Pu Sen Wang

Effects of soxhlet extraction on the surface oxide layer of silicon nitride powders

Abstract An aqueous soxhlet extraction procedure was used to surface-clean five commercial silicon nitride powders. The solid-solution interface properties were characterized before and after extraction by electrokinetic sonic amplitude measurements. The isoelectric point (pHiep) was found to increase significantly for some powders after treatment. The powder surface was analyzed using X-ray photoelectron spectroscopy and X-ray induced Auger electron spectroscopy before and after extraction. The surface oxygen content and oxide layer thickness decrease after treatment. A linear correlation was found between oxide thickness and (pHiep), which yields a pristine (pHiep) of about 9.7 for the unoxidized Si3N4 particle.

1H nuclear magnetic resonance characterization of Portland cement: molecular diffusion of water studied by spin relaxation and relaxation time-weighted imaging

Water molecular dynamics in a hardened Portland cement were characterized by proton Fourier transform nuclear magnetic resonance (NMR) at 400 MHz. Three different types of water molecule (physically bound, chemically bound and porous trapped) were observed. When the hardened cement sample was heated at 105 °C, the physically bound water diffused out of the sample as a function of the heating time while the chemically bound water remained in a stable form. A trace amount of the porously trapped water was also detected to remain in the cavities of the hardened cement even after heating for up to 20 h at this temperature. The loss of the physically bound water proved to be a diffusion-controlled process as evidenced from the NMR data and from a gravimetric technique. A Pake doublet was observed in the NMR spectra. This is a result of the oscillation of the water molecules with hindered molecular motions due to their entrapment in the cement pores. Soaking the dried samples in water resulted in the diffusion of water back into the hardened cement as physically bound water. Nuclear spin–spin relaxation time, T2-weighted imaging showed that the distribution of the physically bound water inside the cylindrical sample formed a doughnut shape after overnight soaking. The residual air in the cement pores may have slowed down the diffusion rate of the water molecules back into the dried cement.

OXIDATION KINETICS OF SILICON CARBIDE WHISKERS STUDIED BY X-RAY PHOTOELECTRON SPECTROSCOPY

Silicon carbide whisker surfaces were characterized by X-ray photoelectron spectroscopy (XPS) to determine changes in the surface oxide film which occurred as a result of heating in air at temperatures from 600 to 800 °C. Equations were derived for the calculation of surface oxide film thickness from the SiC to SiO2 2p intensity ratios. Oxidation was found to follow a linear rate law in this temperature range for the first 10 nm of oxide growth. An activation energy of 17.2 ± 2.8 kcal mol−1 (72 ± 12 kJ mol−1) was measured.

The oxidation of an aluminum nitride powder studied by bremsstrahlung-excited Auger electron spectroscopy and x-ray photoelectron spectroscopy

Bremsstrahlung-excited Auger electron spectroscopy (AES) was used to study the oxidation kinetics of an aluminum nitride powder oxidized in air at 750, 800, 850, and 900 ‐ C. An equation was derived to calculate the average surface oxide film thickness from the aluminum AES spectra. The oxidation of this powder was found to follow a parabolic rate law within this temperature range. The measured activation energy was 230 6 17 kJymol (55 6 4 kcalymol). Analysis with x-ray photoelectron spectroscopy (XPS) showed that in addition to the nitride N 1s peak, there was a second N 1s peak. This peak has been observed in previous studies and can be attributed to N‐ O bonding either within the growing oxide film or at the Al 2O3yAlN interface.

Binder distribution in Si3N4 ceramic green bodies studied by stray-field NMR imaging

Nuclear magnetic resonance (NMR) imaging is an emerging technology which provides a unique material-diagnostic technique by in situ internal mapping. It can provide information not only on material distribution, but also on the chemical and physical characteristics of materials. However, due to the nuclear dipole dipole interaction in solid state materials, NMR spectroscopic signals are normally very broad. NMR imaging based on these unresolved broad lines is extremely difficult, and resolution is poor. The binder distribution was studied in ceramic green bodies with a stray-field NMR imaging facility at a proton frequency of 163 MHz near the edge of a 9.394 T superconducting magnet. The 1H nuclear spin echo signal from silicon nitride green bodies containing 10 wt% of either polyethylene glycol or polyvinyl alcohol as a binder was detected at 163 MHz. NMR images show a good homogeneity of the binder distribution in the cross-sections of the samples. Overall results show that the distribution of polyethylene glycol in Si3N4 green bodies is more homogeneous than that of polyvinyl alcohol under similar processing parameters. NMR spectroscopic results also indicate a higher moisture content in the green bodies containing a polyvinyl alcohol binder.

Surface oxidation kinetics of Si3N4-4%Y2O3 powders studied by Bremsstrahlung-excited Auger spectroscopy

Samples of silicon nitride powder containing 4.0% Y2O3 in weight were heated in air at temperatures between 900 and 1000 °C. The average SiO2 layer thickness on the Si3N4 powder particles, as a function of time at a particular temperature, was measured by Bremsstrahlung-excited Auger electron spectroscopy. Oxidation was found to follow a linear rate law with an activation energy of 56±1.5 kcal mol−1. The yttrium level measured by X-ray photoelectron spectroscopy was also found to decrease as a function of the oxide layer thickness. This suggests that there is a reaction between the Si3N4 and Y2O3 particles which results in the formation of an yttrium-rich phase at the interface between the surface SiO2 layer and the underlying Si3N4 particle.

The Aggregation Phenomena of a Calcium Sulfonate Additive in Hydrocarbon Media by Dynamic Light Scattering

The aggregation phenomena of a calcium sulfonate additive in several hydrodynamic solvents was studied by dynamic light scattering. The Stokes-Einstein equation was used to calculate the average hydrocarbon radii. The aggregate size of calcium sulfonate in hexadecane was found to increase moderately with increasing concentrations. No critical micellar concentration was observed at even concentrations as low as 0.2 millimole/liter. Within the temperature range studied, 15.8° — 47.1°C, the aggregate size decreased moderately as the temperature increased. Hexane, heptane, nonane, dodecane, 1-heptene, 1-heptanol, 2-heptanone, and heptanoic acid were used to study the effect of the solvent. A solvent with a longer chain length tended to result in a smaller aggregate size. The polarity of the solvent also had a critical effect on the aggregate size. The results were discussed by using a thermodynamic model for the aggregation of amphiphiles in nonaqueous solvents. Presented at the 50th Annual Meeting in Chicago...

Aluminium hydroxide growth on aluminium surfaces exposed to an air/1% NO2 mixture

Aluminium samples were exposed to a mixture of 1% NO2 in air at 100%, 52% and <5% relative humidity. Auger electron spectroscopy depth profiling showed that for the 100% and 52% r.h. environments, the thickness of the surface oxide/hydroxide layer increased linearly with exposure time. X-ray photoelectron spectroscopy was used to study the surface composition as a function of exposure time. The XPS data clearly showed that a transformation from AlOOH to Al(OH)3 was occurring with time for high humidity exposures. Thick hydroxide layers which formed after prolonged exposure, were also analysed with scanning electron microscopy, thermogravimetric analysis, and X-ray diffraction.[/p]

XPS study of the dehydration of clay and kaolin powders

X-ray photoelectron spectroscopy (XPS) was used to analyze samples of a commercial clay powder and a kaolin standard. Samples were analyzed both in the as-received condition and after heating to temperatures of ≥750 °C. There was a loss of water during heating that corresponded to the transformation from a clay to an aluminosilicate. The XPS measurements showed that there was a decrease in the O/(Al + Si) ratio and a decrease of 0.2–0.3 eV in the aluminum Auger parameter value. The silicon Auger parameter value was unchanged after heating. Copyright

NMR characterization of injection-moulded alumina green compacts: Part II T2-weighted proton imaging

Injection-moulded alumina green compacts containing polypropylene, wax, and stearic acid were studied for binder distribution by proton nuclear magnetic resonance (1H NMR) imaging. The solid imaging technique of nuclear spin-spin relaxation time,T2, weighted imaging at 400 MHz was used. This imaging technique utilizes a multiple pulse sequence ofD0-(π/2)±x-Τ-D10-D7-(π)x-D7-D10-AQ-D0 for echo detection and phase encoding. Two-and three-dimensional images were constructed from the intensities of these nuclear echo signals. Spatially resolved two-dimensional images obtained by the application of this technique indicated that the green compacts fabricated from the same nominal binder composition did not contain the same amounts of binder. This observation agrees well with our previous conclusion drawn from nuclear spin echo studies by Hahns pulse sequence. A 64×64×64 three-dimensional imaging revealed that the inhomogeneity of binder distribution and internal imperfection do exist at certain locations of the samples. A binder-rich folding line was also detected in one of these green compacts.