Inorganic and organometallic polymers are macromolecular systems in which the polymer backbone contains elements other than the carbon, oxygen and nitrogen usually found in organic polymers [1]. To take as an example silicon-containing polymers, in the silicones the polymer backbone is composed of the Si-O repeat unit; in polysilazanes, of the Si-N unit; in polysilmethylenes, of the Si-C unit. In the polysilanes there are only silicon atoms in the polymer backbone. Many of the other metalloids and metals among the elements in the Periodic Table have been or, in principle, can be incorporated into polymeric systems, so it is clear that the field of inorganic and organometallic polymers is a very large one. Inorganic and organometallic polymers have been of interest to chemists for a long time. It was the commercial development of the silicones in the 1940's that gave this field of research its modem impetus [2]. Once it was appreciated how useful these versatile organosilicon polymers could be, chemists became interested in the possibility of developing other organometallic (and also inorganic) polymers, ones that might complement or even surpass the silicones as far as useful applications were concerned. Research on inorganic and organometallic polymers became very active in the 1950's and 1960's. Work in this area became an international effort, prompted by the need for new materials that would meet the exacting demands of the jet age that had effectively commenced around the end of World War II. Even greater demands, in terms of materials that would still be useful under extreme conditions, came with the space age.

Boron carbide is a ceramic material with excellent high temperature physical properties. As compared to conventional techniques, the preparation of boron carbide from polymeric precursors is attractive as this technique offers a number of unique advantages. In this paper, the screening of polymeric precursors to boron carbide will be discussed. Two promising boron carbide, carborane containing polymeric precursors have resulted in 60-70 wt.% ceramic yields. The chemistry of polymer synthesis and the transformations from the polymer to amorphous and crystalline boron carbide were investigated with infrared spectroscopy, NMR spectroscopy, thermal analysis, and x ray diffraction.

Crystalline SiC was prepared by carbothermic reduction of alkoxide-derived polymeric precursors under a flow of argon at 1400-1600°C. The pyrolysis products from the precursors with a molecular source of carbon, i.e., (R1)x(R2)ySiO1.5(Rl=phenyl; R2=propyl, vinyl), showed different morphologies from those of the precursors derived from TEOS and a polymeric source of carbon (polyfurfuryl alcohol). Furthermore, the pyrolysis chemistry of theses precursors and the microstructure of their corresponding pyrolysis products were studied by means of FTIR, TGA, XRD, SEM, BET, elemental analyses, etc.

Homogeneously mixed nanocrystalline composites incorporating SiC, Si3N4, AIN, BN and TiN, and SiC/AIN solid solutions were prepared from mixtures of known precursors to the separate components and specially prepared singlesource precursors. These novel materials are of potential interest as tough, abrasion-resistant coatings, continuous fibers and matrices for high temperature composites and, possibly, superplastic ceramics. Solid state NMR spectroscopy along with other chemical and materials characterization methods have been employed in studies of the precursor-to-ceramic conversion process and the characterization of the final ceramic products. The results of these studies are described and their implications with respect to the relationship between precursor structure, pyrolysis chemistry and the final ceramic composition and microstructure are discussed.

The reaction between furfuryl alcohol (FuOH) and the hydrolysis product of a mixture of Ti(O-n-Bu)4 and Al(O-sec-Bu)3 (AI:Ti=1:1) resulted in the formation of soluble polymeric solids. Pyrolysis of these polymers in argon yielded TiC/A2O3 composite at 1250 °C. When these polymers were pyrolyzed in ammonia, TiN/AI2O3 or TiN/AIN composite was formed. All these composites showed fine microstructure and excellent homogeneity. Furthermore, the structure and pyrolysis chemistry of these polymeric precursors as well as the microstructure of the pyrolysis products were studied by means of FTIR, TGA, XRD, SEM, TEM, STEM, Auger, elemental analyses etc.

Reaction of Et2AIOEt with ethylene glycol or catechol produced polymers of the general form -[-AI(OEt)-O-R-O-]-n, for R = CH2CH2 or C6H4, respectively. Pyrolytic conversion of these polymers to ceramic materials produced A12O3, at mild (∼500°C) temperatures under a flowing atmosphere of dry air. The crystal phase obtained from the thermolysis is highly dependent upon the degree of cross-linking present in the initial polymer. These results are discussed in terms of the required solid-state atomic reorganization necessary to proceed from polymer to ceramic.

In recent years considerable progress has been made in electronic packaging substrate technology. The future need of miniaturization of devices to increase the signal processing speeds calls for an increase in the device density requiring the substrates to be designed for better thermal, mechanical and electrical efficiency.Fast signal propagation with minimum delay requires the substrate to possess very low dielectric constant. Several glasses and glass-ceramic materials have been identified over the years which show good promise as candidate substrate materials. Among these borophosphate and borophosphosilicate glass-ceramics have been recently identified to have the lowest dielectric constant (3.8). Sol-gel processing has been used to synthesize borosilicate, borophosphosilicate and borophosphate glasses and glass-ceramics using inexpensive boron oxide and phosphorus pentoxide precursors. Preliminary results of the processing of these gels and the effect of volatility of boron alkoxide and its modification on the gel structure are described. X-ray diffraction, Differential thermal analyses and FTIR have been used to characterize the as-prepared and heat treated gels.

The factors affecting the production of homogeneous aluminosilicate gels from alkoxide precursors were investigated. Aluminosilicate gels were made in acidic conditions with a prepolymerization of the silicate precursor followed by the addition of the alumina precursor.

Confirming earlier studies, we found that the gel homogeneity is increased when the rate of gelation decreases. The reduction of the water content and/or the aluminum precursor, and more surprisingly the increase of the acid concentration contributed to the gel time increase and thus the gel homogeneity improvement.

Reaction of boehmite, [AI(O)(OH)]n, with an excess of carboxylic acid (HO2CR) results in the formation of the carboxy substituted alumoxanes, [AI(O)x(OH)y(O2CR)z)]n where 2x + y + z = 3 and R = alkyl substituents. The alumoxanes have been fully characterized by SEM, elemental analysis, IR and multinuclear NMR spectroscopy. The physical properties of the alumoxanes are highly dependent on the identity of R, and range from insoluble crystalline powders, e.g. R = CH3, to powders which readily form solutions or gels in hydrocarbon solvents, e.g. R = C5H11. All of the alumoxanes decompose under mild thermolysis to yield γ-alumina.

Bimetallic glycolato silicates of the form M2 Si2(OCH2CH2O)5 where M is an alkali metal, can be synthesized directly from SiO2, ethylene glycol and MOH. Further reaction with HO(CH2CH2O)4H, PEG4, transforms these compounds into rheologically useful polymers. These polymers offer considerable potential for processing novel silicate glass shapes, e.g. thin films, fibers and membranes, from relatively inexpensive precursors materials. In this paper, we describe pyrolysis studies on the model compound Li2 Si2(OCH2CH2O)5 as a prelude to determing the utility of the polymeric precursors. These studies delineate the effects of temperature on the decomposition processes whereby the glycolato silicate transforms into almost phase pure, crystalline Li2O-2SiO2. This work is contrasted with previous pyrolysis studies on K2 Si2(OCH2CH2O)5.

Stepwise transmetalation of the neutral molecular target (µ4-O)N4 Cu4 C16(N is a monodentate pyridine) with M(NS)n transmealators (NS is a monanionic S-methyl hydrazinecarbodithioate ligand, M is Co (n = 2 or 3), Ni or Zn (n = 2)) in aprotic solvents gives a large family of heteropolymetallic products (µ4 - O)N4 (Co,Ni,Cu,Zn)4 C16 that contain from one to four different metals. Thermolysis of these solid products in flowing O2 at 220 - 250°C gives uniform, highly dispersed mixtures of the respective metal oxides. For example, thermolysis of bulk precursor (µ4 -O)N4 CoNiCuZnCl6 gives a uniform mixture of 1 - 2° particles of Co3 O4, NiO, CuO and ZnO.

Well-crystallized Y2O3 -ZrO2 powder of 12nm crystallite size was synthesized by RoAogenious precipitation under hydrothermal condition at 180°C for 1 hour. This powder consisted of tetragonal zirconia. After calcination and ball milling, the crystiilite size was 22 nm and the tetragonal phase was reduced to 55% by ball milling. The average grain size was 0.5 µm and specific: surface area was 20 m /g. Highly dense TZP(> 99%) with a homogeneous microstructure was obtained by sintering this powder at 1400°C for 2 hours.

Ba/Sr titanate powders were obtained by reacting TiO2.xH2O gel in Ba(OH)2 and/or Sr(OH)2 aqueous solution. Different reaction temperatures between 68°C and 98°C and different mole ratios of Ba(OH)2 and Sr(OH)2 were used.

X-ray diffraction analysis showed that the titanate powders are cubic, and they are solid solutions of barium and strontium. The lattace spaces of the titanates are affected by the Ba(OH)g/ Sr(OH)g, mole ratio. TEM analysis showed that the titanate powders were spherical, ultrafine, and almost monodispersed. The particle size of the powders is about from 41 to 50 nm depending on the reaction temperature.

The titanate powders were compacted and sintered at various temperatures. The best sintering temperature of the powders is about 150°C lower than that of powders made by solid state sintering method.

The reaction of SiO2 with basic magnesium compounds, in the presence of excess ethylene glycol, provides access to the hexacoordinate silicate complex, MgSi(OCH2CH2O)3. Alkali metal hydroxides provide access to pentacoordinate silicates of the type M2Si2(OCH2CH2O)5, where M = Li, Na, K, and Cs. Alumina also reacts with alkali metal hydroxides and ethylene glycol to give compounds of the type MAI(OCH2CH2O)2. HCl neutralization of the alkali metal silicates and aluminates provides the neutral metal glycolates of Si and Al. Stoichiometrically correct mixtures of these complexes form homogeneous, cordierite precursor solutions that are potentially useful for forming films, fibers and membranes of cordierite. These polymer-like materials can be heated directly in air to generate first α-cordierite and then µ-cordierite. Hydrolysis and condensation reactions, which occur in sol-gel processing, are not required. The phase transformations and chemical changes that occur during pyrolytic transformation of the preceramic materials to their target ceramic products were characterized. Methods developed during the course of these studies led to the development of heating schedules wherein well crystallized g.-cordierite could be formed at 850 °C.

Aluminum titanate ceramic powder, used in plasma spray coating as a wear resistant material, was prepared from a gel precursor followed by high temperature calcination. The chemically derived gel was made from aluminum sulfate and titanium isopropoxide in an oxalic acid aqueous system. The soluble complex was studied by 13C NMR spectroscopy which showed that the mole ratio of oxalic acid bound to Al and Ti was 2:1. A coprecipitation took place when ammonium hydroxide was added. The gel was examined by solid-state NMR spectroscopy which suggested that aluminum titanium hydroxyl oxalate was present with Al in a distorted octahedral environment. EDX mapping showed that Al and Ti were evenly distributed throughout the gel particles. Thermal analysis indicated that dehydration and combustion of organic groups took place between 100 and 400 ° C. Gel which was heat treated at 1500 °C was identified as a pure AI2TiO5 crystalline phase by XRD.

A wide variety of compounds, e.g., superconductors, chromites, and manganites, can be synthesized by spray drying a mixture of salts or a combination of salts and sols in a flash dryer. The process ensures good control of stoichiometry, morphology, particle size, and surface area. Appropriate particle-growth and synthesis heat treatments of these fine powders have been conducted, and the resultant powders have been evaluated for possible applications in fuel cells and superconductors. The process has been scaled up to a pilot-plant capacity of ≈40 kg per day.

This investigation involves the thermal conversion of an organosilazane precursors into ultrafine ceramic particles. In this process, aerosols of a reactive organosilazane precursor, [CH3SiHNH]n n = 3 or 4, are injected into a hot furnace to obtain Si3N4/SiC ceramic powders. One critical feature examined in this process was the rapidity of the powder synthesis, in a reaction which involves (i) elimination of ligand groups, (ii) formation of ceramic species, and (iii) condensation of ceramic species into ultrafine ceramic particles. Accompanying studies a model has been formulated to determine the gas cooling rate and particle size.

We describe a new laboratory-scale aerosol process for producing AIN powder. A two-stage reactor design is used. In the first stage, triethyl aluminum (TEA = AI(CC2H5 )3) and NH3 react to form an aerosol adduct in a laminar flow diffusive mixing zone. The aerosol then enters the furnace stage, where it is converted to AIN. We have examined the influence of the major operating variables (e.g., inlet TEA concentration, reactor residence time, and furnace temperature) on the particle size and distribution, yield, and efficiency. For example, increasing the TEA concentration from 0.12 to 1.30 µmol/cm3 causes an increase in the mean particle diameter (from 0.07 to 0.13 Pim), a slight increase in polydispersity (from 0.31 to 0.43), and a decrease in yield efficiency (from 90% to 73%). In contrast, decreasing the reactor residence time (by increasing the flow rate) has little effect on mean particle diameter, but causes a significant increase in yield efficiency (approaching 100%). The overall behavior of the reactor suggests a model in which the particle size distribution of the final product is determined primarily by the aerosol formation steps in the mixing stage (i.e., nucleation, growth, and coalescence), while the composition and crystallinity of the product are determined by furnace conditions.

Vapor phase synthesis of titania particles by oxidation of titanium tetrachloride (TiCI4) was studied in an aerosol reactor between 1200 K and 1723 K. The effect of process variables (reactor residence time, temperature, reactant concentration) on powder size and phase characteristics was investigated using the differential mobility particle sizer, scanning electron microscopy and X-ray diffraction. The morphology of the particles remained unchanged under the process conditions investigated; titania particles were primarily anatase though the rutile weight fraction increased with increase in reactor temperature. The geometric number average diameter of the particles was between 0.13 µm and 0.35 [m and the geometric standard deviation of the particle size distribution was about 1.4. The average particle size increased with increasing temperature, TiCI4 concentration and residence time. The observed changes in the particle size distribution were compared with those predicted by solving the aerosol dynamic equation by a sectional method and accounting for coagulation and first order chemical reaction.

In this paper we report a new approach to the problem of high rate formation of nanophase powders. In our experiments we were able to make aluminum oxide particles in the size range from 5 to 140 nm (peaking sharply at 35 nm) at a rate of 3 g/min. The starting material was a mixture of aluminum-tri-sec-butoxide and sec-butanol. An aerosol was made from this solution and subsequently burned in a special torch, described below. The resulting particles were spherical and no necked regions were observable between them. In a practical application, our technique allows a large production rate while still approaching the ideal of nano-scale monodispersed particles. The work was extended to the formation of zirconium oxide particles with quite similar results in the size distribution.