Porosity control in ceramic membranes has been achieved by controlling particle packing densities in sol-gel processing. TiO2 xerogels with two mean pore radii of 0.7 and 1.7 nm and a porosity varying from 30% to 52% have been obtained. ZrO2 xerogels with a mean pore radius of 0.7 nm and a porosity varying from 7% to 34% have also been prepared. The principle of controlling porosity is to make spongy aggregates and to control the degree of aggregation. Experiments have been conducted to show that spongy aggregates can be produced by gradually removing protons from the strongly charged particles. Viscosity techniques have been used to measure the relative volume fraction of the dispersed phase which, in turn, provides information on aggregate structures. Two aggregation models have been proposed to explain different structural aggregates formed through thermal destabilization in the highly charged system and through charge neutralization by gradually removing charge from the particles in the system.

Ion irradiations of the rare-earth orthosilicate, Ca2Nd8(SiO4)6O2, have been carried out using both alpha particles (emitted from a 238PuO2 source) and 3 MeV argon ions. The unit cell exhibits anisotropic expansion under irradiation, consistent with expectations based on the polyhedral connectivity within the structure. A least-squares analysis of the interatomic distances suggests that the unit-cell expansions are primarily due to changes in oxygen-oxygen distances and cation separations between neighboring polyhedra rather than to bonds within polyhedra. The irradiation-induced change in unit-cell volume is proportional to 1 – exp (BD), where B is an annealing rate constant and D is the dose, in agreement with a model for the accumulation of isolated point defects in the structure. The volume expansion saturates at 2.56% and 1.40% for the alpha and argon irradiations, respectively. Analysis of the results suggests that a significant fraction of the defects produced in the argon-ion displacement cascades are lost to in-cascade recombination. Differential scanning calorimetry of powder irradiated with 3 MeV argon ions to 20 ions/nm2 reveals an exothermic recovery peak at 350 °C with an activation energy of 1.3 ± 0.1 eV and average stored energy release of 28.2 J/g. There is no evidence for amorphization of this material under alpha or argon irradiation.

The microstructure of air voids in both air-entrained and non air-entrained paste, mortar, and concrete has been studied at different ages (5 min to 60 days) in order to understand how air-entrained voids form in portland-cement systems. Scanning electron micrographs of air voids are presented for different ages. The solidification process of portland-cement paste and mortar was frozen at different ages using a low temperature scanning electron microscope and freeze drying. At very early ages the air voids show thin shells made of very fine particles. The packing of the cement grains behind the air void varies, depending on the water-to-cement ratio. Air voids appear to have the same interface with the cement paste matrix as aggregates. No readily visible difference was found between entrapped and entrained air voids.

The tensile properties, fracture behavior, and surface chemical composition of the rigid-rod heterocyclic aromatic polymer fiber poly-(p-phenylene benzobisthiazole), PBT, have been measured as a function of contact with nominally zero valent aluminum overlayers. The samples were produced by immersion of suitable fiber or film specimens in a molten aluminum-12.7 wt. % silicon alloy or by aluminum-vapor deposition followed by heat treatment. The strength of uncoated PBT fiber was 3.0 GPa. After 5 min immersion in the molten aluminum alloy, strengths drop to 80% and 25% of the uncoated fiber values for 600°and 700 °C immersion, respectively. Fiber strengths after aluminum immersion are from zero to 20% lower than the strength for corresponding uncoated fibers heated at an equivalent temperature in argon. Coated fibers exhibit tensile strengths after heating intermediate to similarly heated uncoated or immersed fibers. For all types of samples, the fiber fracture mode changes from fibrillar at failure strengths >1.9 GPa (independent of the environment) to planar at failure strengths <1.9 GPa. X-ray photoelectron spectroscopy of PBT fiber and film surfaces indicates oxidation of the polymer surface has occurred, most likely during fabrication of the fiber or film. The oxygen content of the surface layer is decreased when the film is immersed in molten aluminum-silicon alloy. The lack of corresponding changes in the relative intensities of the polymer C, S, or N photoemission peaks after this immersion suggests that the surface oxidized layer plays some role in protecting the polymer from degradation by molten aluminum. These results strongly suggest that it is possible to fabricate low-density, high-strength PBT/metal composites by liquid aluminum alloy infiltration if the melt temperature is kept below 680 °C and the contact time with molten aluminum kept below 3 min.

A systematic investigation of the microstructural evolution of fast fired, sol-gel derived Pb(Zr, Ti)O3 films (Zr/Ti = 54/46) was performed by analytical transmission electron microscopy (TEM). It was found that the nucleation and growth of the sol-gel PZT films were influenced by the precursor chemistry. The precursor solution was composed of Pb 2-ethylhexanoate, Ti isopropoxide, and Zr n-propoxide in n-propanol. Porous and spherulitic perovskite grains nucleated and grew from a pyrochlore matrix for NH4OH-modified films, but no chemical segregation was found. These thin films consisted completely of porous spherulitic PZT grains (∼2 μm) when the firing temperature was increased. Chemical phase separation with regions of Zr-rich pyrochlore particles separated by Zr-deficient perovskite grains was observed in the initial stages of nucleation and growth for CH3COOH-modified PZT films. This phase separation is attributed to the effect of acetate ligands on the modification of molecular structure of the PZT precursor. Firing the acid-modified films at higher temperatures for long times resulted in porous perovskite grain structures. The residual porosity in these films is suggested to be a result of differential evaporation/condensation rates during the deposition process and the gas evolution at high temperatures due to trapped organics in the films. Dielectric and ferroelectric properties were correlated to the microstructure of the films. Lower dielectric constants (∼500) and higher coercive fields (∼65 kV/cm) were found for the acid-modified PZT films with phase separation in comparison to those measured from the sol-gel films with a uniform microstructure (∽ > 600, Ec < 50 kV/cm). All films fired at 650 °C showed relatively good remanent polarization on the order of 20 μC/cm2.

This paper describes the preparation and compositional analysis of multilayer thin-film coatings prepared using sol-gel techniques. Alternate layers were labeled with an iron tag, derived from hydrolyzed 1,1'-bis(triethoxysilyl)ferrocene. Iron-free layers were composed of SiO2 derived from hydrolyzed tetraethylorthosilicate (TEOS). Analyses of these systems were based on Rutherford Backscattering Spectrometry (RBS) and Cross-Sectional Transmission Electron Microscopy (XTEM). The depth profile of iron, measured by RBS, yielded thicknesses (1000–1600 Å) for the individual layers that could be verified independently by XTEM.

We present a first principles model for abnormal grain growth in either two or three dimensions that has the virtue of simplicity on the one hand and of close agreement with recent computer simulations on the other. The model is based on Gladman's picture of one grain growing in a matrix of similar grains, retaining its shape as it grows. The grain must have a critical size advantage over its neighbors before the process is spontaneous. Once growth is under way the kinetics are identical to those in Hillert's classical model of grain growth. In the competition between abnormal growth of one grain and the normal growth in the matrix, the important parameters are the ratios γa/γm and μa/μm, where γ is the energy and μ is the mobility of a grain boundary and the subscripts a and m refer to abnormal and matrix, respectively. Values of these three factors: size advantage, energy advantage, and mobility advantage define the conditions under which abnormal grain growth may occur. The results agree closely with recent computer simulations and therefore permit interpolations to be made between computer answers. A hypothesis on the role of second phase particles as a trigger for abnormal grain growth is proposed.

Ion beam treatment studies have been carried out to investigate the potential for improvements in surface-sensitive properties of polymers. Kapton, Teflon, Tefzel, and Mylar have been implanted with boron, nitrogen, carbon, silicon, and iron ions, singly or simultaneously with dual or triple beams. The implanted materials were characterized by optical microscopy, transmission electron microscopy, nano-hardness indentation, wear testing, scanning tunneling microscopy, x-ray analysis, nuclear reaction analysis, Fourier transform infrared spectroscopy, and Raman spectroscopy. Although the polymers showed a color change and varying degrees of measurable surface depression in the bombarded area, the implanted surface revealed substantial improvements in surface smoothness, hardness, and wear resistance. In particular, B, N, C triple-beam implanted Kapton showed over 30 times larger hardness than unimplanted material, making it more than three times harder than stainless steel. Sliding wear properties were characterized using an oscillating nylon or high carbon steel wear ball. Severe wear tracks were observed in virgin Kapton, but no appreciable wear was observed in ion implanted Kapton. Mechanisms underlying the improved surface properties are addressed.

Highly concentrated solid-in-liquid suspensions find applications in a number of areas such as electronics, ceramics, paints, coatings, etc. Highly loaded, stable suspensions which exhibit desirable rheological characteristics (moderate viscosity, shear thinning behavior, thixotropy, and a small yield stress, for example), and which have high thermal or electrical conductivities are frequently sought after. We describe here some techniques which can be used to obtain such highly concentrated suspensions. These involve employing mixed size grades of particles and effective dispersants. For thermal paste applications, for example, compliant pastes of up to 78 vol. % solids with thermal conductivity values as high as 6 W/mK (hence, a few times greater than the values reported previously by others), low electrical conductivity, and moderate viscosity have been prepared by mixing different particle size grades of materials such as Al2O3, SiC, AlN, Al, and diamond. Effective dispersants, both commercial as well as those synthesized in our laboratory as novel variations of previously known molecular architectures, have been used to facilitate the achievement of these very high loading and stable suspensions.

A well-characterized Synroc was subjected to durability tests in de-ionized water at 70 °C and 150 °C, and silicate and bicarbonate solutions at 70 °C, to study the effect of temperature and solution composition on the mechanisms of aqueous alteration. SEM and TEM were used before and after durability experiments to characterize the primary and secondary phases in and on the Synroc samples, and to describe changes in morphology and chemistry. Leachant compositions after durability testing were analyzed using ICP/OES and ICP/MS and have been reported elsewhere.8 After durability testing, titaniferous surface layers were observed primarily on perovskite, the most soluble Synroc phase. Electron microscopy demonstrates that the surface layer formed at 70 °C is an amorphous Ti–O film and is fine grained anatase at 150 °C. Congruent dissolution is the major mechanism of perovskite alteration. However, at 150 °C, selective loss of Ca and Sr may occur locally. A number of additional secondary phases were identified, including Al–oxide/hydroxide, Fe–oxide, REE-bearing Ti–oxides, and several silicate phases. The abundance and composition of the secondary phases are related to solution composition and temperature.

The objective of this investigation was to develop a triphasic PTC thermistor composite which incorporated a phase capable of absorbing heat at a critical temperature, and thus limiting deleterious effects associated with thermal runaway. The system chosen for study was pentaerythritol incorporated into a carbon black–polyethylene thermistor system. Pentaerythritol exhibits a first order tetragonal to cubic phase transition at 185 °C, with a 1.87 to 3.18 J/°C · g change in specific heat and a 425 J/cm3 heat of transition. Composites with room temperature resistivities as low as 0.1 Ω · m, a PTCR effect of up to six orders of magnitude, and reproducible temperature-cycling behavior were developed. The pentaerythritol introduced thermal delays up to 7 min at 185 °C and substantially increased the electrical and mechanical stability of the composites with temperature and voltage cycling. High fields imparted irreversible effects in these composites as reflected by an increase in the room temperature and high temperature resistivity.

Crystallization kinetics and phase transformation characteristics of μ- or α-cordierite seeded gels were compared with those of unseeded monophasic gels. The α-cordierite seeding modified the sequence of phase transformation and lowered the temperature of α-cordierite formation from amorphous gel by ∽150 °C. The μ-seeded transformation was characterized by 3-dimensional isotropic growth with essentially zero activation energy for the nucleation step. Combining the nucleation theory with the transition state theory, we have separately estimated the activation free energy of nucleation and that of growth step in the crystallization of amorphous cordierite gel. The estimated activation free energies showed that the crystallization of unseeded cordierite gel is mainly growth controlled.

Isothermal capacitance transient spectroscopy measurements were performed to study the reoxidizing induced change in interface states in (Sr, Ca)TiO3−x based ceramics. A discrete acceptor type trap level was detected with reoxidation below the oxidation temperature of grain surfaces, but two deep levels were detected with reoxidation above this temperature. These ICTS results are in good agreement with the previous report with the DLTS measurements. These interface states are considered to originate from chemisorbed oxygen (O2−, O−) on grain surfaces. The O− and O2− chemisorption levels contribute to the non-ohmic conduction with reoxidizing anneals below and above the oxidation temperature, respectively.

Colloidal amorphous lead niobate particles of different composition and morphologies were obtained by a metal-chelate decomposition method. The nature of the dispersions depended on the conditions of the preparation of the complexes, pH, aging time, and temperature. The structural and chemical changes of the so prepared powders on calcination were investigated by thermal and x-ray diffraction analyses. Chemical mechanisms of the formation and transformation of the particles are suggested.

Small-angle neutron scattering measurements were used to examine the pore microstructure evolution of glassy silica and polycrystalline alpha-alumina as a function of sintering. It was shown that the two major sintering mechanisms, viscous flow and surface and volume diffusion, lead to very different microstructure evolution signatures in terms of the average pore size as a function of density. However, with respect to topology, the evolution of the porosity per unit surface area as a function of density is remarkably similar in the two systems.

Basic thin film deposition processes for controlled deposition of the high-Tc superconductors of the Bi-systems are described. The layered structures of Bi-oxide superconductors are fabricated by a multitarget sputtering process. The multitarget sputtering process realizes the controlled deposition of single phase Bi-oxide superconductors, Bi2O2 · 2SrO · (n −1)CaO · nCuO2 for n = 1 to 5. The minimum thickness controlled by the multitarget sputtering is a half crystal unit-cell of around 15 Å, and the superlattices comprising (AkBk) · m, where A and B denote the Bi-oxide superconductors with different numbers of Cu–O layers, could be fabricated for k > 1, although ion mixing takes place during the sputtering deposition due to the bombardment of the highly energetic sputtered adatoms. Multitarget sputtering will be available for the fabrication of the artificially-made layered oxide superconductors (ALOS).

Fe3Al-based iron aluminides have been of interest for many years because of their excellent oxidation and sulfidation resistance. However, limited room temperature ductility (<5%) and a sharp drop in strength above 600 °C have limited their consideration for use as structural materials. Recent improvements in tensile properties, especially improvements in ductility produced through control of composition and microstructure, and advances in the understanding of environmental embrittlement in intermetallics, including iron aluminides, have resulted in renewed interest in this system for structural applications. The purpose of this paper is to summarize recent developments concerning Fe3Al-based aluminides, including alloy development efforts and environmental embrittlement studies. This report will concentrate on literature published since about 1980, and will review studies of fabrication, mechanical properties, and corrosion resistance that have been conducted since that time.

Fluorozirconate glasses are stable with respect to devitrification but have poor chemical durability and only fair mechanical strength. AlF3-based glasses with improved chemical durability and enhanced mechanical strength are reported here. The optical, mechanical, and thermal properties of these glasses are contrasted to the more familiar ZBLAN composition. The infrared edge of these glasses lies at shorter wavelengths than ZrF4-based glasses, but aluminum fluoride glasses offer some interesting opportunities for short-range IR fiber applications such as sensing, remote spectroscopy, and laser power propagation.

The capacitive probe method for noncontact characterization and monitoring of dielectric materials is analyzed theoretically. An analytical method based upon the Hilbert transform technique and a numerical method using the finite element technique for calculating the potential distribution and change in admittance of the probe caused by presence of the dielectric material as a function of liftoff (distance between the probe plane and the surface of the dielectric material) are described. The two methods are compared with each other and their relative advantages discussed. The possibility of extracting useful information about the dielectric constant of the material from experimental data is also discussed in the light of the proposed theory.