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Two-component oxide powders were prepared in the systems Al2O3-ZrO2. Al2O3-TiO2. and Al2O3-SiO2 by a CVD (chemical vapor deposition) technique using a combustion flame for the purpose of fabricating respective composite or compound ceramics. The CVD powders were spherical and ultrafine (av. 30–70 nm) with log-normal size distribution, and exhibited either very homogeneous or nanoheterogeneous structure and crystallographic metastability in phases and solid solutions. By starting from these CVD powders, not only uniform and excellent microstructure was achieved in the sintered products, but also better sinterability and novel microstructure became possible owing to the metastability which reflected the formation mechanism of two-component powders at high temperatures. CVD powders, which were previously considered difficult to sinter, proved to be excellent starting materials for fabrication of composite and compound ceramics when improved processing techniques were applied.
Spherical titania particles can be prepared in a short time period from an aqueous solution of TiOSO4 by homogeneous precipitation using urea at 70–90°C. Although the hydrolysis of TiOSO4 continues to a considerable extent even in the absence of urea, the presence of urea is essential for the formation of spherical particles. The average particle size produced was l–4μm and changed mainly with the concentration of TiOSO4. As-synthesized powders are amorphous hydrates of titania containing sulfate group and crystallize by calcination into anatase(>500°C) and rutile(>900°C). In calcination, the particles shrink, but hold their original spherical shapes.
The synthesis of mixed oxide ceramic powders that consist of very fine (submicron)-monodisperse particles with uniform composition is desirable in the improvement of existing ceramics and the development of new ceramic materials. Metal alkoxide hydrolysis is a very attractive method for the synthesis of ultrapure composite powders at low temperatures by the sol-gel process.
The present study investigates the effects of the hydrolysis of aluminum alkoxides and the condensation products on the growth kinetics and morphology of composite particles containing alumina and titania. Alkoxides of titanium and aluminum are employed; therefore, powders of high purity are produced. Since various solvents are used as media for powder synthesis, the effect of the solvent on particle morphology will also be discussed.
The preparation of particles of uniform size, shape, and composition has been of interest to colloid chemists and has received serious attention in the ceramic processing community recently. I interest in making monodispersed sols is not based solely on esthetic appeal of such systems but has evolved from high tech applications which place stringent requirements on the properties of ceramic components and thus on starting powders. Uniform particle size and shape are also essential for the evaluation of validity of various sintering models as well as optical, magnetic and electrokinetic properties of colloidal systems developed assuming uniform particle size and shape.
We have formed powders of a strongly ionic compound, RbCl by freezedrying. Stock solutions, varying from very low concentrations with 4 gm RbCl reagent dissolved in 100 cc water to saturated solutions with 80 gm reagent in 100 cc water, were sprayed into isopentane at -160°C. The droplet size in the spray was varied from relatively large 4-mm diameter droplets to extremely small droplets in an aerosol spray. It was determined that both the concentration of the stock solution and the droplet size affect the average size and the size distribution of the primary particles formed and the way in which these primary particles are bonded together. Unlike the powders of many less ionic compounds that are produced by freeze-drying, the primary particles in these RbCl powders are crystalline rather than amorphous. Analysis with an x-ray diffractometer with a cold stage indicates that crystallization occurs during the freezing cycle rather than during the sublimation period of the freeze-drying process.
Highly dielectric ceramics make it possible to markedly miniaturize passive microwave components. We have successfully synthesized ceramic powders of the most important microwave ceramic systems (ZrTiSn)O2, Ba(TiNiZnTa)O3 and Ba(MgTa)O3 by using the sol-gel process. Nonaqueous solutions of metal organics together with different inorganic metal salts were used as precursors. The gels derived from these solutions are homogeneous and transparent. Different drying procedures were investigated in order to optimize powder morphology. Thermoanalytic methods and high -temperature x-ray diffraction were used to study the decomposition of xerogels and the formation of crystal phases, respectively. The calcination process was found to be complete at temperatures well below 750 °C.
For the production of barium-containing powders, wet milling must be avoided because this washes out barium and thus causes undesired phases to appear. A dry milling step of the xerogel solves this problem. The prepared (ZrTiSn)O2 and Ba(TiNiZnTa)O3 already equal or surpass the quality of conventionally prepared ceramics.
A discussion of the formation of periodic colloid structures, liquid-like ordering, and compact and fractal aggregates caused by colloidal forces in ceramic dispersions is presented. Construction of phase diagrams based on simple forms of repulsive potentials is often not adequate, and it is important to include appropriate attractive interactions in the theoretical analyses. Examples of radial distribution functions, osmotic prsueadphase diagrams are given for dispersions interacting through Derjaguin-Landau-Verwey-Overbeek potentials. Densification of colloidal aggregates dlue to positional relaxation and the effects of such densification on the structure of the aggregates are discussed.
The flow of hard sphere-like suspensions near close packing is explored. The change in viscosity with stress and volume fraction shows that at volume fractions above 0.5 shear thickening occurs and that the characteristic shear rates for shear thinning and shear thickening decrease rapidly above this volume fraction. The creep compliance is well characterized by a stretched exponential relaxation time spectrum above volume fractions of 0.52. These results suggest that the limiting volume fraction where the zero shear rate viscosity diverges is determined by a liquid/glass phase transition very similar to that predicted for hard sphere liquids.
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.
A colloidal suspension can be either dispersed or flocculated depending on the interaction between the colloidal particles. If the interaction is repulsive, particles can relax to the minimum of the potential due to their neighboring particles, and the system can reach an equilibrium dispersed state. In the case of attractive interaction, particles form aggregates that settle to the bottom of the container. As the concentration of particles is increased, the overcrowding of the aggregates produces a continuous network throughout the suspension before they settle and a colloidal gel is formed. A major difference between a colloidal gel and a colloidal suspension is that the gel can sustain finite stress and is therefore viscoelastic. Previously we studied the storage modulus and the yield strain of boehmite gels and found that they are related to the particle concentration in a power-law fashion . Similar scaling behavior of the shear modulus was found for other colloidal particulate networks by Buscall et al. . We developed a scaling theory  which successfully explains the experimental results on boehmite gels. The theory further predicts that there can be two types of power-law behavior depending on the relative elastic strength of the clusters to that of the links between clusters within the gel network. Furthermore, there can be a crossover from one type of behavior to the other as the particle concentration is varied.
Superconducting thick films of yttrium-based cuprates can be prepared using a sol-gel technique. Rheological measurements on the precursors are used to identify processing parameters that can lead to better film properties. Steady and dynamic experiments reveal a direct correlation between the properties of the precursor, the final product, and the “age” of the yttrium component used in the precursor. Measurements support a proposed mechanism relating the “age” of the yttrium to the other properties and help to establish a processing methodology to obtain effective superconducting properties.
Solutions have been prepared containing silicon ethoxide, titanium butoxide, and zirconium propoxide as metal organic compounds. Additions of water (for hydrolysis), solvents (ethanol, formamide), and hydrochloric acid (as a catalyst) have been varied systematically. The reaction kinetics of the hydrolysis and condensation steps in these mixtures have been studied by measuring viscosity and 29Si NMR data as a function of time. After gelling, the products have been dried and then converted to glassy materials by firing. Temperatures well below 1000 °C have been used for that. Glasses from the title system have been obtained both in bulk and in thin film form. Properties like density, specific surface, and hardness have been investigated and are reported as well.
Standard sol-gel procedures have been modified to permit sol-gel synthesis to be performed at varying hydrogen ion concentrations and in a variety of organic solvents. These modifications greatly expand the variety of guest molecules that can be incorporated into nondense glassy environments. Processing conditions can be controlled such that the guest compounds generally retain their solution properties thereby allowing new molecular composites to be formed. Reported herein is the incorporation of both porphyrins and metalloporphyrins into silica matrices. Electronic absorption, vibrational, and excited state luminescence properties of the resulting composites clearly demonstrate that the molecular and electronic structure of the porphyrins and metalloporphyrins are preserved during sol-gel processing and in the ceramic matrices. This retention of properties has been shown for guest molecules doped into monoliths as well as films.
Molecular composites that possess unique emission properties have been prepared by introducing luminescent molecules into sol-gel based silica glasses. The lumophores chosen for study are terbium(III) and europium(III) ions whose emissions are quenched in aqueous and sol-gel environments owing to coordination of water molecules to the lanthanide ion. To overcome the quenching process the ions have been encapsulated within cryptands. The resulting terbium(IU) and europium(III) cryptates possess long-lived excited states in both aqueous solution and solgel glassy matrices. Measurements of the excited state dynamics of these molecularly engineered composites demonstrate the feasibility of tailoring molecules to retain their excited state properties in sol-gel derived glasses.
Metallo-organic materials are often used as precursors for highly refractory oxides and hightemperature composites. The feasibility of producing hafnium metallo-organic films and gels by sol-gel techniques is demonstrated in the present study. Since hafnium alkoxides are not commercially available, their preparation is an obstacle in the development of their sol-gel processing applications. A common synthesis technique was used to produce hafnium isopropoxide, which was subsequently hydrolyzed under acid catalyzed conditions to produce films and gels. The films were prepared by spin casting and remained optically transparent. Initially, the gels were also optically transparent, but became translucent upon drying. Raman, IR, and light scattering spectroscopies were used to characterize this sol-gel process, and it is compared with the analogous processing of titanium and zirconium alkoxides.
The low-temperature sol-gel process opens a number of new materials possibilities for generation of glasses with predetermined properties by the incorporation of organic modifiers into the network. Polysiloxanes are potentially interesting organic modifiers for toughening and possibly surface-modifying the silicate networks. Some fundamental studies of the hydrolysis and condensation processes in a tetramethylorthosilicate (TMOS) system, in the absence of added catalyst, have been conducted using 1H and 29Si NMR. The effects of some of the reaction parameters and processing conditions for the subsequent conversion of the gel to monolithic dried gels by heat treatment have been investigated by techniques such as thermal analysis and mass spectroscopy. Procedures which employ mild pressures have been established that permit the generation of monolithic products which show greatly reduced cracking tendencies. Finally, methoxy functionalized poly(dimethylsiloxane) oligomers that can react into the sol-gel network have been prepared. The intermediates are commercially accessible and the process is scaleable. Utilization of a catalyst-free system eliminates the tendency of the siloxane modifier to undergo undesired rearrangements that are known to occur in the presence of strong acids or bases.
Aluminum nitride (AIN), alumina and aramid fibers have been incorporated into epoxy and fluoropolymer matrices. The fluoropolymer composites have dielectric constants less than 3.4 and losses below 0.3%, measured out-of-plane. In-plane and out-of-plane thermal conductivities of the AIN-fluoropolymer composites averaged 5.2 and 1.3 W/mK, respectively, at fiber volume fractions of 0.26 to 0.29. In-plane thermal conductivities of woven fabric composites were accurately predicted by mixing rules; for non-woven and short fiber composites, thermal conductivities were less than predicted. These composites had higher out-of-plane thermal conductivities due to out-of-plane components of the fiber orientations.
Inorganic polymers are used in the formation of green compacts via sedimentation of colloidal alumina suspended in chloroform. Polymers containing highly polar components tend to produce constant density profiles of greater than 55% density, while those containing nonpolar, reactive components produce profiles with a large gradient in packing density. Density profiles describing the sedimentation behavior versus time and the final dried density of the compacts are generated via the use of gamma-ray densitometry. These polymers have the potential not only to increase green compact density but also to reduce weight losses due to “burnout” and subsequent sintering requirements by pyrolyzing to a ceramic phase.
The evaporation of organic by-products released during drying of 1-mm thick silicon tetramethoxide gels was analyzed using gas chromatography. The evaporation kinetics of methanol depended on the drying rate achieved by flowing dry air over the gel. For drying at flow rates less than 50 cm 3/min, exponential kinetics were observed initially with a long time constant (about 100- to 400-min). For drying rates greater than 70 cm3/min, diffusional (t−1/2) kinetics were observed initially. Cracking of the gel during drying was used to indicate the degree of stress. At low drying rates, minor cracking was observed near the edges of the gel. At high flow rates, extensive cracking was observed in samples that exhibited early t−1/2 kinetics. Monitoring the kinetics of drying is essential to optimizing the drying conditions to minimize stress and cracking in gels.
Fundamental issues in the removal of processing aids from ceramic compacts prior to sintering have been investigated, both experimentally and theoretically. A general theoretical model has been developed that couples simultaneous momentum, heat, and mass transfer phenomena in disordered porous materials with the mechanical response predicted by an appropriate poroelasticity theory for partially saturated porous granular materials. The kinetics of pyrolytic degradation of organic processing aids were studied using a thermogravimetric analysis-mass spectrometry (TGA-MS) system.