Current methods for analysis of unknown powders in suspicious packages involve sending samples to laboratory facilities where a variety of time-consuming tests are performed. We have developed and investigated the use of a lead zirconate titanate - titanium (PZT-Ti) microcantilever for in situ detection of the common food- and water-born pathogen, Salmonella typhimurium. Using a bifunctional linking molecule to immobilize antibody on the titanium surface of the microcantilever, we can directly detect salmonella cells in suspensions of differing concentration. This novel surface functionalization technique along with the sub-nanogram sensitivity of the cantilever has allowed for direct quantification of S. typhimurium cells in suspension.

To improve the processing of silicon nitride, boehmite and yttrium oxide were coated on silicon nitride particles by an aqueous sol-gel method. It is shown that boehmite and Y2O3 can be coated on Si3N4 particles from aqueous solutions. However, the Y2O3 coating is not as complete as boehmite coating. The coating of a yttria coating followed by a boehmite coating was attempted and the TEM showed a coating twice as thick as the Y2O3 or AlOOH coating. Hydrothermal treatment of yttrium nitrate hydrous hydroxide results in the formation and growth of Y(OH)3 fibers which can be calcined and transformed into Y2O3 fibers at 500°C.

Large quantity of fly ash is generated every year from the electric power plants. Due to the shortage of disposal sites and tighter regulations, new viable ways of utilizing fly ash are needed. The formation of a porous material comprising fly ash was investigated. It was found that a porous material can be made by mixing fly ash with phosphoric acid, followed by heat treatment. The porous material is light-weight (bulk density ˜1 g/cm3), machinable, and with reasonable strength. Due to the high percentage of close porosity, the porous material has a great potential for thermal insulation applications.

The rheological properties of boehmite-coated silicon nitride aqueous suspensions and gels are reported. In unidirectional rheological tests, it was found that the boehmite coating reduces the viscosity of the suspensions over a wide range of shear rates and volume fractions of particles. The suspension shear stress as a function of shear rate can be described by the Bingham model, and the Bingham yield stresses of boehmite-coated silicon nitride suspensions are lower than those of the uncoated suspensions. The reduction in the viscosity and the Bingham yield stress is attributed to a shallower secondary minimum in the Derjaguin-Landau-Verwey-Overbeek (DLVO) potential between coated particles than that for uncoated silicon nitride particles. Moreover, at low values of pH, the coated silicon nitride suspensions gelled over time, and the viscoelastic behavior of the gels was studied by dynamic oscillatory tests. It was found that the shear modulus (G′) and loss modulus (G″) remain constant up to a certain strain amplitude, γ°, beyond which G′ and G″ begin to vary. The value of G′ in the linear region increases exponentially, whereas γ° decreases exponentially with the volume fraction of coated silicon nitride particles. The exponential behavior of the shear modulus G′ of the gels is similar to the exponential pressure-density relationship found in the previous pressure filtration study, indicating that particulate rearrangement occurs as volume fraction of particles is increased.

The effect of grain size on the elimination of an isolated pore was investigated both by the Monte Carlo simulations and by a scaling analysis. The Monte Carlo statistical mechanics model for sintering was constructed by mapping microstructures onto domains of vectors of different orientations as grains and domains of vacancies as pores. The most distinctive feature of the simulations is that we allow the vacancies to move. By incorporating the outer surfaces of the sample in the simulations, sintering takes place via vacancy diffusion from the pores to the outer sample surfaces. The simulations were performed in two dimensions. The results showed that the model is capable of displaying various sintering phenomena such as evaporation and condensation, rounding of a sharp corner, pore coalescence, thermal etching, neck formation, grain growth, and growth of large pores. For the elimination of an isolated pore, the most salient result is that the scaling law of the pore elimination time tp with respect to the pore diameter dp changes as pore size changes from larger than the grains to smaller than the grains. For example, in sample-size-fixed simulations, tp ∼ d3p for dp < G and tp ∼ d2p for dp > G with the crossover pore diameter dc increasing linearly with G where G is the average grain diameter. For sample-size-scaled simulations, tp ∼ d4p for dp < G and tp ∼ d3p for dp > G. That tp has different scaling laws in different grain-size regimes is a result of grain boundaries serving as diffusion channels in a fine-grain microstructure such as those considered in the simulations. A scaling analysis is provided to explain the scaling relationships among tp, dp, and G obtained in the simulations. The scaling analysis also shows that these scaling relationships are independent of the dimensionality. Thus, the results of the two-dimensional simulations should also apply in three dimensions.

To improve the dispersion of silicon nitride whiskers in aqueous suspensions, a standard dispersant used in glass fiber industry, 3-aminopropyltriethoxysilane (APS) is added. It was found that the viscosity of the whisker suspensions is lowered and the centrifuged density of the suspensions is increased with the addition of the APS. The pH values of suspensions before and after the addition of APS indicate that APS extracts H+ ions from the solutions and the adsorption of APS on silicon nitride is saturated in our experiment. Our results indicate a colloidal route to the processing of ceramic composites with silicon nitride whiskers as reinforcements.

Fly ash was chemically converted into zeolites. The conversion process not only eliminates the disposal problem of coal wastes but more importantly turns the waste material into a useful one. It was shown that fly ash can be converted into two forms of zeolites: P and faujasite, by varying the treatment condition. In addition, zeolite P was found to form earlier with higher treatment temperatures and adding quartz increased the amount of zeolite P formed. Seeding of faujasite was also studied.

The molecular structure of precursors in sol-gel processing of PbTiO3 is modified via β-diketone substitution for the isopropyl groups of titanium isopropoxide. Crystallite size and crystallization behavior are studied using x-ray diffractometry and differential thermal analysis. The crystallite size decreases with increasing amount of β-diketone substitution. Whereas the weight percent of crystalline phase first decreases and then increases with increasing β-diketone substitution. Our results indicate that the β-diketone substitution reduces the size of the structural units in the gel network resulting in smaller crystallite size and more nucleation sites in the powders produced.

Many advanced ceramics are formed by colloidal consolidation using techniques such as sedimentation, centrifugation, or pressure filtration. In all these, the minimization of density variations of various colloidally-consolidated cakes has not been examined systematically until recently. Schilling et al. used γ-ray densitometry to study the density variations in a sedimented cake and showed that the sediment of a flocculated alumina suspension exhibited significant density variations within the cake whereas the sediment of a dispersed alumina suspension showed a constant density profile. Auzerais et al. used a medical X-ray computer tomography (CT) to study the settling of dispersed and flocculated silica suspensions and obtained similar results: The sedimented cake of a dispersed silica suspension was fairly uniform whereas those of flocculated silica suspensions exhibited significant density variations. However, Shih et al. examined pressure-filtered cakes of flocculated alumina suspensions with γ-ray densitometry and showed that the density profiles of pressure-filtered cakes of flocculated alumina suspensions were uniform, in contrast to their sedimentation counterparts which showed significant density variations. These studies illustrate that how the local density varies within a consolidated cake depends not only on the suspension conditions but also on how the suspensions are consolidated, e.g., the consolidation pressure.

In a colloidal suspension with attractive interaction, particles form aggregates that settle to the bottom of the container. As the concentration of particles is increased, the overlapping of the aggregates (flocs) produces a continuous network throughout the suspension before settling occurs and a colloidal gel is formed. Colloidal gels may be divided into hard gels and soft gels.1 Hard gels are those formed by fine particles such as silica or boehmite of about 0.01 µm in size with high interparticle attraction energies. Under small shear rates, there is little restructuring in the gel network. The flocs that pack to form the gel network still retain their fractal structure, which gives rise to the scaling behavior of various mechanical properties of a gel with respect to particle concentration. For example, the storage modulus G' of a hard gel remains constant at small strain and increases in a power-law fashion with particle concentration, (φ, as G' - φn. There can be two regimes, a strong-link regime and a weak-link regime. In both regimes, the exponent n can be expressed in terms of the fractal dimension of the flocs, D, and the fractal dimension of the backbone of the flocs, x, while the form of n depends on what regime the system is in. Furthermore, a gel may crossover from the strong-link regime to the weak-link regime as the particle volume fraction is increased2.

A key step in the processing of ceramics is the consolidation of powders into engineered shapes. Colloidal processing uses solvents (usually water) and dispersants to break up powder agglomerates in suspension and thereby reduce the pore size in a consolidatd compact. However, agglomeration and particle rearrangement leading to pore enlargement can still occur during drying. Therefore, it is beneficial to consolidate the compact asdensely as possible during the suspension stage. The consolidation techniques of pressure filtration and centrifugation were studied and the results are reported in this paper. In particular, the steady-state pressure-density relationship was studied, and information was obtained regarding the consolidation process, the microstructure, and the average density profile of consolidated cakes. We found that the compaction processes in these two consolidation methods are quite different. In general, a consolidated cake is a particulate network made up of many structural units which are fractal objects formed during aggregation in the suspension. In pressure filtration, compaction is a process of breaking up the fractal structural units in the particulate network by applied pressure; the resulting partie rearrangement is produced by overcoming energetic barriers which are related to tee packing density of the compact. Recently, we performed Monte Carlo simulations on a cluster-cluster aggregation model with restructuring,1 and found the exponential relationship between pressure and density is indeed the result of the breaking up of the fractal structural units.2 On the other hand, in centrifugation, compaction involves the rearrangement of the fractal structural units without breaking them so that the self-similar nature of the aggregates is preserved. Furthermore, we calculated density profiles fom the bottom to the top of the cakes by solving the local static force balance equation n the continuum particulate network. In pressure filtration of alumina and boehmite, the cakes are predicted have uniform density. The results of γ-ray densitometry3 on a pressure-filtrated alumina cake confirmed this prediction. In contrast, in centrifugation, the density profiles are predicted to show significant variation for cakes on the order of one centimeter high. Moreover, the pressure-filtered boehmite cakes showed no cracking during drying. This indicated that pressure filtration is a good consolidation technique for nanometer-sized particles such as boehmite. The improved drying property is probably a result of a minimal shrinkage due to the formation of higher packing densities during filtration.

A semi-dilute polymer solution is one in which polymers overla1p. The bulk properties of semi-dilute polymer solutions have been studied extensively and much has been known. The scaling theory has been very successful. 1,2 The predictions of the scaling theory about various physical quantities as a function of polymer concentration, c, have been confirmed experimentally. 1,3

The stability of a colloidal suspension plays an important role in colloidal processing of materials. The stability of the colloidal fluid phase is especially vital in achieving high green densities. By colloidal fluid phase, we refer to a phase in which colloidal particles are well separated and free to move about by Brownian motion, By controlling parameters such as pH, salt concentration, and surfactants, one can achieve high packing (green) densities in the repulsive regime where the suspension is well dispersed as a colloidal fluid, and low green densities in the attractive regime where the suspensions are flocculated [1,2]. While there is increasing interest in using bimodal suspensions to improve green densities, neither the stability of a binary suspension as a colloidal fluid nor the stability effects on the green densities have been studied in depth as yet. Traditionally, the effect of using bimodal-particle-size distribution has only been considered in terms of geometrical packing developed by Furnas and others [3,4]. This model is a simple packing concept and is used and useful for hard sphere-like repulsive interparticle interactions. With the advances in powder technology, smaller and smaller particles are available for ceramic processing. Thus, the traditional consideration of geometrial packing for the green densities of bimodal suspensions may not be enough. The interaction between particles must be taken into account.

In this paper we are concerned with the morphology of the polymers adsorbedon surfaces, in particular di-block copolymers. Our work is motivated by the experimental findings of Fladziioannou et al. [1] on the steric forces between two adsorbed layers of di-block poly(vinyl-2-pyridine)\ polystyrene (PV2P\ PS) copolymer on mica surfaces. The PV2P block binds strongly on the mica surfaces and the PS block extends into thesolvent toluene (good solvent for PS). Hadziiouannou et al. found that the repulsive forces between the two surfaces start at a distance 1) larger than 10 times the radius of gyration RG of a free P' in toluene. Furthermore, the starting distance D increases with increasing degree of polymerization N of PS in a fashion I) ~ Na with a close to I. We,tudy the adsorption of di-block copolymer with Monte Carlo simulations. The Monte Carlo simulations are especially powerful in dealing with kinetics which is important in systems where hysteresis is observed II1 and cannot be appropriately taken into account by analytical (or numerical) calculations based onequilibrium assumptions.

Microstructures play an important role in sintering. Hlowever, most existing sintering theories are macroscopic and do not consider the topological constraints in a microstructure. To fully take into account the microstructural effect, Anderson et al. [l] developed a Monte Carlo statistical mechanics model to study the grain growth behavior of a fully dense system. lHere we developed a generalized model which allows both densification and grain growth to occur. The densification occurs through the diffusion of matter and is incorporated in the Monte Carlo procedure. Thus we can monitor both the microstructural evolution and the densification at the same time during heat treatment. To our knowledge, this is the first statistical mechanics model constructed for sintering. As a quantitative example, the sintering behavior of an isolated pore of various sizes and shapes at different grain growth rates is studied.