T. W. Zerda

SiC–CNT nanocomposites: high pressure reaction synthesis and characterization

Silicon carbide–carbon nanotube composite was fabricated using the high pressure reactive sintering technique. Samples were synthesized at high pressures, 2 and 8 GPa, and temperatures, 1770 and 1970 K. Their structures were studied using x-ray diffraction, x-ray photoelectron spectroscopy, transmission electron microscopy, and Raman scattering techniques. The composites produced at high pressure have pronounced nanocrystalline structure (the mean crystallite size of the SiC matrix was 32–37 nm) and very promising mechanical properties: fracture toughness of 6.8–7.1 MPa m0.5 and Vickers hardness of 20–21 GPa.

Diffusion and optical properties of Nd-doped sol-gel silica glasses

Abstract Water and acetone neodymium nitrate solutions of up to 2.3 mol/l were used to impregnate porous sol-gel glass by immersing outgassed samples in the liquid for a specified time. Diffusion coefficients of the solvents were determined using the diaphragm technique. Concentrations of the solute in the pores of samples having various average pore diameters were determined from the intensity of 740 nm absorption band. The time dependence of the concentrations at various penetration depths was used to calculate the diffusion coefficients from Ficks law. Concentration gradients inside the materials were produced by controlling the time the sample was exposed to the solution. To maintain the concentration profile during the drying stage of the process, the mobility of the solvent must be faster than that of the solute. When the sample was sintered at 1000 K neodymium was incorporated into the glass network. It was shown that both diverging and converging gradient index lenses can be produced.

In situ X-ray diffraction study of germanium at pressures up to 11 GPa and temperatures up to 950 K

Abstract In situ X-ray diffraction measurements on germanium were conducted in the pressure range of 5–11 GPa and temperatures up to 950 K. Using our data a better defined P–T diagram for germanium is presented. The coordinates of the triple point between GeI–GeII–GeL have been determined to a better degree of precision. The onsets of the GeI–GeII transition were found both under hydrostatic and non-hydrostatic conditions. Anisotropy of thermal expansion coefficient for the GeII is characterized from the c/a ratios in the temperature interval 473–823 K. Phases GeIII and GeIV are shown to be metastable forms of germanium.

Properties of nanostructured diamond-silicon carbide composites sintered by high pressure infiltration technique

A high-pressure silicon infiltration technique was applied to sinter diamond–SiC composites with different diamond crystal sizes. Composite samples were sintered at pressure 8 GPa and temperature 2170 K. The structure of composites was studied by evaluating x-ray diffraction peak profiles using Fourier coefficients of ab initio theoretical size and strain profiles. The composite samples have pronounced nanocrystalline structure: the volume-weighted mean crystallite size is 41–106 nm for the diamond phase and 17–37 nm for the SiC phase. The decrease of diamond crystal size leads to increased dislocation density in the diamond phase, lowers average crystallite sizes in both phases, decreases composite hardness, and improves fracture toughness.

Microstructure of nanocrystalline diamond powders studied by powder diffractometry

High resolution x-ray diffraction peaks of diamond nanosize powders of nominal sizes ranging from 5 to 250nm were analyzed and provided information on grain structure, average size of crystallites, and concentration of dislocations. Selected samples were heat treated at 1670K at pressures 2.0 and 5.5GPa or had surface modified by outgassing, heat treatment at vacuum conditions, and by controlled adsorption of gases. The apparent lattice parameter method was applied to characterize the structure of a shell-core model of nanosize particles. The multiple whole profile fitting provided information on crystallite sizes and density of dislocations. Population of dislocations increased with applied pressure, while strain and interplanar distances in the surface layers decreased. Adsorption of foreign gases on the grain surface modified the structure of the surface layers but did not affect dislocations near the center of the grains.

Diffusion of Solvents and Cations in Porous Sol-Gel Glass

Diffusion coefficients for water, cyclohexane, toluene, chloroform, acetone and acetonitrile in porous sol-gel glass were determined using the diaphragm and radioactive tracer techniques. Polar solvents were found to diffuse faster than nonpolar solvent within porous sol-gel glass. The diffusion coefficients of Nd3+ and Er3+ inside porous sol-gel glass were determined from concentration profiles within monoliths impregnated by 1.6M rare earth salts dissolved in either acetone or water. To study the effects of ligands on the diffusion, four different erbium salts were used: nitrate, chloride, bromide, and perchloride. It was found that the diffusion rate increases with decreasing radius of rare earth coordination sphere.

Solid-solid phase transitions of cyclohexane in porous sol-gel glass

Abstract Raman spectra of solid cyclohexane inside porous sol-gel glass were measured as a function of temperature and pore diameter. Samples with average pore diameters of 3.5 and 22 nm were used. The transition between the cubic and monoclinic phases of cyclohexane was accompanied by changes in the v21 band shape. From those changes, the depression of the phase transition temperature was estimated and was found to depend on the size of the pores. At low temperatures, cyclohexane in the porous glass formed a mixture of monoclinic crystallites and an amorphous phase. It is suggested that the amorphous structure is formed by molecules within a surface layer near the pore walls. It is shown that the extent of the amorphous structure can be altered by replacing surface hydroxyl groups with trimethylsilyl groups, i.e., by modifying surface interactions.

The mechanism of the solid-state reaction between carbon nanotubes and nanocrystalline silicon under high pressure and at high temperature

An x-ray powder diffraction method was used to study the reaction between carbon nanotubes (CNT) and silicon (Si) nanosize powder at 2 GPa and temperatures varying from 1273 to 1370 K with different sintering times. Samples were obtained using the piston–cylinder system. On the basis of the Avrami–Erofeev model, we found the activation energy of silicon carbide (SiC) formation from CNT and Si to be 96 ± 30 kJ mol−1. Analysis of x-ray diffraction patterns provided information on the domain sizes and microstrain in SiC. Extending the sintering time increased the grain sizes and decreased the microstrain in SiC, and increasing the temperature resulted in larger crystallites.

Microstructure evaluations of carbon nanotube/diamond/silicon carbide nanostructured composites by size?strain line-broadening analysis methods

Efforts have been made to manufacture novel composite materials with improved properties by incorporating multi-wall carbon nanotubes (MWNTs) into metal or brittle ceramic. Here we prepared dense MWNT/diamond/silicon carbide (SiC) composites under high-pressure and high-temperature (HPHT) conditions by anchoring MWNTs into a diamond/SiC matrix. The measured mechanical properties indicate that the composites have both superior hardness and enhanced fracture toughness. Moreover, we have undertaken an x-ray diffraction line-broadening analysis to elucidate how the microstructures including domain sizes and micro-strains depend on the externally applied temperature and how the microstructures correlate to the macroproperties of the as-fabricated composites.

Reaction kinetics of nanostructured silicon carbide

SiC nanowires were produced from carbon nanotubes and silicon by two different methods at high temperature. X-ray powder diffraction was used to determine SiC concentration. The reaction rate using the Avrami–Erofeev method was determined for samples sintered at temperatures ranging from 1313 to 1823 K. The activation energy was found to be (254 ± 36) kJ mol−1. The limiting factor in SiC formation is diffusion of silicon and carbon atoms through the produced layer of SiC.