Ultrafine-grained ceramics have been synthesized by the production of ultrafine (2–20 nm) particles, using the gas-condensation method, followed by their in-situ, ultra-high vacuum consolidation at room temperature. These new nanophase ceramics have properties that are significantly improved relative to those of their coarser-grained, conventionally-prepared counterparts. For example, nanophase rutile (TiO2) with an initial mean grain diameter of 12 nm sinters at 400 to 600°C lower temperatures than conventional powders, without the need for compacting or sintering aids. The sintered nanophase rutile exhibits both improved microhardness and fracture characteristics. These property improvements result from the reduced scale of the grains and the increased cleanliness of the particle surfaces and the subsequently-formed grain boundaries. Research completed on the synthesis, characterization, and properties of nanophase ceramics is reviewed and the potential for using the nanophase synthesis method for engineering new and/or improved ceramics and composites is considered.

Samples of nanophase TiO2 were prepared by the condensation of Ti vapors into clusters, their in situ oxidation to TiO2, and their consolidation into thin disks. Small angle neutron scattering was measured in the as-consolidated condition and after selected isothermal sintering anneals of up to 23 h at 550°C. The maximum entropy analysis method was used to obtain the size distributions of the scattering centers from the scattering curves. The results are interpreted in terms of a microstructural model consisting of nanometer sized grains of TiO2 separated by about 0.5 nm wide boundary regions, which contain voids and TiO2 of ≤60–70% of bulk density.

Nanophase titanium, prepared by the gas-condensation method both as aggregated powder and in lightly compacted discs, has been studied by conventional small angle neutron scattering, and by use of contrast variation methods. The contrast has been changed (a), isotopically, by means of deuterated/protonated solvents distilled into the specimen and (b) by progressive incremental oxidation of the Ti particles using fixed doses of low-pressure oxygen. It was shown that some evolution of the small angle pattern for lightly compacted nanophase Ti occurred over a period of several months at 300 K. Contrast matching by external solvent works well and has allowed the scattering lengths of oxidized and unoxidized specimens to be followed. The results imply that the scattering from metal and oxide can be separated under suitable conditions. The partial oxidation experiments indicate that there is both a fast and slow oxidation at 300 K. Also, during slow oxidation, high scattering length density scattering centers were formed whose number increased, but whose size remained the same at about 2 nm; these centers are tentatively assumed to be TiO2.

The effect of He gas pressure during evaporation and post-evaporation O2 exposure on phase formation in nanophase materials has been examined. Ultrafine powders of Ti and Pd were prepared by the inert gas-condensation technique and bulk nanophase samples were formed by consolidation of these powders with or without prior exposure to O2. Evaporation of Ti in He pressures greater than 500 Pa followed by exposure to O2 results in the formation of ultrafine powders of crystalline rutile (TiO2) which are compacted to form nanocrystalline TiO2. Surprisingly, reducing the He pressure used during evaporation to less than approximately 500 Pa results in the formation of ultrafine powders of an amorphous phase. Room temperature consolidation of this powder under vacuum, however, results in nanocrystalline Ti being formed if the powder is not exposed to O2 prior to consolidation, and a mixture of rutile and an unidentified crystalline phase if the powder has been previously exposed to O2. Further reduction of the He pressure during evaporation of Ti to less than approximately 10 Pa results in the formation of crystalline Ti having a film-like morphology rather than than the desired ultrafine particles. Experiments on Pd evaporated in 10 to 6000 Pa of He have yielded ultrafine powders and consolidated samples of only the crystalline fcc phase, regardless of the He pressure

The microstructure of nanocrystalline (n-) TiO2 was studied as a function of sintering temperature up to 1273 K. Grain growth was monitored using x-ray diffraction and scanning electron microscopy. Measurements of density and permeability of He and Ar were also conducted. The specific surface area and the total pore volume were determined quantitatively using the nitrogen adsorption method. These measurements revealed that highly compacted n-TiO2 had green body densities as high as 75 % of bulk density and that sintering occurred at much lower temperatures than in conventional powder. Densification proceeded by loss of the small pores first. The possibilities of achieving high densities with limited grain growth will be discussed.

Particulate TiO2 membranes have been prepared by sol-gel techniques from alkoxide precursors. The degree of aggregation, which is controlled by physical chemical processes during sol preparation influences the gelling volume and gel structure. While three types of gel structures have been proposed, the porosity of the final membrane seems to be little affected by these hydrogel structures. Firing temperature is a much more critical parameter. It is concluded that the primary particle size determines the final membrane porosity in this TiO2 system.

Ultrafine ceramic structures based on the nitrides and carbides of titanium and silicon have been synthesized in a computer-controlled hot-wall CVD reactor. Layered deposits have been produced by pulsing the reactant gases judiciously under software control. The development of a columnar structure which is endemic to most CVD materials has been suppressed. Skeletal structures of filaments have also been grown with appropriate catalysts by the vapor-liquid-solid mechanism and immediately infiltrated in situ with different materials to produce filament-reinforced composite coatings.

Ultrafine-grained carbon films and filaments have been grown from methane-hydrogen mixtures by RF plasma-assisted CVD. The microstructural features of these materials are of the order of 20 to 100 nm. The subgrain structure determined by Raman spectroscopy varies from 2 to 3 nm.

A new approach in ultrafine composite synthesis involves rapid condensation of metallic and non-metallic species, produced by laser-induced evaporation. A heated tungsten filament is simultaneously employed for codeposition of W via chemical transport mechanisms. This process occurs in a reducing environment of hydrogen gas, where evaporated species were produced by a laser-induced plume. Composite layers were formed on a Ni alloy substrate surface, at a rate of about 1Pm/sec. The matrix of the composite films was either Al or W, and the dispersed phase was amorphous silica fibers. The diameter of the fibers was between 25nm and 120nm, depending on the interaction times. Various analytical techniques have been employed to characterize the as-synthesized layers. Experimental evidences do not support the Vapor-Liquid-Solid model for fiber growth. An alternative fiber growth mechanism is proposed.

Nanophase materials can be prepared either by physical methods or chemical methods. Physical methods include thermal evaporation, sputtering and melt quenching, whereas chemical methods include glow-discharge decomposition, chemical vapor deposition, sol-gel dehydration and gas-solid reaction. Recently, we have used controlled activity gassolid reactions to prepare nanophase WC-Co cermet powders at different WC loadings. In the process we have discovered some new metastable phases in the W-Co-C ternary system at temperatures below 1000 °C.

The sol-gel technique was used to fabricate α-Fe/mullite composite materials with iron concentrations ranging from 20–60 wt.% Fe. Microscopy studies showed the microstructure to consist of particulate α-Fe dispersed uniformly throughout a mullite matrix. Iron particle size, estimated by TEM, ranged from 20–500 nm. The materials exhibited ferromagnetic hysteresis with saturation magnetization Ms values as high as 115 EMU/g.

In recent years considerable effort has been expended on the development of dispersion strengthened alloys by mechanical alloying. Our research has shown that considerable improvement in microstructure control and properties can be gained by carrying out milling at cryogenic temperatures. We have found that aluminum and dilute aluminum alloys can be dispersion strengthened with aluminum oxy-nitride particles by the use of a slurry milling technique where the fluid medium is liquid nitrogen. The alloyed powders produced by this technique are strengthened by aluminum oxy-nitride particles which are typically 2–10 nm in diameter and with a mean spacing of 50–100 nm. The dispersoids are generated during the milling process by adsorption and reaction with components of the liquid nitrogen bath. On thermal treatment prior to consolidation, the alloyed powders recrystallize to a grain size which is typically in the range 0.05 to 0.3 μm. The alloys exhibit a yield stress in excess of 325 MPa at room temperature and a virtually temperature independent yield stress of about 130 MPa at temperatures greater than 375° C. The paper describes the preparation of dispersion strengthened aluminum by cryomilling, the characteristics of the microstructure and discusses some aspects of the mechanical properties.

A new processing method, the Mixalloy process, has been developed to process alloys with novel microstructures and compositions. In this process, microstructural control is achieved through the use of turbulent mixing of liquid metals in addition to controlling solidification rate and chemical composition. Boride dispersion strengthened copper alloys were produced using the Mixalloy process. Thermally stable and fine (average less than 100 nm) boride dispersoids were formed by in-situ chemical reaction in the copper alloy matrices during mixing. The uniform mixture of the matrix and dispersoids was then rapidly solidified to maintain the fine microstructure. The consolidated material shows exceptional thermal stability and an excellent combination of strength, ductility, and electrical conductivity. Furthermore, the flexibility of the process allows the matrices of these dispersion strengthened coppers to be easily alloyed to fulfill specific needs. The versatility and simplicity of the Mixalloy process provide an economical alternative to other processing means in the manufacturing of high performance alloys such as dispersion strengthened alloys.

A ceramic matrix composite consisting of 100–200 μm Al2O3 grains (90 grit) embedded an Al2O3/metal matrix was examine. The matrix was produced by the directed oxidation of a molten Al alloy. The microstructure was studied by optical methods, x-ray diffraction and transmission electron microscopy (TEM) in conjunction with energy dispersive spectrometry (EDS). The matrix A12O3 was interconnected and present as ∼50–500 nm grains in ∼ 10–50 μm regions. Within these regions, the Al2O3 was separated by low angle. Al2O3/Al2O3 grain boundaries. The boundaries between regions consisted either of high angle Al2O3/Al2O3 boundaries with no grain boundary phase or of tortuous metal channels from 50 nm up to 1 μm in width. The metallic constituent within the regions was only partly interconnected, consisting of similar channels or isolated spheres of 1–500 nm. The metal consisted of Al and Si. Large fractions of the filler Al2O3 surface (∼50%) were directly bonded to the matrix Al2O3.

The reflection of mid- and near-infrared radiation (400-15000 cm−1 by thin Pt/Al2O3 cermet films was measured using a Fourier transform spectrometer. The data were compared with predictions of three models for the effective optical constants of heterogeneous materials: Maxwell-Garnett, Bruggeman, and a simplified version of a probabilistic growth model due to Sheng. Sheng's model provides the best description of the data over the complete range of metallic volume fraction. This result is expected based on the most likely topology of the films and is in agreement with other work on similar systems at higher frequencies.

Transparent silica gel - polymer composites have been prepared by the impregnation of porous silica gels with fluid organic monomer followed by in situ polymerization. These materials constitute an entirely new class of transparent composites.

Ultrafine copper particles were prepared by the thermal decomposition of a copper formate-poly(2-vinylpyridine) complex. At temperatures above 125°C, a redox reaction occurs where Cu+2 is reduced to copper metal and formate is oxidized to CO2 and H2. The decomposition reaction was studied by thermogravimetric analysis, differential scanning calorimetry and mass spectrometry. Copper concentrations up to 23 wt% have been incorporated into the polymer by this technique. The presence of the polymeric ligand induces the redox reaction to occur at a temperature 80°C lower than in uncomplexed copper formate. Incorporation of the reducing agent (formate anion) into the polymer precursor enables the redox reaction to occur in the solid state. Films of the polymer precursor were prepared and the formation of metallic copper particles were studied by visible and infrared spectroscopy, x-ray diffraction techniques, and transmission electron microscopy. Results from these measurements indicate that spherical copper particles with an average diameter of 35angstrom are isolated within the polymer matrix. The particles are thermodynamically stable at temperatures up to the decomposition of the polymer matrix (≈350 °C), but oxidize rapidly upon exposure air.

We show that rod-like particulates structured from interleaving layers of metal and insulator (or semiconductor) can exhibit novel infrared and far-infrared characteristics that are tunable through the variation of their geometric parameters. In particular, a dispersion of these particles can be made to absorb strongly at any prescribed far-infrared frequency by controlling their transverse dimensions and insulator fractions.

Historically, non-periodic models have usually found preference over microcrystalline ones for describing the structure of amorphous materials. Nevertheless, truly microcrystalline materials do exist, and the characteristic, monotonically decreasing isothermal calorimetric signal associated with the growth of the grains, can sometimes be used to identify them unambiguously. An example of the identification of the micro-quasicrystalline structure of some sputtered aluminum-transition metal alloys is discussed.

We investigated through X- ray diffraction and transmission electron microscopy the crystal refinement of the intermetallic compound AIRu by high- energy ball milling. The deformation process causes a decrease of crystal size to 5–7 rum and an increase of atomic level strain. This deformation is localized in shear bands with a thickness of 0.5 to 1 micron. Within these bands the crystal lattice breaks into small grains with a typical size of 8–14 rum. Further deformation leads to a final nanocrystalline structure with randomly oriented crystallite grains separated by high- angle grain boundaries.

Granular alloys of the form (Fe50Ni50)x(Al2O3)1−x, where x is the volume percent, have been produced by sputtering from a single composite target. The microstructure has been verified by X-ray diffraction and by transmission electron microscopy (TEM). The grains are single phase FeNi alloys with the fcc structure and range in size from 15 Å to 50 Å. Superparamagnetism has been observed, and the coercivity has been found to decrease with increasing grain size. The results are compared to previous work on granular Fex(SiO2)1−x.