The intensity I(q) of the small-angle x-ray or neutron scattering has been calculated for a system of randomly oriented, independently scatter-ing pores with a number distribution of pore diameters which has the form of a power law. As has already been shown, [P. W. Schmidt, J. Appl. Cryst. 15, 567–569 (1982)], when the number distribution of the maximum diameters a of the pores is proportional to a−γ, I(q) is proportional to q−(7−γ), where q = 4πλ−1sin(θ/2), θ is the scattering angle, and λ is the wavelength. The coefficient of the power-law intensity has been expressed in terms of some of the constants which determine the diameter distribu-tion. Equations have been obtained for the scattered intensity I(q) at q values larger and smaller than those at which power-law scattering occurs. The intensity scattered by this system is compared with the intensity from a system of pores with fractal pore-boundary surfaces which have a fractal dimension D.

The reaction of trialkylaluminum compounds with ammonia has been examined as a potential route to high purity AlN powder and to AlN thin films. This reaction proceeds in stages in which the initially formed Lewis acid/base adduct undergoes thermal decomposition to a series of intermediate alkylaluminum-amide and -imide species with increasing Al-N bonding, i.e.,

The structure and properties of several of these species have been studied using various physical and chemical methods, leading to a better understanding of the chemistry of this novel AlN precursor system. The structure of the intermediate organoaluminum amide, (CH3)2AlNH2, has been determined by single crystal X-ray diffraction methods and found to contain molecular trimer units with a six-membered Al-N ring structure similar to those which make up the wurzite structure of AlN. This compound is readily volatile and has been used to deposit AIN thin films on Si surfaces by a low-pressure CVD process. This approach has also been used to prepare AlN as a high surface area, high purity powder.

The controlled reductive dehalogenation of elemental halides has been studied as a low temperature approach to boride and carbide precursors. We have shown that SiCl4 and CCl4 can be reacted with metallic sodium at 130° in a non-polar solvent to give the precursor to SiC. Similar type reactions with TiCl4 and BCl3 or with BCl3 and CC14 have produced the precursors to TiB2 and B4C respectively. This procedure has also been used to generate the precursors to the two-phase composites SiC/TiC and SiC/TiN. The method is generally applicable to any combination of elemental halides which can be reduced by alkali metals or alkali metal alloys.

The synthesis of designed organometallic compounds and their selective activation and transformation into materials of high purity (for electronic applications), high strength and/or high temperature stability (for refrac-tory or structural applications), represents a potential area of extreme growth in organometallic chemistry. Research in this area could provide entirely new, inexpensive, fabrication methods for common and exotic materials.

In this paper, we develop design principles for the preparation of organometallic precursors, “premetallics” that can be selectively con-verted, in high yields, to a desired refractory metal. We also describe our preliminary efforts to prepare tungsten carbides (WCx) from a pre-metallic. Pyrolysis studies using Cp2W2(CO)4(DMAD) [DMAD = Dimethyl-acetylene dicarboxylate], 5, as the premetallic demonstrate that 5 will decompose at temperatures of ∼ 700°C to give good yields of W2C. 5 is soluble and decomposes fully in 10–20 minutes, without the need of another reactant.

Boron nitride has been prepared in the past from classical high temperature reactions and more recently by CVD methods. Few attempts have been made to prepare this important material from pyrolyses of preceramic oligomers or polymers. In the present study oligomerization reactions of substituted borazenes with silyamine crosslinking groups have been found to provide useful gel materials which upon pyrolysis form boron nitrogen materials. Selected aspects of this chemistry and some characterization of the materials is presented.

A transition metal (e.g., Ru3 (CO)12, Pt/C) catalyzed process for Si-N bond formation is discussed that provides a new route to mono-, oligo-, and polysilazanes. The catalysts function by activating Si-H bonds in the pres-ence of ammonia. Polymeric silazanes can also be produced from oligomers in the presence of ammonia at low temperatures. This method allows us to control or modify the composition of the polysilazane during or after the polymeriza-tion. A variety of polysilazanes were prepared and converted to Si3 N4 with ceramic yields ranging from 55%-85%. By varying the monomers and reaction conditions, we can control the nitrogen and carbon content in the preceramic polymers, which enables us to obtain ceramic products that are primarily Si3N4and simultaneously minimizes the coproduction of SiC and C.

Sol-gel precursors to silicon carbide have been prepared using trifunctional chloro and alkoxysilanes which contain both the silicon and carbon necessary for SiC formation. Crosslinked gels having the ideal formula [RSiO1 5].]n have been synthesized by a hydrolysis/condensation scheme for a series of saturated and unsaturated R groups. The starting gels have been characterized by a variety of elemental analysis, spectroscopic and physical measurements including IR. XRD. TGA.. surface area and pore volume. A particularly powerful method for characterizing these gels is the combination of 13C and 29 Si solid state NMR which can provide information about the degree of crosslinking as well as residual hydroxy/alkoxy content.

The controlled pyrolysis of these gels has been used to prepare silicon carbide-containing ceramic products with surface areas in excess of 600m2/gm. The pyrolysis products are best described as a partially crystalline, partially amorphous mixture of β-SiC, silica and carbon. The effect of carbon chain length and the degree of unsaturation in the R group on the composition and surface area of the product has been determined. The origin of the high surface area of the pyrolysis products has been identified and its implications on potential uses of these materials is discussed.

A series of novel compounds was isolated in the course of exploratory work on the chemistry of titanium halides in liquid ammonia. This work was undertaken to study synthetic approaches to titanium-containing precursors of ceramic materials. Representative of these compounds is a mixed valence Ti(III)-Ti(IV) tetramer, [NH4 +·NH3]2[Ti4Br4 (NH2)12]−2, which was produced by the reaction of potassium borohydride and titanium IV bromide in liquid ammonia at room temperature. Similar ammonium salts of either Ti(IV) or Ti(III) were also prepared. The reaction of the ammonium salts with sodium acetylide in liquid ammonia evolves acetylene in an amount equivalent to the ammonium ion present. This provided the charge of the complex and yielded novel titanium acetylide derivatives. The acetylides convert into titanium carbonitrides upon thermal treatment to 800°C.

The reaction of titanium halides of their ammonolytic products with sodium acetylide in liquid ammonia to yield halogen-free acetylide precur-sors having a relatively high titanium content appears to be a convenient synthetic approach. This is made possible by the fact that the alkali metal halide by-products are soluble and easily separated in that reaction medium. This approach appears to be a generalized route applicable to a number of transition metal elements of interest.

Characterization of the pyrolysis products of preceramic polysilazanes synthesized by reactions of the oligomer-[H2SiNMe]x in the presence of Ru3 (CO)12 catalyst has demonstrated that these polymers have great potential as precursors of Si3N4 for several applications. The polysilazanes studied are viscous liquids that can be converted to ceramic material with a yield of >65 wt %. The vitreous product contains regions of fully dense material and large cavities that indicate of considerable gas evolution. A preliminary pyrolysis sequence has been constructed based on a combination of TGA of the polymer, SEM investigation of the product, and mass spectroscopic analysis of the gases and heavy fragments released during conversion of the polysilazanes to ceramic material. This understanding of pyrolysis mechanisms will aid in developing even more effective polymeric precursors to Si3N4 and in optimizing pyrolysis procedures for a variety of useful appl ications.

In recent years, there has been steadily increasing research activity directed towards preparation of ceramic materials via polymer pyrolysis. Among the most thermomechanically stable structural ceramics are SiC, Si3N4, and their solid solutions. These materials are widely used as ceramic fiber reinforcements in composites owing to their high mechanical strength, stiffness, and oxidative stability. Processes have now been developed to make these fibers by utilizing melt spinning. Fine diameter continuous filaments are formed, and further processing involves the steps of mild reactive cure and high-temperature pyrolysis. The approach in this review is to summarize the fiber making process with emphasis on improving fired properties by utilizing known mechanisms of fiber failure and process modification procedures. Fiber characteristics, applications of high performance fiber, and perceived trends will also be discussed.

Properties of a system at equilibrium depend on pressure, temperature and composition. Thus for a melt produced by the sol-gel (SG) process to be different from an identical composition melted from batch (MB), both cannot be at equilibrium. Non equilibrium melts can be associated with structural differences or homogeneity differences. The former have been suggested for SG melts while the latter is always possible in MB melts. Appropriate relaxation times are presented for structural and heterogeneity relaxation. From this it is concluded that structural differences will not persist unless the SG melt is metastable with respect to the equilibrium melt. A method for testing this unlikely premise is proposed.

Melts which are imperceptibly different from equilibrium may have non-equilibrium crystal embryo distributions that relax toward equilibrium with a time constant longer than that for structural relaxation. A difference in embryo and stable nuclei distribution will result in different crystallization kinetics.

A review is presented of some of the previously reported differences and similarities between comparable gel glasses (and gels) and ordinary glasses. In this regard, considerations are made with respect to such factors as structure, physical and thermal properties, and phase transformation behavior. A variety of silicate lass compositions are used for illustrative purposes. The discussion is roughly ivided into two sections; low and high temperature behavior. At low temperatures one anticipates that differences between gel and conventional glasses will exist, but such dissimilarities are not expected to persist to high temperatures. However, experimental evidence is presented which indicates the perpetuation of such differences to very high temperatures. A partial resolution for this anomalous behavior is offered.

Studies on the structures and crystallization behavior of gels in TiO2, GeO2, SiO2-GeO2 and PbO-GeO2 systems are presented to analyze the differences in structures and crystallization behavior of gel-derived glasses and conventional glasses. The kinetic parameters for crystallization such as activation energies and frequency factors of gels, gel-derived glasses and conventional glasses in the GeO2-PbO system are presented to elucidate their difference in nucleation and crystallization behaviors. The influence of gel processing parameters such as pH and water concentration on the crystallization behavior of gels are discussed.

The molecular structure of sol-gel processed silica can be studied using electron-spin resonance (ESR) spectroscopy. Spectra reveal changes in structure from sol to gel and from gel to glass. Gels prepared under various conditions contain intrinsic and extrinsic defects. Other spectroscopies that have been used to investigate gels are nuclear magnetic resonance (NMR), Raman, and visible.

The principles underlying controlled heterogeneous nucleation are reviewed. The application of these principles for the preparation of ceramic powders with equiaxed morphology and narrow size distribution is illustrated with examples from the literature and our recent work on alumina. It is concluded that controlled heterogeneous nucleation will be an important factor in fully realizing the benefits of chemically derived polycrystalline ceramics.

Alumina sols made by the hydrolysis of aluminum sec-butoxide in an acidic medium have been found to show behavior during drying and aging which appears similar to that reported for gels in a number of clay-water systems. Structural and phenomenological characteristics of these two classes of materials are compared. The extension to aluminum hydroxide systems of the models proposed to explain the gelation of clay sols are discussed, and a model is presented to describe the gelation behavior of aluminum hydroxide sols.

Ceramics can be prepared by pyrolysis of organosilicon polymers. Advantages of this method of ceramics preparation are; the ability to prepare shapes difficult to achieve by other methods such as fibers and films; the ability to achieve high purity because reagents used to prepare the polymer can be purified by well established chemical methods; processing at lower temperature than conventional methods [2].

The molecular dynamics computer simulation technique has been used to study silicic acid and pyrosilicic acid molecules (H4 SiO4 and H6 Si 2 O7, respectively). The structure of the simulated molecules are compared to those found by molecular orbital calculations as well as structures inferred from silicate hydrates. The potentials used to simulate the molecules were also used in simulations of bulk silicates and compared with experimental data. Results indicate good correlation.

Molecular Dynamics (MD) methods for modeling the structure and kinetic behavior of ceramics and glasses are reviewed and discussed while emphasizing the effect of various potential functions on the structure developed and the relevance of model calculations. Two dynamic experiments from the authors’ research are used as examples of modeling technique and interpretation of results. In the first, a Lennard-Jones force function is used to model the behavior of viscous liquids under large deformations. In the second, the brittle response of amorphous SiO2 modeled with an ionic Born-Mayer-Huggins interparticle potential is investigated at very high deformation rates. A comparison of results with experiment show the limitations and the great promise of the MD approach.

Molecular orbital calculations on hydroxyacid molecules with first- and secondrow X-cations (X = Li through N and Na through S) yield bond lengths and angles that mimic those of chemically similar minerals. These bond lengths are used to find a formula giving bond length as a function of a bond-strength parameter that reproduces XO bond lengths in crystals with main-group X-cations from all six rows of the periodic table within 0.05Å on average. The molecular orbital calculations also provide insights into reaction energies, physical properties of crystals such as electron density distributions, and data not amenable to direct measurement. They also provide a basis from which computational models for mineral structures may be constructed.