic (fcc) phase, and no trace of the bulk equilibrium phases of body-centered cubic (Fe) and hexagonal close-packed (Co) were found. Various analytical techniques have revealed that the encased nanocrystals are pure metal (some carbide was found in the case of Fe), ferromagnetic, and generally spherical. The nanocrystals are protected by turbostratic graphite, regardless of the size of the nanocrystals. The turbostratic graphite coating is usually made up of between 2 and 10 layers. No trace of any unwanted elements (e.g., oxygen) was found. The low carbon: metal ratio arc technique is a relatively clean process for the production of graphite encapsulated nanocrystals.

Microwave and plasma sintering of ceramics

Abstract Both microwave and plasma sintering hold forth the promise of rapid processing as well as refinements in microstructure and enhanced properties. A new approach to sintering modeling provides a means of quantifying any athermal effects under nonisothermal as well as isothermal conditions. The well-understood benefits of fast heating are corroborated. Beyond that, evidence is mounting from a number of sources that sintering rates for both techniques are enhanced beyond those anticipated from rapid heating. Of course, the causes of enhancement in the two techniques are unrelated. Possible explanations are considered.

Microwave curing effects on the 28-day strength of cementitious materials

Abstract Microwave energy can be applied on the curing of the cementitious materials because of their water content. Isothermal heating, achieved best by feedback temperature control, is required to investigate properly the temperature effects on the properties. Type I Portland cement mortars, some with additions of appropriate amounts of pozzolanic material, such as slag, silica fume, or class F fly ash, were cured isothermally with feedback temperature control at several temperatures. The curing times were determined by the instrumented penetration test, and 28-day compressive strengths were measured. Optimum curing conditions were found for 40 °C and 60 °C, but 80 °C was unsuitable for microwave curing.

Solid-State Sintering

The theory of solid-state sintering is reviewed, with particular emphasis on the effects of several concurrent mechanisms on sintering behavior. Computer synthesis of both non-isothermal and isothermal sintering data has been used to predict initial sintering behavior under a wide variety of conditions. This approach has been successful in describing the sintering of compacts of spherical particles of Fe, Cu, Fe2 O3, Ag, and LiF. The relative importance of surface, grain boundary, and volume diffusion and vapor transport are discussed. Experimentally observed sintering behavior is discussed in terms of the theoretical model.

Ultra-Rapid Sintering

A number of highly sinterable ceramic powders have been rapidly sintered by insertion of small samples into preheated furnaces, rapid heating of small specimens in low thermal mass furnaces, and passing tube-shaped samples through short hot zone furnaces. More recently, aluminum oxide has been sintered by passing through gas plasmas. Heating rates in the neighborhood of 100 oK/s and densification rates >1%/s have been achieved in the plasma sintering. Sintering models and computer simulation shed some light on the effect of rapid heating on the various sintering mechanisms and the interplay among the sintering mechanism.

Silica-encapsulated magnetic nanoparticles formed by a combined arc evaporation/chemical vapor deposition technique

A multistep technique has been developed for the generation of metallic/alloy nanoparticles coated with amorphous silica. As a proof of concept, an inert-gas blown-arc geometry was used to create nanoparticles from a bulknickel source, and silica coating formation was accomplished via tetraethyloxysilane (TEOS) decomposition over the nanoparticles in an adjacent chemical vapor deposition chamber. The composite particles exhibit resistance to hydrochloric acid attack over extended times, thereby confirming the protective nature of the silica coating, and magnetic measurements indicate a superparamagnetic transition temperature of 41 K. TEOS flow rate was found to have a profound effect on particle morphology, and individually coated dispersed particles were observed for the intermediate flow rate studied. These results, combined with the well-established field of silica functionalization, offer the possibility that a variety of industrially significant coated magnetic nanostructures may be synthesized with this versatile approach.

MODEL OF CHEMICAL VAPOR INFILTRATION USING TEMPERATURE GRADIENTS

An optimized chemical vapor infiltration (CVI) process has conditions that promote complete densification at the fastest allowable reaction rate. In order to help define optimum conditions, a model has been developed to simulate the CVI of a fibrous specimen for determining the effects of temperature gradients along with the other processing parameters such as pressure, size, chemistry, rate of reaction, and porosity on the resulting deposition profiles. This model simulates the deposition of alumina matrix within fibers wrapped around a tube. This symmetry reduces the model to a simple one-dimensional problem. Parameters for transport properties, calculated using a local microstructure model, are used in this macroscopic model. The model is applied as a guideline for choosing optimum conditions for producing a dense ceramic matrix composite. From this model, process diagrams are constructed that can help an experimentalist to choose the best conditions for the CVI process using temperature gradients.