Xiaohe Chen

Ftir study of a nanostructured aluminum nitride powder surface: Determination of the acidic/basic sites by CO, CO2 and acetic acid adsorptions

Abstract The surface contamination of nanosized powders is a critical factor influencing the overall material properties. For non-oxide ceramics, hydrolysis leads to a surface reactivity close to that of the corresponding oxide. However, the surface behavior of these non-oxide nanostructured ceramics often results in chemical specificities. A thorough investigation of the surface composition and reactivity of a nanostructured aluminum nitride powder, already started in previous papers, is pursued here. The Fourier transform infrared spectrometric study of carbon monoxide, carbon dioxide and acetic acid adsorptions on the activated aluminum nitride surface revealed the presence of Bronsted and Lewis basic sites. Furthermore, the aluminum nitride surface chemical specificity with respect to the γ-alumina surface was demonstrated.

SYNTHESIS AND PROPERTIES OF AN ALUMINUM NITRIDE/POLYIMIDE NANOCOMPOSITE PREPARED BY A NONAQUEOUS SUSPENSION PROCESS

A nanocomposite of a chemically synthesized nanostructured aluminum nitride (AlN) and a polyimide has been studied. Using a nonaqueous polar solvent, N -methylpyrrolidinone (NMP), as the suspension media, the degree of particle agglomeration of AlN was reduced dramatically from micron to nanoscale size. Upon the addition of poly(amic acid) to the AlN/NMP suspension, a further deagglomeration of the particles was observed. The surface physicochemical interactions have been investigated by characteristic model reactions using FTIR spectroscopy. A mechanistic interpretation for the deagglomeration and stabilization behavior is discussed. The formation of the AlN/PI nanocomposite was achieved by the rapid solidification of the precursor suspension followed by compression molding. Such an approach for nanocomposites exhibits better homogeneity with ultrafine fillers and allows a tailorable composition and property at the nanoscale level. Finally, AlN/PI nanocomposites with an increased ceramic loading, up to 65% by volume, were attained and their thermal and mechanical properties, along with the compositional effects, have been investigated.

Mechanistic investigation of the preparation of polymer/ceramic nanocomposites

Abstract Polyimide/aluminum nitride (AlN) nanocomposites with very high packing density were obtained through an efficient solution mixing method. The homogeneous composites showed a trend of decreasing thermal expansion, and increasing hardness and Youngs modulus, as the ceramic loading increased. A detailed investigation of the preparation process, via in-situ FT-IR analysis of a series of probe molecule reactions on the particle surface, suggested a strong chemisorption of amide groups on the nanoparticle surface. Therefore, a reaction mechanism for the chemisorption is proposed. Considering the structural similarity of the probe molecules and poly(amic acid), a precursor to polyimide, indications are that (1)chemisorption plays an important role for mixing processing of a nanoparticle/organic medium; and (2) amide group adsorption is one of the most efficient interaction during the suspension processing for the chemically synthesized nanostructured AlN powders.

Synthesis and nonlinear optical characterization of nanostructured gold/polymer composites and suspensions

Abstract Nanometric gold particles were synthesized by a liquid/liquid phase-transfer reaction. Composites of these particles were prepared by free-radical polymerization of suspensions of the particles in styr ene and methyl methacrylate monomer. Concentrations of 1.0, 1.3, and 4.4 mg/ml were prepared,with no noticeable agglomeration of particles during processing. These composites and toluene suspensions (0.18 mg/ml) showed a linear absorption peak at 530 nm, which is characteristic of nanosized gold. High-resolution electron microscopy measurement showed that the particle diameters varied from 5 to 10 nm. Degenerate four-wave mixing experiments at 532 nm yielded, on 10-micron films, a maximum value of 1.0 × 10 −10 e.s.u. for the third-order nonlinear optical susceptibility. The toluene suspension had a susceptibility of 7.7 × 10 −12 e.s.u.

Studies on the Structure and Properties of Ceramic/Polymernanocomposites

Unique synthetic approaches for the synthesis of homogeneously dispersed and highly loaded aluminum nitride (AIN) / polyimide (PI) nanocomposites have been developed. The effective interactions at the solid-liquid interface during the preparation of stable dispersions of ceramic/polymers have been investigated. In particular, the surface chemical composition of the nanoparticles has been analyzed. Characteristic model reactions on the surface have been carried out, which revealed the mechanisms for the deagglomeration and stabilization of nanoparticles via chemisorption reactions. Moreover, compared to other synthetic approaches, this method demonstrates the capability of preparing extremely highly loaded nanocomposites and being applicable to a wide range of materials. The thermal and mechanical properties of the A1N/PI nanocomposite have also been studied.

FTIR analysis of the surface of nanostructured aluminium nitride powder prepared via chemical synthesis

Surface chemistry analysis of nanostructured aluminium nitride powder, obtained by a chemical synthesis method, has been conducted using FTIR spectroscopy. The study compared the spectra of the as-synthesized powder, the activated powder treated at temperatures from 573 to 873 K, the surface-deuteriated powder, and the powder treated under ammonia. Hydroxy and amine functional groups were determined on the surface of the as-synthesized powder. The activity and the types of these groups were examined. The surface analysis results were compared with those of γ-Al2O3. The unique surface characteristics of the nanostructured AlN are discussed.

Tem and hrtem characterization of nanostructured M50 type steel

Nanostructured M50 type steel materials were prepared by hot pressing the precursor powders. The precursor powders were chemically synthesized using two different techniques, namely, thermal decomposition and co-reduction. During the hot press process, the precursor powders were transformed to nanocrystalline phases with the precipitation of fine carbides. Simultaneously, the crystalline powders were densified. The densified samples were studied using both conventional and high resolution transmission electron microscopy. The effects of hot pressing parameters such as temperature, time and pressure on the development of nanostructures, grain growth and formation of defects are discussed.

FT-IR characterization of the acidic and basic sites on a nanostructured aluminum nitride surface

A nanostructured aluminum nitride powder prepared by sol-gel type chemical synthesis is analyzed by Fourier transform infrared spectrometry. The surface acidic and basic sites are probed out by adsorption of several organic molecules. Resulting from the unavoidable presence of oxygen, the aluminum nitride surface is an oxinitride layer in fact, and its surface chemistry should present some analogies with alumina. Therefore, a thorough comparison between the acido-basicity of aluminum nitride and aluminum oxide is discussed. The remaining nitrogen atoms in the first atomic layer modify the acidity-basicity relative balance and reveals the specificity of the aluminum nitride surface.